EPICS R3.14 Channel Access Reference Manual
Jeffrey O. Hill
Los Alamos National
Laboratory, SNS Division
Ralph Lange
BESSY
Copyright © 2002 The
University of Chicago, as Operator of Argonne National Laboratory.
Copyright © 2002 The Regents of the University of California, as Operator of
Los Alamos National Laboratory.
Copyright © 2002 Berliner Elektronenspeicherringgesellschaft für
Synchrotronstrahlung.
EPICS BASE Versions
3.13.7 and higher are distributed subject to a Software License Agreement found
in the file LICENSE that is included with this distribution.
Modified on
$Date: 2009-03-20$
Table of Contents
Functionality Index
Deprecated Function Call Interface Function Index
Configuration
Why Reconfigure Channel Access
Typically reasons to reconfigure EPICS Channel Access:
- Two independent control systems must share a network without fear of
interaction
- A test system must not interact with an operational system
- Use of address lists instead of broadcasts for name resolution and server
beacons
- Control system occupies multiple IP subnets
- Nonstandard client disconnect time outs or server beacon intervals
- Specify the local time zone
- Transport of large arrays
All Channel Access (CA) configuration occurs through EPICS environment
variables. When searching for an EPICS environment variable EPICS first looks
in the environment using the ANSI C getenv() call. If no matching variable
exists then the default specified in the EPICS build system configuration files
is used.
Name |
Range |
Default |
EPICS_CA_ADDR_LIST |
{N.N.N.N N.N.N.N:P ...} |
<none> |
EPICS_CA_AUTO_ADDR_LIST |
{YES, NO} |
YES |
EPICS_CA_CONN_TMO |
r > 0.1 seconds |
30.0 |
EPICS_CA_BEACON_PERIOD |
r > 0.1 seconds |
15.0 |
EPICS_CA_REPEATER_PORT |
i > 5000 |
5065 |
EPICS_CA_SERVER_PORT |
i > 5000 |
5064 |
EPICS_CA_MAX_ARRAY_BYTES |
i >= 16384 |
16384 |
EPICS_CA_MAX_SEARCH_PERIOD |
r > 60 seconds |
300 |
EPICS_TS_MIN_WEST |
-720 < i <720 minutes |
360 |
Environment variables are set differently depending on the command line
shell that is in use.
C shell |
setenv EPICS_CA_ADDR_LIST 1.2.3.4 |
bash |
export EPICS_CA_ADDR_LIST=1.2.3.4 |
vxWorks shell |
putenv ( "EPICS_CA_ADDR_LIST =1.2.3.4" ) |
DOS command line |
set EPICS_CA_ADDR_LIST=1.2.3.4 |
Windows NT / 2000 / XP |
control panel / system / environment tab |
CA and Wide Area Networks
Normally in a local area network (LAN) environment CA discovers the address
of the host for an EPICS process variable by broadcasting frames containing a
list of channel names ( CA search messages ) and waiting for responses from the
servers that host the channels identified. Likewise CA clients efficiently
discover that CA servers have recently joined the LAN or disconnected from the
LAN by monitoring periodically broadcasted beacons sent out by the servers.
Since hardware broadcasting requires special hardware capabilities, we are
required to provide additional configuration information when EPICS is extended
to operate over a wide area network (WAN).
Channel Access is implemented using internet protocols (IP). IP addresses
are divided into host and network portions. The boundary between each portion
is determined by the IP netmask. Portions of the IP address corresponding to
zeros in the netmask specify the hosts address within an IP subnet. Portions of
the IP address corresponding to binary ones in the netmask specify the address
of a host's IP subnet. Normally the scope of a broadcasted frame will be
limited to one IP subnet. Addresses with the host address portion set to all
zeros or all ones are special. Modern IP kernel implementations reserve
destination addresses with the host portion set to all ones for the purpose of
addressing broadcasts to a particular subnet. In theory we can issue a
broadcast frame on any broadcast capable LAN within the interconnected internet
by specifying the proper subnet address combined with a host portion set to all
ones. In practice these "directed broadcasts" are frequently limited by the
default router configuration. The proper directed broadcast address required to
reach a particular host can be obtained by logging into that host and typing
the command required by your local operating environment. Ignore the loop back
interface and use the broadcast address associated with an interface connected
to a path through the network to your client. Typically there will be only one
Ethernet interface.
UNIX |
ifconfig -a |
vxWorks |
ifShow |
Windows |
ipconfig |
IP ports are positive integers. The IP address, port number, and protocol
type uniquely identify the source and destination of a particular frame
transmitted between computers. Servers are typically addressed by a well known
port number. Clients are assigned a unique ephemeral port number during
initialization. IP ports below 1024 are reserved for servers that provide
standardized facilities such as mail or file transfer. Port number between 1024
and 5000 are typically reserved for ephemeral port number assignments.
The two default IP port numbers used by Channel Access may be reconfigured.
This might occur when a site decides to set up two or more completely
independent control systems that will share the same network. For instance, a
site might set up an operational control system and a test control system on
the same network. In this situation it is desirable for the test system and the
operational system to use identical PV names without fear of collision. A site
might also configure the CA port numbers because some other facility is already
using the default port numbers. The default Channel Access port numbers have
been registered with IANA.
Purpose |
Default |
Environment Variable |
CA Server |
5064 |
EPICS_CA_SERVER_PORT |
CA Beacons (sent to CA repeater daemon) |
5065 |
EPICS_CA_REPEATER_PORT |
If a client needs to communicate with two servers that are residing at
different port numbers then an extended syntax may be used with the
EPICS_CA_ADDR_LIST environment variable. See WAN
Environment below.
When the CA client library connects a channel it must first determine the IP
address of the server the channels Process Variable resides on. To accomplish
this the client sends name resolution (search) requests to a list of server
destination addresses. These server destination addresses can be IP unicast
addresses (individual host addresses) or IP broadcast addresses. Each name
resolution (search) request contains a list of Process Variable names.If one of
the servers reachable by this address list knows the IP address of a CA server
that can service one or more of the specified Process Variables, then it sends
back a response containing the server's IP address and port number.
During initialization CA builds the list of server destination addresses
used when sending CA client name resolution (search) requests. This table is
initialized by introspecting the network interfaces attached to the host. For
each interface found that is attached to a broadcast capable IP subnet, the
broadcast address of that subnet is added to the list. For each point to point
interface found, the destination address of that link is added to the list.
This automatic server address list initialization can be disabled if the EPICS
environment variable "EPICS_CA_AUTO_ADDR_LIST" exists and its value is either
of "no" or "NO". The typical default is to enable network interface
introspection driven initialization with "EPICS_CA_AUTO_ADDR_LIST" set to "YES"
or "yes".
Following network interface introspection, any IP addresses specified in the
EPICS environment variable EPICS_CA_ADDR_LIST are added to the list of
destination addresses for CA client name resolution requests. In an EPICS
system crossing multiple subnets the EPICS_CA_ADDR_LIST must be set so that CA
name resolution ( search requests ) frames pass from CA clients to the targeted
CA servers unless a CA proxy (gateway) is installed. The addresses in
EPICS_CA_ADDR_LIST may be dotted IP addresses or host names if the local OS has
support for host name to IP address translation. When multiple names are added
to EPICS_CA_ADDR_LIST they must be separated by white space. There is no
requirement that the addresses specified in the EPICS_CA_ADDR_LIST be a
broadcast addresses, but this will often be the most convenient choice.
C shell |
setenv EPICS_CA_ADDR_LIST "1.2.3.255 8.9.10.255" |
bash |
export EPICS_CA_ADDR_LIST="1.2.3.255 8.9.10.255" |
vxWorks |
putenv ( "EPICS_CA_ADDR_LIST=1.2.3.255 8.9.10.255" ) |
If a client needs to communicate with two servers that are residing at
different port numbers then an extended syntax may be used with the
EPICS_CA_ADDR_LIST environment variable. Each host name or IP address in the
EPICS_CA_ADDR_LIST may be immediately followed by a colon and an IP port number
without intervening whitespace. Entries that do not specify a port number will
default to EPICS_CA_SERVER_PORT.
C shell |
setenv EPICS_CA_ADDR_LIST "1.2.3.255 8.9.10.255:10000" |
Frequently vxWorks systems boot by default with routes limiting access only
to the local subnet. If a EPICS system is operating in a WAN environment it may
be necessary to configure routes into the vxWorks system which enable a vxWorks
based CA server to respond to requests originating outside it's subnet. These
routing restrictions can also apply to vxWorks base CA clients communicating
with off subnet servers. An EPICS system manager can implement an rudimentary,
but robust, form of access control for a particular host by not providing
routes in that host that reach outside of a limited set of subnets. See
"routeLib" in the vxWorks reference manual.
If the CA client library does not see a beacon from a server that it is
connected to for EPICS_CA_CONN_TMO seconds then an state-of-health message is
sent to the server over TCP/IP. If this state-of-health message isn't promptly
replied to then the client library will conclude that channels communicating
with the server are no longer responsive and inform the CA client side
application via function callbacks. The parameter EPICS_CA_CONN_TMO is
specified in floating point seconds. The default is typically 30 seconds. For
efficient operation it is recommended that EPICS_CA_CONN_TMO be set to no less
than twice the value specified for EPICS_CA_BEACON_PERIOD.
Prior to EPICS R3.14.5 an unresponsive server implied an immediate TCP
circuit disconnect, immediate resumption of UDP based search requests, and
immediate attempts to reconnect. There was concern about excessive levels of
additional activity when servers are operated close to the edge of resource
limitations. Therefore with version R3.14.5 and greater the CA client library
continues to inform client side applications when channels are unresponsive,
but does not immediately disconnect the TCP circuit. Instead the CA client
library postpones circuit shutdown until receiving indication of circuit
disconnect from the IP kernel. This can occur either because a server is
restarted or because the IP kernel's internal TCP circuit inactivity keep alive
timer has expired after a typically long duration (as is appropriate for IP
based systems that need to avoid thrashing during periods of excessive load).
The net result is less search and TCP circuit setup and shutdown activity
suring periods of excessive load.
The CA client library will continuously attempt to connect any CA channels
that an application has created until it is successful. The library
periodically queries the server destination address list described above with
name resolution requests for any unresolved channels. Since this address list
frequently contains broadcast addresses, and because nonexistent process
variable names are frequently configured, or servers may be temporarily
unavailable, then it is necessary for the CA client library internals to
carefully schedule these requests in time to avoid introducing excessive load
on the network and the servers.
When the CA client library has many channels to connect, and most of its
name resolution requests are responded to, then it sends name resolution
requests at an interval that is twice the estimated round trip interval for the
set of servers responding, or at the minimum delay quantum for the operating
system - whichever is greater. The number of UDP frames per interval is also
dynamically adjusted based on the past success rates.
If a name resolution request is not responded to, then the client library
doubles the delay between name resolution attempts and reduces the number of
requests per interval. The maximum delay between attempts is limited by
EPICS_CA_MAX_SEARCH_PERIOD (see Configuring the Maximum
Search Period). Note however that prior to R3.14.7, if the client library
did not receive any responses over a long interval it stoped sending name
resolution attempts altogether until a beacon anomaly was detected (see
below).
The CA client library continually estimates the beacon period of all server
beacons received. If a particular server's beacon period becomes significantly
shorter or longer then the client is said to detect a beacon anomaly. The
library boosts the search interval for unresolved channels when a beacon
anomaly is seen or when any successful search response is received,
but with a longer initial interval between requests than is used when the
application creates a channel. Creation of a new channel does not
(starting with EPICS R3.14.7) change the interval used when searching for
preexisting unresolved channels. The program "casw" prints a message on
standard out for each CA client beacon anomaly detect event.
See also When a Client Does not See the Server's
Beacon.
The rate at which name resolution (search) requests are sent exponentially
backs off to a plateau rate. The value of this plateau has an impact on network
traffic because it determines the rate that clients search for channel names
that are miss-spelled or otherwise don't exist in a server. Furthermore, for
clients that are unable to see the beacon from a new server, the plateau rate
may also determine the maximum interval that the client will wait until
discovering a new server.
Starting with EPICS R3.14.7 this maximum search rate interval plateau in
seconds is determined by the EPICS_CA_MAX_SEARCH_PERIOD environment
variable.
See also When a Client Does not See the Server's
Beacon.
When several client processes run on the same host it is not possible for
all of them to directly receive a copy of the server beacon messages when the
beacon messages are sent to unicast addresses, or when legacy IP kernels are
still in use. To avoid confusion over these restrictions a special UDP server,
the CA Repeater, is automatically spawned by the CA client library when it is
not found to be running. This program listens for server beacons sent to the
UDP port specified in the EPICS_CA_REPEATER_PORT parameter and fans any beacons
received out to any CA client program running on the same host that have
registered themselves with the CA Repeater. If the CA Repeater is not already
running on a workstation, then the "caRepeater" program must be in your path
before using the CA client library for the first time.
If a host based IOC is run on the same workstation with standalone CA client
processes, then it is probably best to start the caRepeater process when the
workstation is booted. Otherwise it is possible for the standalone CA client
processes to become dependent on a CA repeater started within the confines of
the host based IOC. As long as the host based IOC continues to run there is
nothing wrong with this situation, but problems could arise if this host based
IOC process exits before the standalone client processes which are relying on
its CA repeater for services exit.
Since the repeater is intended to be shared by multiple clients then it
could be argued that it makes less sense to set up a CA repeater that listens
for beacons on only a subset of available network interfaces. In the worst case
situation the client library might see beacon anomalies from servers that it is
not interested in. Modifications to the CA repeater forcing it to listen only
on a subset of network interfaces might be considered for a future release if
there appear to be situations that require it.
Note: Starting with EPICS R3.14 all of the libraries in the EPICS base
distribution rely on facilities built into the operating system to determine
the correct time zone. Nevertheless, several programs commonly used with EPICS
still use the original "tssubr" library and therefore they still rely on proper
configuration of EPICS_TS_MIN_WEST.
While the CA client library does not translate inbetween the local time and
the time zone independent internal storage of EPICS time stamps, many EPICS
client side applications call core EPICS libraries which provide these
services. To set the correct time zone users must compute the number of
positive minutes west of GMT (maximum 720 inclusive) or the negative number of
minutes east of GMT (minimum -720 inclusive). This integer value is then placed
in the variable EPICS_TS_MIN_WEST.
Time Zone |
EPICS_TS_MIN_WEST |
USA Eastern |
300 |
USA Central |
360 |
USA Mountain |
420 |
USA Pacific |
480 |
Alaska |
540 |
Hawaii |
600 |
Japan |
-540 |
China |
-420 |
Germany |
-120 |
United Kingdom |
0 |
Starting with version R3.14 the environment variable
EPICS_CA_MAX_ARRAY_BYTES determines the size of the largest array that may pass
through CA. Prior to this version only arrays smaller than 16k bytes could be
transfered. The CA libraries maintains a free list of 16384 byte network
buffers that are used for ordinary communication. If EPICS_CA_MAX_ARRAY_BYTES
is larger than 16384 then a second free list of larger data buffers is
established and used only after a client send its first large array request.
The CA client library uses EPICS_CA_MAX_ARRAY_BYTES to determines the
maximum array that it will send or receive. Likewise, the CA server uses
EPICS_CA_MAX_ARRAY_BYTES to determine the maximum array that it may send or
receive. The client does not influence the server's message size quotas and
visa versa. In fact the value of EPICS_CA_MAX_ARRAY_BYTES need not be the same
in the client and the server. If the server receives a request which is too
large to read or respond to in entirety then it sends an exception message to
the client. Likewise, if the CA client library receives a request to send an
array larger than EPICS_CA_MAX_ARRAY_BYTES it will return ECA_TOLARGE.
A common mistake is to correctly calculate the maximum datum size in bytes
by multiplying the number of elements by the size of a single element, but
neglect to add additional bytes for the compound data types (for example
DBR_GR_DOUBLE) commonly used by the more sophisticated client side
applications. Based on this confusion, one could arrive at the conclusion
that EPICS_CA_MAX_ARRAY_BYTES might have been better named
EPICS_CA_MAX_DATUM_BYTES, or that the software should be changed internally to
round the users request up by the size of the maximum scalar datum (nothing has
been done to address this issue so far).
Name |
Range |
Default |
EPICS_CAS_SERVER_PORT |
i > 5000 |
EPICS_CA_SERVER_PORT |
EPICS_CAS_AUTO_BEACON_ADDR_LIST |
{YES, NO} |
EPICS_CA_AUTO_ADDR_LIST |
EPICS_CAS_BEACON_ADDR_LIST |
{N.N.N.NN.N.N.N:P...} |
EPICS_CA_ADDR_LIST1 |
EPICS_CAS_BEACON_PERIOD |
r > 0.1 seconds |
15.0 |
EPICS_CAS_BEACON_PORT |
i > 5000 |
EPICS_CA_REPEATER_PORT |
EPICS_CAS_INTF_ADDR_LIST |
{N.N.N.NN.N.N.N:P...} |
<none> |
EPICS_CAS_IGNORE_ADDR_LIST |
{N.N.N.NN.N.N.N:P...} |
<none> |
Server Port
The server configures its port number from the EPICS_CAS_SERVER_PORT
environment variable if it is specified. Otherwise the EPICS_CA_SERVER_PORT
environment variable determines the server's port number. Two servers can share
the same UDP port number on the same machine, but there are restrictions - see
a discussion of unicast addresses and two servers sharing
the same UDP port on the same host.
Server Beacons
The EPICS_CAS_BEACON_PERIOD parameter determines the server's beacon period
and is specified in floating point seconds. The default is typically 15
seconds. See also EPICS_CA_CONN_TMO and Dynamic Changes in the CA Client Library Search
Interval.
CA servers build a list of addresses to send beacons to during
initialization. If EPICS_CAS_AUTO_BEACON_ADDR_LIST has the value "YES" then the
beacon address list will be automatically configured to contain the broadcast
addresses of all LAN interfaces found in the host and the destination address
of all point-to-point interfaces found in the host. However, if the user also
defines EPICS_CAS_INTF_ADDR_LIST then beacon address list automatic
configuration is constrained to the network interfaces specified therein, and
therefore only the broadcast addresses of the specified LAN interfaces, and the
destination addresses of all specified point-to-point links, will be
automatically configured.
If EPICS_CAS_BEACON_ADDR_LIST is defined then its contents will be used to
augment any automatic configuration of the beacon address list. Individual
entries in EPICS_CAS_BEACON_ADDR_LIST may override the destination port number
if ":nnn" follows the host name or IP address there. Alternatively, when both
EPICS_CAS_BEACON_ADDR_LIST and EPICS_CAS_INTF_ADDR_LIST are not defined then
the contents of EPICS_CA_ADDR_LIST is used to augment the list. Otherwise, the
list is not augmented.
The EPICS_CAS_BEACON_PORT parameter specifies the destination port for
server beacons. The only exception to this occurs when ports are specified in
EPICS_CAS_BEACON_ADDR_LIST or possibly in EPICS_CA_ADDR_LIST. If
EPICS_CAS_BEACON_PORT is not specified then beacons are sent to the port
specified in EPICS_CA_REPEATER_PORT.
Binding a Server to a Limited Set of Network Interfaces
The parameter EPICS_CAS_INTF_ADDR_LIST allows a ca server to bind itself to,
and therefore accept messages only over, a limited set of the local host's
network interfaces (each specified by it's IP address). On UNIX systems type
"netstat -i" (type "ipconfig" on windows) to see a list of the local host's
network interfaces. Specifically, UDP search messages addressed to both the IP
addresses in EPICS_CAS_INTF_ADDR_LIST and also to the broadcast addresses of
the corresponding LAN interfaces will be accepted by the server. By default,
the CA server is accessible from all network interfaces configured into its
host. In R3.14 and previous releases the CA server employed by iocCore does
not implemet this feature.
Ignoring Process Variable Name Resolution Requests From Certain Hosts
Name resolution requests originating from any of the IP addresses specified
in the EPICS_CAS_IGNORE_ADDR_LIST parameter are not replied to.In R3.14 and
previous releases the CA server employed by iocCore does not implemet this
feature.
Client Configuration that also Applies to Servers
See also Configuring the Maximum Array Size.
See also Routing Restrictions on vxWorks Systems.
An application that uses the CA client library functions described in this
document will need to include the cadef.h header files as follows.
#include "cadef.h"
This header file is located at "<EPICS base>/include/". It includes
many other header files (operating system specific and otherwise), and
therefore the application must also specify "<EPICS
base>/include/os/<arch>" in its header file search path.
An application that uses the Channel Access Client Library functions
described in this document will need to link with the EPICS CA Client Library
and also the EPICS Common Library. The EPICS CA Client Library calls the EPICS
Common Library. The following table shows the names of these libraries on UNIX
and Windows systems.
|
UNIX Object |
UNIX Shareable |
Windows Object |
Windows Shareable |
EPICS CA Client Library |
libca.a |
libca.so |
ca.lib |
ca.dll |
EPICS Common Library
|
libCom.a |
libCom.so |
Com.lib |
Com.dll |
The above libraries are located in "<EPICS
base>/lib/<architechture>".
If you do not use the EPICS build environemnt (layered make files) then it
may be helpful to run one of the EPICS make files and watch the compile/link
lines. This may be the simplest way to capture the latest system and compiler
specific options required by your build environment. I have included some
snapshots of typical build lines below, but expect some risk of this
information becoming dated.
Typical Linux Build Options
/usr/bin/gcc -c -D_POSIX_C_SOURCE=199506L -D_POSIX_THREADS
-D_XOPEN_SOURCE=500 -DOSITHREAD_USE_DEFAULT_STACK -D_X86_ -DUNIX -D_BSD_SOURCE
-Dlinux -D_REENTRANT -ansi -O3 -Wall -I. -I.. -I../../../include/os/Linux
-I../../../include ../acctst.c
/usr/bin/g++ -o acctst
-L/home/user/epicsR3.14/epics/base/lib/linux-x86/
-Wl,-rpath,/mnt/bogart_home/hill/epicsR3.14/epics/base/lib/linux-x86
acctstMain.o acctst.o -lca -lCom
Typical Solaris Build Options
/opt/SUNWspro/bin/cc -c -D_POSIX_C_SOURCE=199506L -D_XOPEN_SOURCE=500
-DOSITHREAD_USE_DEFAULT_STACK -DUNIX -DSOLARIS=9 -mt -D__EXTENSIONS__ -Xc -v
-xO4 -I. -I.. -I./../../../include/os/solaris -I./../../../include
../acctst.c
/opt/SUNWspro/bin/CC -o acctst
-L/home/phoebus1/JHILL/epics/base/lib/solaris-sparc/ -mt -z ignore -z combreloc
-z lazyload -R/home/disk1/user/epics/base/lib/solaris-sparc acctstMain.o
acctst.o -lca -lCom
Typical Windows Build Options
cl -c /nologo /D__STDC__=0 /Ox /GL /W3 /w44355 /MD -I. -I..
-I..\\..\\..\\include\\os\\WIN32 -I..\\..\\..\\include ..\\acctst.c
link -nologo /LTCG /incremental:no /opt:ref /release /version:3.14
-out:acctst.exe acctstMain.obj acctst.obj
d:/user/R3.14.clean/epics/base/lib/WIN32-x86/ca.lib
d:/user/R3.14.clean/epics/base/lib/WIN32-x86/
Typical vxWorks Build Options
/usr/local/xcomp/ppc/bin/ccppc -c -D_POSIX_SOURCE -DCPU=PPC603
-DvxWorks -include /home/vx/tornado20/target/h/vxWorks.h -ansi -O3 -Wall
-mcpu=603 -mstrict-align -fno-builtin -I. -I.. -I../../../include/os/vxWorks
-I../../../include -I/home/vx/tornado20/target/h ../acctst.c
Other Systems and Compilers
Contributions gratefully accepted.
acctst <PV name> [progress logging level] [channel duplication count]
[test repetition count] [enable preemptive callback]
Description
Channel Access Client Library regression test.
The PV used with the test must be native type DBR_DOUBLE or DBR_FLOAT, and
modified only by acctst while the test is running. Therefore, periodically
scanned hardware attached analog input records do not work well. Test failure
is indicated if the program stops prior to printing "test complete". If
unspecified the progress logging level is zero, and no messages are printed
while the test is progressing. If unspecified, the channel duplication count is
20000. If unspecified, the test repetition count is once only. If unspecified,
preemptive callback is disabled.
catime <PV name> [channel count] [append number to pv name if true]
Description
Channel Access Client Library performance test.
If unspecified, the channel count is 10000. If the "append number to pv name
if true" argument is specified and it is greater than zero then the channel
names in the test are numbered as follows.
<PV name>000000, <PV name>000001, ... <PV name>nnnnnn
casw [-i <interest level>]
Description
CA server "beacon anomaly" logging.
CA server beacon anomalies occur when a new server joins the network, a
server is rebooted, network connectivity to a server is reestablished, or if a
server's CPU exits a CPU load saturated state.
CA clients with unresolved channels reset their search request scheduling
timers whenever they see a beacon anomaly.
This program can be used to detect situations where there are too many
beacon anomalies. IP routing configuration problems may result in false beacon
anomalies that might cause CA clients to use unnecessary additional network
bandwidth and server CPU load when searching for unresolved channels.
If there are no new CA servers appearing on the network, and network
connectivity remains constant, then casw should print no messages at all. At
higher interest levels the program prints a message for every beacon that is
received, and anomalous entries are flagged with a star.
caEventRate <PV name> [subscription count]
Description
Connect to the specified PV, subscribe for monitor updates the specified
number of times (default once), and periodically log the current sampled event
rate, average event rate, and the standard deviation of the event rate in Hertz
to standard out.
ca_test <PV name> [value to be written]
Description
If a value is specified it is written to the PV. Next, the current value of
the PV is converted to each of the many external data type that can be
specified at the CA client library interface, and each of these is formated and
then output to the console.
caget [options] <PV name> ...
Description
Get and print value for PV(s).
The values for one or multiple PVs are read and printed to stdout. The
DBR_... format in which the data is read, the output format, and a number of
details of how integer and float values are represented can be controlled using
command line options.
When getting multiple PVs, their order on the command line is retained in
the output.
Option |
Description |
-h |
Print usage information |
|
CA options: |
-w <sec> |
Wait time, specifies CA timeout, default is 1.0 second(s) |
-c |
Asynchronous get (use ca_get_callback and wait for completion) |
-p <prio> |
CA priority (0-99, default 0=lowest) |
|
Format and data type options: |
|
Default output format is "name value" |
-t |
Terse mode - print only value, without name |
-a |
Wide mode "name timestamp value stat sevr" (read PVs as
DBR_TIME_xxx) |
-n |
Print DBF_ENUM values as number (default are enum strings) |
-d <type> |
Request specific dbr type; use string (DBR_ prefix may be omitted)
or number of one of the following types:
DBR_STRING |
0 |
DBR_STS_FLOAT |
9 |
DBR_TIME_LONG |
19 |
DBR_CTRL_SHORT |
29 |
DBR_INT |
1 |
DBR_STS_ENUM |
10 |
DBR_TIME_DOUBLE |
20 |
DBR_CTRL_INT |
29 |
DBR_SHORT |
1 |
DBR_STS_CHAR |
11 |
DBR_GR_STRING |
21 |
DBR_CTRL_FLOAT |
30 |
DBR_FLOAT |
2 |
DBR_STS_LONG |
12 |
DBR_GR_SHORT |
22 |
DBR_CTRL_ENUM |
31 |
DBR_ENUM |
3 |
DBR_STS_DOUBLE |
13 |
DBR_GR_INT |
22 |
DBR_CTRL_CHAR |
32 |
DBR_CHAR |
4 |
DBR_TIME_STRING |
14 |
DBR_GR_FLOAT |
23 |
DBR_CTRL_LONG |
33 |
DBR_LONG |
5 |
DBR_TIME_INT |
15 |
DBR_GR_ENUM |
24 |
DBR_CTRL_DOUBLE |
34 |
DBR_DOUBLE |
6 |
DBR_TIME_SHORT |
15 |
DBR_GR_CHAR |
25 |
DBR_STSACK_STRING |
37 |
DBR_STS_STRING |
7 |
DBR_TIME_FLOAT |
16 |
DBR_GR_LONG |
26 |
DBR_CLASS_NAME |
38 |
DBR_STS_SHORT |
8 |
DBR_TIME_ENUM |
17 |
DBR_GR_DOUBLE |
27 |
|
|
DBR_STS_INT |
8 |
DBR_TIME_CHAR |
18 |
DBR_CTRL_STRING |
28 |
|
|
|
|
Arrays: |
|
Value format: Print number of requested values, then list of
values |
Default: |
Print all values |
-# <count> |
Print first <count> elements of an array |
|
Floating point type format: |
Default: |
Use %g format |
-e <nr> |
Use %e format, with <nr> digits after the decimal point |
-f <nr> |
Use %f format, with <nr> digits after the decimal point |
-g <nr> |
Use %g format, with <nr> digits after the decimal point |
-s |
Get value as string (may honour server-side precision) |
|
Integer number format: |
Default: |
Print as decimal number |
-0x |
Print as hex number |
-0o |
Print as octal number |
-0b |
Print as binary number |
camonitor [options] <PV name> ...
Description
Subscribe to and print value updates for PV(s).
Option |
Description |
-h |
Print usage information |
|
CA options: |
-w <sec> |
Wait time, specifies CA timeout, default is 1.0 second(s) |
-m <mask> |
Specify CA event mask to use, with <mask> being any combination
of 'v' (value), 'a' (alarm), 'l' (log). Default: va |
-p <prio> |
CA priority (0-99, default 0=lowest) |
|
Timestamps: |
Default: |
Print absolute timestamps (as reported by CA server) |
-t <key> |
Specify timestamp source(s) and type, with <key> containing
's' = CA server (remote) timestamps
'c' = CA client (local) timestamps (shown in '()'s)
'n' = no timestamps
'r' = relative timestamps (time elapsed since start of program)
'i' = incremental timestamps (time elapsed since last update)
'I' = incremental timestamps (time elapsed since last update, by
channel) |
|
Enum Format: |
-n |
Print DBF_ENUM values as number (default are enum strings) |
|
Arrays: |
|
Value format: Print number of requested values, then list of
values |
Default: |
Print all values |
-# <count> |
Print first <count> elements of an array |
|
Floating point type format: |
Default: |
Use %g format |
-e <nr> |
Use %e format, with <nr> digits after the decimal point |
-f <nr> |
Use %f format, with <nr> digits after the decimal point |
-g <nr> |
Use %g format, with <nr> digits after the decimal point |
-s |
Get value as string (may honour server-side precision) |
|
Integer number format: |
Default: |
Print as decimal number |
-0x |
Print as hex number |
-0o |
Print as octal number |
-0b |
Print as binary number |
caput [options] <PV name> <value>
caput -a [options] <PV name> <no of elements> <value> ...
Description
Put value to a PV.
The specified value is written to the PV (as a string). The PV value is read
before and after the write operation and printed as "Old" and "new" values on
stdout.
The array variant writes an array to the specified PV. The first numeric
argument specifying the number of array elements is kept for compatibility with
the array data format of caget - the actual number of values specified on the
command line is used.
Option |
Description |
-h |
Print usage information |
|
CA options: |
-w <sec> |
Wait time, specifies CA timeout, default is 1.0 second(s) |
-c |
Asynchronous put (use ca_put_callback and wait for completion) |
-p <prio> |
CA priority (0-99, default 0=lowest) |
|
Format options: |
-t |
Terse mode - print only sucessfully written value, without name |
|
Enum Format: |
|
Auto - try value as ENUM string, then as index number |
-n |
Force interpretation of values as numbers |
-s |
Force interpretation of values as strings |
|
Arrays: |
-a |
Put array data |
|
Value format: Print number of requested values, then list of
values |
cainfo [options] <PV name> ...
Description
Get and print channel and connection information for PV(s).
All available Channel Access related information about PV(s) is printed to
stdout.
The -s option allows to specify an interest level for calling Channel
Access' internal report function ca_client_status(), that prints lots of
internal informations on stdout, including environment settings, used CA ports
etc.
Option |
Description |
-h |
Print usage information |
|
CA options: |
-w <sec> |
Wait time, specifies CA timeout, default is 1.0 second(s) |
-s <level> |
Call ca_client_status with the specified interest level |
-p <prio> |
CA priority (0-99, default 0=lowest) |
Verify that the broadcast addresses are identical on the server's host and
on the client's host. This can be checked on UNIX with "netstat -i" or
"ifconfig -a"; on vxWorks with ifShow; and on windows with ipconfig. It is
normal for the broadcast addresses to not be identical if the client and server
are not directly attached to the same IP subnet, and in this situation the
EPICS_CA_ADDR_LIST must be set. Otherwise, if the client and server are
intended to be on the same IP subnet, then the problem may be that the IP
netmask is incorrectly set in the network interface configuration. On most
operating systems, when the host's IP address is configured, the host's IP
subnet mask is also configured.
Verify that the client and server are using the same UDP port. Check the
server's port by running "netstat -a | grep nnn" where nnn is the port number
configured in the client. If you do not set EPICS_CA_SERVER_PORT or
EPICS_CAS_SERVER_PORT then the default port will be 5064.
Two servers can run on the same host with the same server port number, but
there are restrictions. If the host has a modern IP kernel it is possible to
have two or more servers share the same UDP port. It is not possible for these
servers to run on the same host using the same TCP port. If the CA server
library detects that a server is attempting to start on the same port as an
existing CA server then both servers will use the same UDP port, and the 2nd
server will be allocated an ephemeral TCP port. Clients can be configured to
use the same port number for both servers. They will locate the 2nd server via
the shared UDP port, and transparently connect to the 2nd server's ephemeral
TCP port. Be aware however that If there are two server's running on the same
host sharing the same UDP port then they will both receive UDP search requests
sent as broadcasts, but unfortunately (due to a weakness of most IP kernel
implementations) only one of the servers will typically receive UDP search
requests sent to unicast addresses (i.e. a single specific host's ip
address).
Two conclusions deserve special emphasis. First, if a client does not
see the server's beacons, then it will use additional network and server
resources sending periodic state-of-health messages. Second, if a
client does not see a newly introduced server's beacon, then it will take up to
EPICS_CA_MAX_SEARCH_PERIOD to find that newly introduced server. Also,
starting with EPICS R3.14.7 the client library does not suspend
searching for a channel after 100 unsuccessful attempts until a beacon anomaly
is seen. Therefore, if the client library is from before version R3.14.7 of
EPICS and it timed out attempting to find a server whoose beacon cant be seen
by the client library then the client application might need to be restarted in
order to connect to this new beacon-out-of-range server. The typical situation
where a client would not see the server's beacon might be when the client isnt
on the same IP subnet as the server, and the client's EPICS_CA_ADDR_LIST was
modified to include a destination address for the server, but the server's
beacon address list was not modified so that it's beacons are received by the
client.
When communication over a virtual circuit times out, then each channel
attached to the circuit enters a disconnected state and the disconnect callback
handler specified for the channel is called. However, the circuit is not
disconnected until TCP/IP's internal, typically long duration, keep alive timer
expires. The disconnected channels remain attached to the beleaguered circuit
and no attempt is made to search for, or to reestablish, a new circuit. If, at
some time in the future, the circuit becomes responsive again, then the
attached channels enter a connected state again and reconnect call back
handlers are called. Any monitor subscriptions that received an update message
while the channel was disconnected are also refreshed. If at any time the
library receives an indication from the operating system that a beleaguered
circuit has shutdown or was disconnected then the library will immediately
reattempt to find servers for each channel and connect circuits to them.
A well known negative side effect of the above behavior is that CA clients
will wait the full (typically long) duration of TCP/IP's internal keep alive
timer prior to reconnecting under the following scenario (all of the following
occur):
- An server's (IOC's) operating system crashes (or is abruptly turned off)
or a vxWorks system is stopped by any means
- This operating system does not immediately reboot using the same IP
address
- A duplicate of the server (IOC) is started appearing at a different IP
address
It is unlikely that any rational organization will advocate the above
scenario in a production system. Nevertheless, there are opportunities
for users to become confused during control system development, but it
is felt that the robustness improvements justify isolated confusion during the
system integration and checkout activities where the above scenarios are most
likely to occur.
Contrast the above behavior with the CA client library behavior of releases
prior to R3.14.5 where the beleaguered circuit was immediately closed when
communication over it timed out. Any attached channels were immediately
searched for, and after successful search responses arrived then attempts were
made to build a new circuit. This behavior could result in undesirable resource
consumption resulting from periodic circuit setup and teardown overhead
(thrashing) during periods of CPU / network / IP kernel buffer congestion.
Short lived CA client applications that issue a CA put request and then
immediately exit the process (return from main or call
exit ) may find that there request isn't executed. To guarantee
that the request is sent call ca_flush followed by
ca_context_destroy prior to terminating the process.
Many Berkley UNIX derived Internet Protocol (IP) kernels use a memory
management scheme with a fixed sized low level memory allocation quantum called
an "mbuf". Messages about "ENOBUFS" are an indication that your IP kernel is
running low on mbuf buffers. An IP kernel mbuf starvation situation may lead to
temporary IP communications stalls or reduced throughput. This issue has to
date been primarily associated with vxWorks systems where mbuf starvation on
earlier vxWorks versions is rumored to lead to permanent IP communications
stalls which are resolved only by a system reboot. IP kernels that use mbufs
frequently allow the initial and maximum number of mbufs to be configured.
Consult your OS's documentation for configuration procedures which vary between
OS and even between different versions of the same OS.
Contributing Circumstances
- The total number of connected clients is high. Each active socket
requires dedicated mbufs for protocol control blocks, and for any data that
might be pending in the operating system for transmission to Channel Access
or to the network at a given instant. If you increase the vxWorks limit on
the maximum number of file descriptors then it may also be necessary to
increase the size of the mbuf pool.
- The server has multiple connections where the server's sustained event
(monitor subscription update) production rate is higher than the client's
or the network's sustained event consumption rate. This ties up a per
socket quota of mbufs for data that are pending transmission to the client
via the network. In particular, if there are multiple clients that
subscribe for monitor events but do not call ca_pend_event() or ca_poll()
to process their CA input queue, then a significant mbuf consuming backlog
can occur in the server.
- The server does not get a chance to run (because some other higher
priority thread is running) and the CA clients are sending a high volume of
data over TCP or UDP. This ties up a quota of mbufs for each socket in the
server that isn't being reduced by the server's socket read system
calls.
- The server has multiple stale connections. Stale connections occur when a
client is abruptly turned off or disconnected from the network, and an
internal "keepalive" timer has not yet expired for the virtual circuit in
the operating system, and therefore mbufs may be dedicated to unused
virtual circuits. This situation is made worse if there are active monitor
subscriptions associated with stale connections which will rapidly increase
the number of dedicated mbufs to the quota available for each circuit.
- When sites switch to the vxWorks 5.4 IP kernel they frequently run into
network pool exhaustion problems. This may be because the original vxWorks
IP kernel expanded the network pool as needed at runtime while the new
kernel's pool is statically configured at compile time, and does
not expand as needed at runtime. Also, at certain sites problems
related to vxWorks network driver pool exhaustion have also been reported
(this can also result in ENOBUF diagnostic messages).
Related Diagnostics
- The EPICS command "casr [interest level]" displays information about the
CA server and how many clients are connected.
- The vxWorks command "inetstatShow" indicates how many bytes are pending
in mbufs and indirectly (based on the number of circuits listed) how many
mbuf based protocol control blocks have been consumed. The vxWorks commands
(availability depending on vxWorks version) mbufShow, netStackSysPoolShow,
and netStackDataPoolShow indicate how much space remains in the network
stack pool.
- The RTEMS command "netstat [interest level]" displays network information
including mbuf consumption statistics.
Server Subscription Update Queuing
If the subscription update producer in the server produces subscription
updates faster than the subscription update consumer in the client consumes
them, then events have to be discarded if the buffering in the server
isn’t allowed to grow to an infinite size. This is a law of nature
– based on queuing theory of course.
What is done depends on the version of the CA server. All server versions
place quotas on the maximum number of subscription updates allowed on the
subscription update queue at any given time. If this limit is reached, an
intervening update is discarded in favor of a more recent update. Depending on
the version of the server, rapidly updating subscriptions are or are not
allowed to cannibalize the quotas of slow updating subscriptions in limited
ways. Nevertheless, there is always room on the queue for at least one update
for each subscription. This guarantees that the most recent update is always
sent.
Adding further complication, the CA client library also implements a
primitive type of flow control. If the client library sees that it is reading a
large number of messages one after another w/o intervening delay it knows that
it is not consuming events as fast as they are produced. In that situation it
sends a message telling the server to temporarily stop sending subscription
update messages. When the client catches up it sends another message asking the
server to resume with subscription updates. This prevents slow clients from
getting time warped, but also guarantees that intervening events are discarded
until the slow client catches up.
There is currently no message on the IOC’s console when a particular
client is slow on the uptake. A message of this type used to exist many years
ago, but it was a source of confusion (and what we will call message noise) so
it was removed.
There is unfortunately no field in the protocol allowing the server to
indicate that an intervening subscription update was discarded. We should
probably add that capability in a future version. Such a feature would, for
example, be beneficial when tuning an archiver installation.
Significant performance gains can be realized when the CA client library
doesn't wait for a response to return from the server after each request. All
requests which require interaction with a CA server are accumulated (buffered)
and not forwarded to the IOC until one of ca_flush_io, ca_pend_io,
ca_pend_event, or ca_sg_pend are called allowing several operations to be
efficiently sent over the network together. Any process variable values written
into your program's variables by ca_get() should not be referenced by your
program until ECA_NORMAL has been received from ca_pend_io().
If successful, the routines described here return the status code
ECA_NORMAL. Unsuccessful status codes returned from the client library are
listed with each routine in this manual. Operations that appear to be valid to
the client can still fail in the server. Writing the string "off" to a floating
point field is an example of this type of error. If the server for a channel is
located in a different address space than the client then the ca_xxx()
operations that communicate with the server return status indicating the
validity of the request and whether it was successfully enqueued to the server,
but communication of completion status is deferred until a user callback is
called, or lacking that an exception handler is called. An error number and the
error's severity are embedded in CA status (error) constants. Applications
shouldn't test the success of a CA function call by checking to see if the
returned value is zero as is the UNIX convention. Below are several methods to
test CA function returns. See ca_signal() and SEVCHK
for more information on this topic.
status = ca_XXXX();
SEVCHK( status, "ca_XXXX() returned failure status");
if ( status & CA_M_SUCCESS ) {
printf ( "The requested ca_XXXX() operation didn't complete successfully");
}
if ( status != ECA_NORMAL ) {
printf("The requested ca_XXXX() operation didn't complete successfully because \"%s\"\n",
ca_message ( status ) );
}
CA channels form a virtual circuit between a process variable (PV) and a
client side application program. It is possible to connect a wide variety of
data sources into EPICS using the CA server library. When a CA channel
communicates with an EPICS Input Output Controller (IOC) then a field is a
specialization of a PV, and an EPICS record is a plug compatible function block
that contains fields, and the meta data below frequently are mapped onto
specific fields within the EPICS records by the EPICS record support (see the
EPICS Application Developer Guide).
Arguments of type chtype specifying the data type you wish to transfer. They
expect one of the set of DBR_XXXX data type codes defined in db_access.h. There
are data types for all of the C primitive types, and there are also compound (C
structure) types that include various process variable properties such as
units, limits, time stamp, or alarm status. The primitive C types follow a
naming convention where the C typedef dbr_xxxx_t corresponds to the DBR_XXXX
data type code. The compound (C structure) types follow a naming convention
where the C structure tag dbr_xxxx corresponds to the DBR_XXXX data type code.
The following tables provides more details on the structure of the CA data type
space. Since data addresses are passed to the CA client library as typeless
"void *" pointers then care should be taken to ensure that you have passed the
correct C data type corresponding to the DBR_XXXX type that you have specified.
Architecture independent types are provided in db_access.h to assist
programmers in writing portable code. For example "dbr_short_t" should be used
to send or receive type DBR_SHORT. Be aware that type name DBR_INT has been
deprecated in favor of the less confusing type name DBR_SHORT. In practice,
both the DBR_INT type code and the DBR_SHORT type code refer to a 16 bit
integer type, and are functionally equivalent.
Channel Access Primitive Data Types
CA Type Code |
Primitive C Data Type |
Data Size |
DBR_CHAR |
dbr_char_t |
8 bit character |
DBR_SHORT |
dbr_short_t |
16 bit integer |
DBR_ENUM |
dbr_enum_t |
16 bit unsigned integer |
DBR_LONG |
dbr_long_t |
32 bit signed integer |
DBR_FLOAT |
dbr_float_t |
32 bit IEEE floating point |
DBR_DOUBLE |
dbr_double_t |
64 bit IEEE floating point |
DBR_STRING |
dbr_string_t |
40 character string |
Structure of the Channel Access Data Type Space
CA Type Code |
Read / Write |
Primitive C Data Type |
Process Variable Properties |
DBR_<PRIMITIVE TYPE> |
RW |
dbr_<primitive type>_t |
value |
DBR_STS_<PRIMITIVE TYPE> |
R |
struct dbr_sts_<primitive type> |
value, alarm status, and alarm severity |
DBR_TIME_<PRIMITIVE TYPE> |
R |
struct dbr_time_<primitive type> |
value, alarm status, alarm severity, and time stamp |
DBR_GR_<PRIMITIVE TYPE> |
R |
struct dbr_gr_<primitive type> |
value, alarm status, alarm severity, units, display precision, and
graphic limits |
DBR_CTRL_<PRIMITIVE TYPE> |
R |
struct dbr_ctrl_<primitive type> |
value, alarm status, alarm severity, units, display precision,
graphic limits, and control limits |
DBR_PUT_ACKT |
W |
dbr_put_ackt_t |
Used for global alarm acknowledgement. Do transient alarms have to be
acknowledged? (0,1) means (no, yes). |
DBR_PUT_ACKS |
W |
dbr_put_acks_t |
Used for global alarm acknowledgement. The highest alarm severity to
acknowledge. If the current alarm severity is less then or equal to
this value the alarm is acknowledged. |
DBR_STSACK_STRING |
R |
struct dbr_stsack_string |
value, alarm status, alarm severity, ackt, ackv |
DBR_CLASS_NAME |
R |
dbr_class_name_t |
name of enclosing interface (name of the record if channel is
attached to EPICS run time database) |
Channel value arrays can also be included within the structured CA data
types. If more than one element is requested, then the individual elements can
be accessed in an application program by indexing a pointer to the value field
in the DBR_XXX structure. For example, the following code computes the sum of
the elements in a array process variable and prints its time stamp. The dbr_size_n function can be used to determine the correct
number of bytes to reserve when there are more than one value field elements in
a structured CA data type.
#include <stdio.h>
#include <stdlib.h>
#include "cadef.h"
int main ( int argc, char ** argv )
{
struct dbr_time_double * pTD;
const dbr_double_t * pValue;
unsigned nBytes;
unsigned elementCount;
char timeString[32];
unsigned i;
chid chan;
double sum;
int status;
if ( argc != 2 ) {
fprintf ( stderr, "usage: %s <channel name>", argv[0] );
return -1;
}
status = ca_create_channel ( argv[1], 0, 0, 0, & chan );
SEVCHK ( status, "ca_create_channel()" );
status = ca_pend_io ( 15.0 );
if ( status != ECA_NORMAL ) {
fprintf ( stderr, "\"%s\" not found.\n", argv[1] );
return -1;
}
elementCount = ca_element_count ( chan );
nBytes = dbr_size_n ( DBR_TIME_DOUBLE, elementCount );
pTD = ( struct dbr_time_double * ) malloc ( nBytes );
if ( ! pTD ) {
fprintf ( stderr, "insufficient memory to complete request\n" );
return -1;
}
status = ca_array_get ( DBR_TIME_DOUBLE, elementCount, chan, pTD );
SEVCHK ( status, "ca_array_get()" );
status = ca_pend_io ( 15.0 );
if ( status != ECA_NORMAL ) {
fprintf ( stderr, "\"%s\" didnt return a value.\n", argv[1] );
return -1;
}
pValue = & pTD->value;
sum = 0.0;
for ( i = 0; i < elementCount; i++ ) {
sum += pValue[i];
}
epicsTimeToStrftime ( timeString, sizeof ( timeString ),
"%a %b %d %Y %H:%M:%S.%f", & pTD->stamp );
printf ( "The sum of elements in %s at %s was %f\n",
argv[1], timeString, sum );
ca_clear_channel ( chan );
ca_task_exit ();
free ( pTD );
return 0;
}
Certain CA client initiated requests asynchronously execute an application
supplied call back in the client process when a response arrives. The functions
ca_put_callback, ca_get_callback, and ca_add_event all request notification of
asynchronous completion via this mechanism. The event_handler_args
structure is passed by value to the application supplied
callback. In this structure the dbr field is a void pointer to any
data that might be returned. The s tatus field will be
set to one of the CA error codes in caerr.h and will indicate the status of the
operation performed in the IOC. If the status field isn't set to ECA_NORMAL or
data isn't normally returned from the operation (i.e. put call back) then you
should expect that the dbr field will be set to a nill pointer
(zero). The fields usr , chid , and type
are set to the values specified when the request was made by the application.
The "dbr" pointer, and any data that it points to, are valid only when
executing within the user's callback function.
typedef struct event_handler_args {
void *usr; /* user argument supplied with request */
chanId chid; /* channel id */
long type; /* the type of the item returned */
long count; /* the element count of the item returned */
const void *dbr; /* a pointer to the item returned */
int status; /* ECA_XXX status of the requested op from the server */
} evargs;
void myCallback ( struct event_handler_args args )
{
if ( args.status != ECA_NORMAL ) {
}
if ( args.type == DBR_TIME_DOUBLE ) {
const struct dbr_time_double * pTD =
( const struct dbr_time_double * ) args.dbr;
}
}
When the server detects a failure, and there is no client call back function
attached to the request, then an exception handler is executed in the client.
The default exception handler prints a message on the console and exits if the
exception condition is severe. Certain internal exceptions within the CA client
library, and failures detected by the SEVCHK macro may also cause the exception
handler to be invoked. To modify this behavior see ca_add_exception_event().
If the Process Variable's server and it's client are colocated within the
same memory address space and the same host then the ca_xxx() operations bypass
the server and directly interact with the server tool component (commonly the
IOC's function block database). In this situation the ca_xxx() routines
frequently return the completion status of the requested operation directly to
the caller with no opportunity for asynchronous notification of failure via an
exception handler. Likewise, callbacks may be directly invoked by the CA
library functions that request them.
For routines that require an argument specifying the number of array
elements, no more than the process variable's maximum native element count may
be requested. The process variable's maximum native element count is available
from ca_element_count() when the channel is connected. If less elements than
the process variable's native element count are requested the requested values
will be fetched beginning at element zero. By default CA limits the number of
elements in an array to be no more than approximately 16k divided by the size
of one element in the array. Starting with EPICS R3.14 the maximum array size
may be configured in the client and in the server.
Application programs should assume that CA servers may be restarted, and
that network connectivity is transient. When you create a CA channel its
initial connection state will most commonly be disconnected. If the Process
Variable's server is available the library will immediately initiate the
necessary actions to make a connection with it. Otherwise, the client library
will monitor the state of servers on the network and connect or reconnect with
the process variable's server as it becomes available. After the channel
connects the application program can freely perform IO operations through the
channel, but should expect that the channel might disconnect at any time due to
network connectivity disruptions or server restarts.
Three methods can be used to determine if a channel is connected: the
application program might call ca_state to
obtain the current connection state, block in ca_pend_io until the channel connects, or install
a connection callback handler when it calls ca_create_channel . The ca_pend_io approach is best suited to simple
command line programs with short runtime duration, and the connection callback
method is best suited to toolkit components with long runtime duration. Use of
ca_state is appropriate only in programs
that prefer to poll for connection state changes instead of opting for
asynchronous notification. The ca_pend_io function blocks only for
channels created specifying a nill connection handler callback function. The
user's connection state change function will be run immediately from within
ca_create_channel if the CA
client and CA server are both hosted within the same address space (within the
same process).
Starting with EPICS R3.14 the CA client libraries are fully thread safe on
all OS (in past releases the library was thread safe only on vxWorks). When the
client library is initialized the programmer may specify if preemptive call
back is enabled. Preemptive call back is disabled by default. If preemptive
call back is enabled then the user's call back functions might be called by
CA's auxiliary threads when the main initiating channel access thread is not
inside of a function in the channel access client library. Otherwise, the
user's call back functions will be called only when the main initiating channel
access thread is executing inside of the CA client library. When the CA client
library invokes a user's call back function it will always wait for the current
callback to complete prior to executing another call back function. Programmers
enabling preemptive callback should be familiar with using mutex locks to
create a reliable multi-threaded program.
To set up a traditional single threaded client you will need code like this
(see ca_context_create and CA Client Contexts and Application Specific Auxiliary
Threads) .
SEVCHK ( ca_context_create(ca_disable_preemptive_callback ),
"application pdq calling ca_context_create" );
To set up a preemptive callback enabled CA client context you will need code
like this (see ca_context_create and CA Client Contexts and Application Specific Auxiliary
Threads).
SEVCHK ( ca_context_create(ca_enable_preemptive_callback ),
"application pdq calling ca_context_create" );
It is often necessary for several CA client side tools running in the same
address space (process) to be independent of each other. For example, the
database CA links and the sequencer are designed to not use the same CA client
library threads, network circuits, and data structures. Each thread that calls
ca_context_create() for the first time either
directly, or implicitly when calling any CA library function for the first
time, creates a CA client library context. A CA client library context contains
all of the threads, network circuits, and data structures required to connect
and communicate with the channels that a CA client application has created. The
priority of auxiliary threads spawned by the CA client library are at fixed
offsets from the priority of the thread that called ca_context_create(). An application specific
auxiliary thread can join a CA context by calling ca_attach_context() using the CA context
identifier that was returned from ca_current_context() when it is called by the
thread that created the context which needs to be joined. A context which is to
be joined must be preemptive - it must be created using ca_context_create(ca_enable_preemptive_callback).
It is not possible to attach a thread to a non-preemptive CA context created
explicitly or implicitly with
ca_create_context(ca_disable_preemptive_callback). Once a thread has joined
with a CA context it need only make ordinary ca_xxxx() library calls to use the
context.
A CA client library context can be shut down and cleaned up, after
destroying any channels or application specific threads that are attached to
it, by calling ca_context_destroy(). The
context may be created and destroyed by different threads as long as they are
both part of the same context.
If preemptive call back is not enabled, then for proper operation CA must
periodically be polled to take care of background activity. This requires that
your application must either wait in one of ca_pend_event(), ca_pend_io(), or
ca_sg_block() or alternatively it must call ca_poll() at least every 100
milli-seconds. In single threaded applications a file descriptor manager like
Xt or the interface described in fdManager.h can be used to monitor both mouse
clicks and also CA's file descriptors so that ca_poll() can be called
immediately when CA server messages arrives over the network.
With the embryonic releases of EPICS it was a common practice to examine a
channel's connection state, its native type, and its native element count by
directly accessing fields in a structure using a pointer stored in type
chid . Likewise, a user private pointer in the per channel
structure was also commonly set by directly accessing fields in the channel
structure. A number of difficulties arise from this practice, which has long
since been deprecated. For example, prior to release 3.13 it was recognized
that transient changes in certain private fields in the per channel structure
would make it difficult to reliably test the channels connection state using
these private fields directly. Therefore, in release 3.13 the names of certain
fields were changed to discourage this practice. Starting with release 3.14
codes written this way will not compile. Codes intending to maintain the
highest degree of portability over a wide range of EPICS versions should be
especially careful. For example you should replace all instances off
channel_id->count with
ca_element_count(channel_id) . This approach should be reliable on
all versions of EPICS in use today. The construct ca_puser(chid) =
xxxx is particularly problematic. The best mechanisms for setting the
per channel private pointer will be to pass the user private pointer in when
creating the channel. This approach is implemented on all versions. Otherwise,
you can also use ca_set_puser(CHID,PUSER) , but this function is
available only after the first official (post beta) release of EPICS 3.13.
Calling CA functions from the vxWorks shell thread is a somewhat
questionable practice for the following reasons.
- The vxWorks shell thread runs at the very highest priority in the system
and therefore socket calls are made at a priority that is above the
priority of tNetTask − a practice that has caused the WRS IP kernel
to get sick in the past. That symptom was observed some time ago, but we
don’t know if WRS has fixed the problem.
- The vxWorks shell thread runs at the very highest priority in the system
and therefore certain CA auxiliary threads will not get the priorities that
are requested for them. This might cause problems only when in a CPU
saturation situations.
- If the code does not call ca_context_destroy (ca_task_exit in past
releases) then resources are left dangling.
- In EPICS R3.13 the CA client library installed vxWorks task exit handlers
behaved strangely if CA functions were called from the vxWorks shell,
ca_task_exit() wasn’t called, and the vxWorks shell restarted. In
EPICS R3.14 vxWorks task exit handlers are not installed and therefore
cleanup is solely the responsibility of the user. With EPICS R3.14 the user
must call ca_context_destroy or ca_task_exit to clean up on vxWorks. This
is the same behavior as on all other OS.
As you might expect, it isnt safe to call the CA client library from a POSIX
signal handler. Likewise, it isnt safe to call the CA client library from
interrupt context.
Function Call Reference
#include <cadef.h>
enum ca_preemptive_callback_select
{ ca_disable_preemptive_callback, ca_enable_preemptive_callback };
int ca_context_create ( enum ca_preemptive_callback_select SELECT );
Description
This function, or ca_attach_context(),
should be called once from each thread prior to making any of the other Channel
Access calls. If one of the above is not called before making other CA calls
then a non-preemptive context is created by default, and future attempts to
create a preemptive context for the current threads will fail.
If ca_disable_preemptive_callback is specified then additional
threads are not allowed to join the CA context using
ca_context_attach() because allowing other threads to join implies that CA
callbacks will be called preemptively from more than one thread.
Arguments
SELECT
- This argument specifies if preemptive invocation of callback functions
is allowed. If so your callback functions might be called when the thread
that calls this routine is not executing in the CA client library. There
are two implications to consider.
First, if preemptive callback mode is enabled the developer must
provide mutual exclusion protection for his data structures. In this mode
it's possible for two threads to touch the application's data structures
at once: this might be the initializing thread (the thread that called
ca_context_create) and also a private thread created by the CA client
library for the purpose of receiving network messages and calling
callbacks. It might be prudent for developers who are unfamiliar with
mutual exclusion locking in a multi-threaded environment to specify
ca_disable_preemptive_callback .
Second, if preemptive callback mode is enabled the application is no
longer burdened with the necessity of periodically polling the CA client
library in order that it might take care of its background activities. If
ca_enable_preemptive_callback is specified then CA client
background activities, such as connection management, will proceed even
if the thread that calls this routine is not executing in the CA client
library. Furthermore, in preemptive callback mode callbacks might be
called with less latency because the library is not required to wait
until the initializing thread (the thread that called ca_context_create)
is executing within the CA client library.
Returns
ECA_NORMAL - Normal successful completion
ECA_ALLOCMEM - Failed, unable to allocate space in pool
ECA_NOTTHREADED - Current thread is already a member of a non-preemptive
callback CA context (possibly created implicitly)
See Also
ca_context_destroy()
#include <cadef.h>
void ca_context_destroy();
Description
Shut down the calling thread's channel access client context and free any
resources allocated. Detach the calling thread from any CA client context.
Any user-created threads that have attached themselves to the CA context
must stop using it prior to its being destroyed. A program running in an IOC
context must delete all of its channels prior to calling ca_context_destroy()
to avoid a crash.
A CA client application that calls epicsExit() must install an
EPICS exit handler that calls ca_context_destroy() only after first
calling ca_create_context(). This will guarantee that the EPICS exit handlers
get called in the correct order.
On many OS that execute programs in a process based environment the
resources used by the client library such as sockets and allocated memory are
automatically released by the system when the process exits and
ca_context_destroy() hasn't been called, but on light weight systems such as
vxWorks or RTEMS no cleanup occurs unless the application call
ca_context_destroy().
Returns
ECA_NORMAL - Normal successful completion
See Also
ca_context_create()
#include <cadef.h>
typedef void ( *pCallBack ) (
struct connection_handler_args );
int ca_create_channel
(
const char *PROCESS_VARIABLE_NAME,
caCh *USERFUNC,
void *PUSER,
capri priority,
chid *PCHID
);
Description
This function creates a CA channel. The CA client library will attempt to
establish and maintain a virtual circuit between the caller's application and a
named process variable in a CA server. Each call to ca_create_channel allocates
resources in the CA client library and potentially also a CA server. The
function ca_clear_channel() is used to release these resources. If successful,
the routine writes a channel identifier into the user's variable of type
"chid". This identifier can be used with any channel access call that operates
on a channel.
The circuit may be initially connected or disconnected depending on the
state of the network and the location of the channel. A channel will only enter
a connected state after server's address is determined, and only if channel
access successfully establishes a virtual circuit through the network to the
server. Channel access routines that send a request to a server will return
ECA_DISCONNCHID if the channel is currently disconnected.
There are two ways to obtain asynchronous notification when a channel enters
a connected state.
- The first and simplest method requires that you call ca_pend_io(), and
wait for successful completion, prior to using a channel that was created
specifying a nil connection call back function pointer.
- The second method requires that you register a connection handler by
supplying a valid connection callback function pointer. This connection
handler is called whenever the connection state of the channel changes. If
you have installed a connection handler then ca_pend_io() will not
block waiting for the channel to enter a connected state.
The function ca_state(CHID) can be used to test the connection state of a
channel. Valid connections may be isolated from invalid ones with this function
if ca_pend_io() times out.
Due to the inherently transient nature of network connections the order of
connection call backs relative to the order that ca_create_channel() calls are
made by the application can't be guaranteed, and application programs may need
to be prepared for a connected channel to enter a disconnected state at any
time.
Example
See caExample.c in the example application created by makeBaseApp.pl.
Arguments
PROCESS_VARIABLE_NAME
- A nil terminated process variable name string. EPICS process control
function block database variable names are of the form "<record
name>.<field name>". If the field name and the period separator
are omitted then the "VAL" field is implicit. For example "RFHV01" and
"RFHV01.VAL" reference the same EPICS process control function block
database variable.
USERFUNC
- Optional address of the user's call back function to be run when the
connection state changes. Casual users of channel access may decide to
set this field to nil or 0 if they do not need to have a call back
function run in response to each connection state change event.
The following structure is passed by value to the user's
connection connection callback function. The op field will
be set by the CA client library to CA_OP_CONN_UP when the
channel connects, and to CA_OP_CONN_DOWN when the channel
disconnects. See ca_puser if the
PUSER argument is required in your callback
handler.
struct ca_connection_handler_args {
chanId chid; /* channel id */
long op; /* one of CA_OP_CONN_UP or CA_OP_CONN_DOWN */
};
PUSER
- The value of this void pointer argument is retained in
storage associated with the specified channel. See the MACROS manual page
for reading and writing this field. Casual users of channel access may
wish to set this field to nil or 0.
PRIORITY
- The priority level for dispatch within the server or network with 0
specifying the lowest dispatch priority and 99 the highest. This
parameter currently does not impact dispatch priorities within the
client, but this might change in the future. The abstract priority range
specified is mapped into an operating system specific range of priorities
within the server. This parameter is ignored if the server is running on
a network or operating system that does not have native support for
prioritized delivery or execution respectively. Specifying many different
priorities within the same program can increase resource consumption in
the client and the server because an independent virtual circuit, and
associated data structures, is created for each priority that is used on
a particular server.
PCHID
- The user supplied channel identifier storage is overwritten with a
channel identifier if this routine is successful.
Returns
ECA_NORMAL - Normal successful completion
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_STRTOBIG - Unusually large string
ECA_ALLOCMEM - Unable to allocate memory
ca_clear_channel()
#include <cadef.h>
int ca_clear_channel (chid CHID);
Description
Shutdown and reclaim resources associated with a channel created by
ca_create_channel().
All remote operation requests such as the above are accumulated (buffered)
and not forwarded to the IOC until one of ca_flush_io, ca_pend_io,
ca_pend_event, or ca_sg_pend are called. This allows several requests to be
efficiently sent over the network in one message.
Clearing a channel does not cause its disconnect handler to be called, but
clearing a channel does shutdown and reclaim any channel state change event
subscriptions (monitors) registered with the channel.
Arguments
CHID
- Identifies the channel to delete.
Returns
ECA_NORMAL - Normal successful completion
ECA_BADCHID - Corrupted CHID
#include <cadef.h>
int ca_put ( chtype TYPE,
chid CHID, void *PVALUE );
int ca_array_put ( chtype TYPE,
unsigned long COUNT,
chid CHID, const void *PVALUE);
typedef void ( *pCallBack ) (struct event_handler_args );
int ca_put_callback ( chtype TYPE,
chid CHID, const void *PVALUE,
pCallBack PFUNC, void *USERARG );
int ca_array_put_callback ( chtype TYPE,
unsigned long COUNT,
chid CHID, const void *PVALUE,
pCallBack PFUNC, void *USERARG );
Description
Write a scalar or array value to a process variable.
When ca_array_put or ca_put are invoked the client will receive no response
unless the request can not be fulfilled in the server. If unsuccessful an
exception handler is run on the client side. If a connection is lost and then
resumed outstanding ca_array_put or ca_put requests are not automatically
reissued following reconnect, and no additional notification are provided to
the user for each put request.
When ca_array_put_callback are invoked the user supplied asynchronous call
back is called only after the initiated write operation and all actions
resulting from the initiating write operation complete. If unsuccessful the
call back function is invoked indicating bad status. If the channel disconnects
before a put callback request can be completed, then the client's call back
function is called with bad status, but this does not guarantee that the server
did not receive and process the request before the disconnect.
All of these functions return ECA_DISCONN if the channel is currently
disconnected.
All put requests are accumulated (buffered) and not forwarded to the IOC
until one of ca_flush_io, ca_pend_io, ca_pend_event, or ca_sg_pend are called.
This allows several requests to be efficiently combined into one message.
Arguments
TYPE
- The external type of the supplied value to be written. Conversion will
occur if this does not match the native type. Specify one from the set of
DBR_XXXX in db_access.h
COUNT
- Element count to be written to the specified channel. This must match
the array pointed to by PVALUE.
CHID
- Channel identifier
PVALUE
- Pointer to a value or array of values provided by the application to be
written to the channel.
PFUNC
- address of user supplied callback function to be
run when the requested operation completes
USERARG
- pointer sized variable retained and then passed back to user supplied
function above
Returns
ECA_NORMAL - Normal successful completion
ECA_BADCHID - Corrupted CHID
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_BADCOUNT - Requested count larger than native element count
ECA_STRTOBIG - Unusually large string supplied
ECA_NOWTACCESS - Write access denied
ECA_ALLOCMEM - Unable to allocate memory
ECA_DISCONN - Channel is disconnected
See Also
ca_flush_io()
ca_pend_event()
ca_get()
#include <cadef.h>
int ca_get ( chtype TYPE,
chid CHID, void *PVALUE );
int ca_array_get ( chtype TYPE, unsigned long COUNT,
chid CHID, void *PVALUE );
typedef void ( *pCallBack ) (struct event_handler_args );
int ca_get_callback ( chtype TYPE,
chid CHID, pCallBack USERFUNC, void *USERARG);
int ca_array_get_callback ( chtype TYPE, unsigned long COUNT,
chid CHID,
pCallBack USERFUNC, void *USERARG );
Description
Read a scalar or array value from a process variable.
When ca_get or ca_array_get are invoked the returned channel value cant be
assumed to be stable in the application supplied buffer until after ECA_NORMAL
is returned from ca_pend_io. If a connection is lost outstanding get requests
are not automatically reissued following reconnect.
When ca_get_callback or ca_array_get_callback are invoked a value is read
from the channel and then the user's callback is invoked with a pointer to the
retrieved value. Note that ca_pend_io will not block for the delivery of values
requested by ca_get_callback. If the channel disconnects before a get callback
request can be completed, then the clients call back function is called with
bad status.
All of these functions return ECA_DISCONN if the channel is currently
disconnected.
All get requests are accumulated (buffered) and not forwarded to the IOC
until one of ca_flush_io, ca_pend_io, ca_pend_event, or ca_sg_pend are called.
This allows several requests to be efficiently sent over the network in one
message.
Example
See caExample.c in the example application created by makeBaseApp.pl.
Arguments
TYPE
- The external type of the user variable to return the value into.
Conversion will occur if this does not match the native type. Specify one
from the set of DBR_XXXX in db_access.h
COUNT
- Element count to be read from the specified channel. Must match the
array pointed to by PVALUE.
CHID
- Channel identifier
PVALUE
- Pointer to an application supplied buffer where the current value of
the channel is to be written.
USERFUNC
- Address of user supplied callback function to be
run when the requested operation completes.
USERARG
- Pointer sized variable retained and then passed back to user supplied
call back function above.
Returns
ECA_NORMAL - Normal successful completion
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_BADCHID - Corrupted CHID
ECA_BADCOUNT - Requested count larger than native element count
ECA_GETFAIL - A local database get failed
ECA_NORDACCESS - Read access denied
ECA_ALLOCMEM - Unable to allocate memory
ECA_DISCONN - Channel is disconnected
See Also
ca_pend_io()
ca_pend_event()
ca_create_subscription()
#include <cadef.h>
typedef void ( *pCallBack ) (
struct event_handler_args );
int ca_create_subscription ( chtype TYPE,
unsigned long COUNT, chid CHID,
unsigned long MASK, pCallBack USERFUNC, void *USERARG,
evid *PEVID );
Description
Register a state change subscription and specify a call back function to be
invoked whenever the process variable undergoes significant state changes. A
significant change can be a change in the process variable's value, alarm
status, or alarm severity. In the process control function block database the
deadband field determines the magnitude of a significant change for for the
process variable's value. Each call to this function consumes resources in the
client library and potentially a CA server until one of ca_clear_channel or
ca_clear_event is called.
Subscriptions may be installed or canceled against both connected and
disconnected channels. The specified USERFUNC is called once immediately after
the subscription is installed with the process variable's current state if the
process variable is connected. Otherwise, the specified USERFUNC is called
immediately after establishing a connection (or reconnection) with the process
variable. The specified USERFUNC is called immediately with the process
variable's current state from within ca_add_event() if the client and the
process variable share the same address space.
If a subscription is installed on a channel in a disconnected state then the
requested count will be set to the native maximum element count of the channel
if the requested count is larger.
All subscription requests such as the above are accumulated (buffered) and
not forwarded to the IOC until one of ca_flush_io, ca_pend_io, ca_pend_event,
or ca_sg_pend are called. This allows several requests to be efficiently sent
over the network in one message.
If at any time after subscribing, read access to the specified process
variable is lost, then the call back will be invoked immediately indicating
that read access was lost via the status argument. When read access is restored
normal event processing will resume starting always with at least one update
indicating the current state of the channel.
A better name for this function might have been ca_subscribe.
Example
See caMonitor.c in the example application created by makeBaseApp.pl.
Arguments
TYPE
- The type of value presented to the call back funstion. Conversion will
occur if it does not match native type. Specify one from the set of
DBR_XXXX in db_access.h
COUNT
- The element count to be read from the specified channel. A count of
zero specifies the native elemnt count.
CHID
- channel identifier
USRERFUNC
- The address of user supplied callback function to
be invoked with each subscription update.
USERARG
- pointer sized variable retained and passed back to user callback
function
RESERVED
- Reserved for future use. Specify 0.0 to remain upwardly compatible.
PEVID
- This is a pointer to user supplied event id which is overwritten if
successful. This event id can later be used to clear a specific
event. This option may may be omitted by passing a nil pointer.
MASK
- A mask with bits set for each of the event trigger types requested. The
event trigger mask must be a bitwise or of one or more of the
following constants.
- DBE_VALUE - Trigger events when the channel value exceeds the
monitor dead band
- DBE_LOG - Trigger events when the channel value exceeds the
archival dead band
- DBE_ALARM - Trigger events when the channel alarm state
changes.
For functions above that do not include a trigger specification,
events will be triggered when there are significant changes in the
channel's value or when there are changes in the channel's alarm state.
This is the same as "DBE_VALUE | DBE_ALARM."
Returns
ECA_NORMAL - Normal successful completion
ECA_BADCHID - Corrupted CHID
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_ALLOCMEM - Unable to allocate memory
ECA_ADDFAIL - A local database event add failed
See Also
ca_pend_event()
ca_flush_io()
ca_clear_subscription()
#include <cadef.h>
int ca_clear_subscription ( evid EVID );
Description
Cancel a subscription.
All ca_clear_event() requests such as the above are accumulated (buffered)
and not forwarded to the server until one of ca_flush_io, ca_pend_io,
ca_pend_event, or ca_sg_pend are called. This allows several requests to be
efficiently sent together in one message.
Arguments
- EVID
- event id returned by ca_add_event()
Returns
ECA_NORMAL - Normal successful completion
ECA_BADCHID - Corrupted CHID SEE ALSO ca_add_event()
#include <cadef.h>
int ca_pend_io ( double TIMEOUT );
Description
This function flushes the send buffer and then blocks until outstanding ca_get requests complete, and until channels created
specifying nill connection handler function pointers connect for the first
time.
- If ECA_NORMAL is returned then it can be safely assumed that all
outstanding ca_get requests have completed
successfully and channels created specifying nill connection handler
function pointers have connected for the first time.
- If ECA_TIMEOUT is returned then it must be assumed for all previous ca_get requests and properly qualified first time
channel connects have failed.
If ECA_TIMEOUT is returned then get requests may be reissued followed by a
subsequent call to ca_pend_io(). Specifically, the function will block only for
outstanding ca_get requests issued, and also any channels
created specifying a nill connection handler function pointer, after the last
call to ca_pend_io() or ca client context creation whichever is later. Note
that ca_create_channel requests generally
should not be reissued for the same process variable unless ca_clear_channel is called first.
If no ca_get or connection state change events are
outstanding then ca_pend_io() will flush the send buffer and return immediately
without processing any outstanding channel access background
activities.
The delay specified to ca_pend_io() should take into account worst case
network delays such as Ethernet collision exponential back off until
retransmission delays which can be quite long on overloaded networks.
Unlike ca_pend_event , this routine will
not process CA's background activities if none of the selected IO requests are
pending.
Arguments
- TIMEOUT
- Specifies the time out interval. A
TIMEOUT interval of
zero specifies forever.
Returns
ECA_NORMAL - Normal successful completion
ECA_TIMEOUT - Selected IO requests didnt complete before specified
timeout
ECA_EVDISALLOW - Function inappropriate for use within an event handler
See Also
ca_get()
ca_create_channel()
ca_test_io()
#include <cadef.h>
int ca_test_io();
Description
This function tests to see if all ca_get requests are
complete and channels created specifying a nill connection callback function
pointer are connected. It will report the status of outstanding ca_get requests issued, and channels created specifying a
nill connection callback function pointer, after the last call to ca_pend_io()
or CA context initialization whichever is later.
Returns
ECA_IODONE - All IO operations completed
ECA_IOINPROGRESS - IO operations still in progress
See Also
ca_pend_io()
#include <cadef.h>
int ca_pend_event ( double TIMEOUT );
int ca_poll ();
Description
When ca_pend_event is invoked the send buffer is flushed and CA background
activity is processed for TIMEOUT seconds.
When ca_poll is invoked the send buffer is flushed and any outstanding CA
background activity is processed.
The ca_pend_event function will not return before the specified
time-out expires and all unfinished channel access labor has been processed,
and unlike ca_pend_io returning from the
function does not indicate anything about the status of pending IO
requests.
Both ca_pend_event and ca_poll return ECA_TIMEOUT
when successful. This behavior probably isn't intuitive, but it is preserved to
insure backwards compatibility.
See also Thread Safety and Preemptive Callback to User
Code.
Arguments
TIMEOUT
- The duration to block in this routine in seconds. A timeout of zero
seconds blocks forever.
Returns
ECA_TIMEOUT - The operation timed out
ECA_EVDISALLOW - Function inappropriate for use within a call back
handler
#include <cadef.h>
int ca_flush_io();
Description
Flush outstanding IO requests to the server. This routine might be useful
to users who need to flush requests prior to performing client side labor in
parallel with labor performed in the server.
Outstanding requests are also sent whenever the buffer which holds them
becomes full.
Returns
ECA_NORMAL - Normal successful completion
#include <cadef.h>
int ca_signal ( long CA_STATUS, const char * CONTEXT_STRING );
void SEVCHK( CA_STATUS, CONTEXT_STRING );
Description
Provide the error message character string associated with the supplied
channel access error code and the supplied error context to diagnostics. If the
error code indicates an unsuccessful operation a stack dump is printed, if this
capability is available on the local operating system, and execution is
terminated.
SEVCHK is a macro envelope around ca_signal which only calls ca_signal() if
the supplied error code indicates an unsuccessful operation. SEVCHK is the
recommended error handler for simple applications which do not wish to write
code testing the status returned from each channel access call.
Examples
status = ca_context_create (...);
SEVCHK ( status, "Unable to create a CA client context" );
If the application only wishes to print the message associated with an error
code or test the severity of an error there are also functions provided for
this purpose.
Arguments
CA_STATUS
- The status (error code) returned from a channel access function.
CONTEXT_STRING
- A null terminated character string to supply as error context to
diagnostics.
Returns
ECA_NORMAL - Normal successful completion
#include <cadef.h>
typedef void (*pCallback) ( struct exception_handler_args HANDLERARGS );
int ca_add_exception_event ( pCallback USERFUNC, void *USERARG );
Description
Replace the currently installed CA context global exception handler call
back.
When an error occurs in the server asynchronous to the clients thread then
information about this type of error is passed from the server to the client in
an exception message. When the client receives this exception message an
exception handler callback is called.The default exception handler prints a
diagnostic message on the client's standard out and terminates execution if the
error condition is severe.
Note that certain fields in "struct exception_handler_args" are not
applicable in the context of some error messages. For instance, a failed get
will supply the address in the client task where the returned value was
requested to be written. For other failed operations the value of the addr
field should not be used.
Arguments
USERFUNC
- Address of user callback function to be executed when an exceptions
occur. Passing a nil value causes the default exception handler to be
reinstalled. The following structure is passed by value to the user's
callback function. Currently, the
op field can be one of
CA_OP_GET, CA_OP_PUT, CA_OP_CREATE_CHANNEL, CA_OP_ADD_EVENT,
CA_OP_CLEAR_EVENT, or CA_OP_OTHER.
struct exception_handler_args {
void *usr; /* user argument supplied when installed */
chanId chid; /* channel id (may be nill) */
long type; /* type requested */
long count; /* count requested */
void *addr; /* user's address to write results of CA_OP_GET */
long stat; /* channel access ECA_XXXX status code */
long op; /* CA_OP_GET, CA_OP_PUT, ..., CA_OP_OTHER */
const char *ctx; /* a character string containing context info */
sonst char *pFile; /* source file name (may be NULL) */
unsigned lineNo; /* source file line number (may be zero) */
};
USERARG
- pointer sized variable retained and passed back to user function
above
Example
void ca_exception_handler (
struct exception_handler_args args)
{
char buf[512];
char *pName;
if ( args.chid ) {
pName = ca_name ( args.chid );
}
else{
pName = "?";
}
sprintf ( buf,
"%s - with request chan=%s op=%d data type=%s count=%d",
args.ctx, pName, args.op, dbr_type_to_text ( args.type ), args.count );
ca_signal ( args.stat, buf );
}
ca_add_exception_event ( ca_exception_handler , 0 );
Returns
ECA_NORMAL - Normal successful completion
#include <cadef.h> int ca_add_fd_registration ( void ( USERFUNC * ) ( void *USERARG, int FD, int OPENED ), void * USERARG )
Description
For use with the services provided by a file descriptor manager (IO
multiplexor) such as ""fdmgr.c". A file descriptor manager is often needed when
two file descriptor IO intensive libraries such as the EPICS channel access
client library and the X window system client library must coexist in the same
UNIX process. This function allows an application code to be notified whenever
the CA client library places a new file descriptor into service and whenever
the CA client library removes a file descriptor from service. Specifying
USERFUNC=NULL disables file descriptor registration (this is the default).
Arguments
USERFUNC
Pointer to a user supplied C function returning null with the above
arguments.
USERARG
User supplied pointer sized variable passed to the above function.
FD
A file descriptor.
OPENED
Boolean argument is true if the file descriptor was opened and false if the
file descriptor was closed.
Example
int s;
static struct myStruct aStruct;
void fdReg ( struct myStruct *pStruct, int fd, int opened )
{
if ( opened ) printf ( "fd %d was opened\n", fd );
else printf ( "fd %d was closed\n", fd );
}
s = ca_add_fd_registration ( fdReg, & aStruct );
SEVCHK ( s, NULL );
Comments
When using this function it is advisable to call it only once prior to
calling any other CA function, or once just after creating the CA context (if
you create the context explicitly). Use of this interface can improve latency
slightly in applications that use non preemptive callback mode at the expense
of some additional runtime overhead when compared to the alternative which is
just polling ca_pend_event periodically. It would probably not be appropriate
to use this function with preemptive callback mode. Starting with R3.14 this
function is implemented in a special backward compatibility mode. if
ca_add_fd_registration is called, a single pseudo UDP fd is
created which CA pokes whenever something significant happens. Xt and others
can watch this fd so that backwards compatibility is preserved, and so that
they will not need to use preemptive callback mode but they will nevertheless
get the lowest latency response to the arrival of CA messages.
Returns
"ECA_NORMAL - Normal successful completion
#include <cadef.h>
typedef int caPrintfFunc ( const char *pFromat, va_list args );
int ca_replace_printf_handler ( caPrintfFunc *PFUNC );
Description
Replace the default handler for formatted diagnostic message output. The
default handler uses fprintf to send messages to 'stderr'.
Arguments
PFUNC
- The address of a user supplied call back handler to be invoked when CA
prints diagnostic messages. Installing a nil pointer will cause the
default call back handler to be reinstalled.
Examples
int my_printf ( char *pformat, va_list args ) {
int status;
status = vfprintf( stderr, pformat, args);
return status;
}
status = ca_replace_printf_handler ( my_printf );
SEVCHK ( status, "failed to install my printf handler" );
Returns
ECA_NORMAL - Normal successful completion
#include <cadef.h>
typedef void ( *pCallBack )( struct access_rights_handler_args );
int ca_replace ( chid CHAN, pCallBack PFUNC );
Description
Install or replace the access rights state change callback handler for the
specified channel.
The callback handler is called in the following situations.
- whenever CA connects the channel immediately before the channel's
connection handler is called
- whenever CA disconnects the channel immediately after the channel's
disconnect call back is called
- once immediately after installation if the channel is connected.
- whenever the access rights state of a connected channel changes
When a channel is created no access rights handler is installed.
Arguments
CHAN
- The channel identifier.
PFUNC
- Address of user supplied call back function. A nil pointer uninstalls
the current handler. The following arguments are passed by value
to the supplied callback handler.
typedef struct ca_access_rights {
unsigned read_access:1;
unsigned write_access:1;
} caar;
/* arguments passed to user access rights handlers */
struct access_rights_handler_args {
chanId chid; /* channel id */
caar ar; /* new access rights state */
};
Returns
ECA_NORMAL - Normal successful completion
See Also
ca_modify_user_name()
ca_modify_host_name()
#include <cadef.h>
chtype ca_field_type ( CHID );
Description
Return the native type in the server of the process variable.
Arguments
CHID
- channel identifier
Returns
TYPE
- The data type code will be a member of the set of DBF_XXXX in
db_access.h. The constant TYPENOTCONN is returned if the channel is
disconnected.
ca_element_count()
#include <cadef.h>
unsigned ca_element_count ( CHID );
Description
Return the maximum array element count in the server for the specified IO
channel.
Arguments
CHID
- channel identifier
Returns
COUNT
- The maximum array element count in the server. An element count of
zero is returned if the channel is disconnected.
#include <cadef.h>
char * ca_name ( CHID );
Description
Return the name provided when the supplied channel id was created.
Arguments
CHID
- channel identifier
Returns
PNAME
- The channel name. The string returned is valid as long as the channel
specified exists.
#include <cadef.h>
void ca_set_puser ( chid CHID, void *PUSER );
Description
Set a user private void pointer variable retained with each channel for use
at the users discretion.
Arguments
- CHID
- channel identifier
- PUSER
- user private void pointer
#include <cadef.h>
void * ca_puser ( CHID );
Description
Return a user private void pointer variable retained with each channel for
use at the users discretion.
Arguments
CHID
- channel identifier
Returns
PUSER
- user private pointer
#include <cadef.h>
enum channel_state {
cs_never_conn, /* valid chid, server not found or unavailable */
cs_prev_conn, /* valid chid, previously connected to server */
cs_conn, /* valid chid, connected to server */
cs_closed }; /* channel deleted by user */
enum channel_state ca_state ( CHID );
Description
Returns an enumerated type indicating the current state of the specified IO
channel.
Arguments
CHID
- channel identifier
Returns
STATE
- the connection state
#include <cadef.h>
const char * ca_message ( STATUS );
Description
return a message character string corresponding to a user specified CA
status code.
Arguments
STATUS
- a CA status code
Returns
- STR
ING
- the corresponding error message string
#include <cadef.h>
char * ca_host_name ( CHID );
Description
Return a character string which contains the name of the host to which a
channel is currently connected.
Arguments
CHID
- the channel identifier
Returns
STRING
- The process variable server's host name. If the channel is disconnected
the string "<disconnected>" is returned.
#include <cadef.h>
int ca_read_access ( CHID );
Description
Returns boolean true if the client currently has read access to the
specified channel and boolean false otherwise.
Arguments
CHID
- the channel identifier
Returns
STRING
- boolean true if the client currently has read access to the specified
channel and boolean false otherwise
#include <cadef.h>
int ca_write_access ( CHID );
Description
Returns boolean true if the client currently has write access to the
specified channel and boolean false otherwise.
Arguments
CHID
- the channel identifier
Returns
STRING
- boolean true if the client currently has write access to the specified
channel and boolean false otherwise
#include <db_access.h>
extern unsigned dbr_size[/*TYPE*/];
Description
An array that returns the size in bytes for a DBR_XXXX type.
Arguments
TYPE
- The data type code. A member of the set of DBF_XXXX in db_access.h.
Returns
SIZE
- the size in bytes of the specified type
#include <db_access.h>
unsigned dbr_size_n ( TYPE, COUNT );
Description
Returns the size in bytes for a DBR_XXXX type with COUNT elements. If the
DBR type is a structure then the value field is the last field in the
structure. If COUNT is greater than one then COUNT-1 elements are appended to
the end of the structure so that they can be addressed as an array through a
pointer to the value field.
Arguments
TYPE
- The data type
COUNT
- The element count
Returns
SIZE
- the size in bytes of the specified type with the specified number of
elements
#include <db_access.h>
extern unsigned dbr_value_size[/* TYPE */];
Description
The array dbr_value_size[TYPE] returns the size in bytes for the value
stored in a DBR_XXXX type. If the type is a structure the size of the value
field is returned otherwise the size of the type is returned.
Arguments
TYPE
- The data type code. A member of the set of DBF_XXXX in db_access.h.
Returns
SIZE
- the size in bytes of the value field if the type is a structure and
otherwise the size in bytes of the type
#include <db_access.h>
const char * dbr_type_text ( chtype TYPE );
Description
Returns a constant null terminated string corresponding to the specified dbr
type.
Arguments
TYPE
- The data type code. A member of the set of DBR_XXXX in db_access.h.
Returns
STRING
- The const string corresponding to the DBR_XXX type.
#include <cadef.h>
Description
void ca_test_event ( struct event_handler_args );
A built-in subscription update call back handler for debugging purposes that
prints diagnostics to standard out.
Examples
void ca_test_event ();
status = ca_add_event ( type, chid, ca_test_event, NULL, NULL );
SEVCHK ( status, .... );
See Also
ca_add_event()
#include <cadef.h>
int ca_sg_create ( CA_SYNC_GID *PGID );
Description
Create a synchronous group and return an identifier for it.
A synchronous group can be used to guarantee that a set of channel access
requests have completed. Once a synchronous group has been created then channel
access get and put requests may be issued within it using ca_sg_get() and
ca_sg_put() respectively. The routines ca_sg_block() and ca_sg_test() can be
used to block for and test for completion respectively. The routine
ca_sg_reset() is used to discard knowledge of old requests which have timed out
and in all likelihood will never be satisfied.
Any number of asynchronous groups can have application requested operations
outstanding within them at any given time.
Arguments
PGID
- Pointer to a user supplied CA_SYNC_GID.
Examples
CA_SYNC_GID gid;
status = ca_sg_create ( &gid );
SEVCHK ( status, Sync group create failed );
Returns
ECA_NORMAL - Normal successful completion
ECA_ALLOCMEM - Failed, unable to allocate memory
See Also
ca_sg_delete()
ca_sg_block()
ca_sg_test()
ca_sg_reset()
ca_sg_put()
ca_sg_get()
#include <cadef.h>
int ca_sg_delete ( CA_SYNC_GID GID );
Description
Deletes a synchronous group.
Arguments
- GID
- Identifier of the synchronous group to be deleted.
Examples
CA_SYNC_GID gid;
status = ca_sg_delete ( gid );
SEVCHK ( status, Sync group delete failed );
Returns
ECA_NORMAL - Normal successful completion
ECA_BADSYNCGRP - Invalid synchronous group
See Also
ca_sg_create()
#include <cadef.h>
int ca_sg_block ( CA_SYNC_GID GID, double timeout );
Description
Flushes the send buffer and then waits until outstanding requests complete
or the specified time out expires. At this time outstanding requests include
calls to ca_sg_array_get() and calls to ca_sg_array_put(). If ECA_TIMEOUT is
returned then failure must be assumed for all outstanding queries. Operations
can be reissued followed by another ca_sg_block(). This routine will only block
on outstanding queries issued after the last call to ca_sg_block(),
ca_sg_reset(), or ca_sg_create() whichever occurs later in time. If no queries
are outstanding then ca_sg_block() will return immediately without processing
any pending channel access activities.
Values written into your program's variables by a channel access synchronous
group request should not be referenced by your program until ECA_NORMAL has
been received from ca_sg_block(). This routine will process pending channel
access background activity while it is waiting.
Arguments
- GID
- Identifier of the synchronous group.
Examples
CA_SYNC_GID gid;
status = ca_sg_block(gid);
SEVCHK(status, Sync group block failed);
Returns
ECA_NORMAL - Normal successful completion
ECA_TIMEOUT - The operation timed out
ECA_EVDISALLOW - Function inappropriate for use within an event handler
ECA_BADSYNCGRP - Invalid synchronous group
See Also
ca_sg_test()
ca_sg_reset()
#include <cadef.h>
int ca_sg_test ( CA_SYNC_GID GID )
Description
Test to see if all requests made within a synchronous group have
completed.
Arguments
GID
- Identifier of the synchronous group.
Description
Test to see if all requests made within a synchronous group have
completed.
Examples
CA_SYNC_GID gid;
status = ca_sg_test ( gid );
Returns
ECA_IODONE - IO operations completed
ECA_IOINPROGRESS - Some IO operations still in progress
#include <cadef.h>
int ca_sg_reset ( CA_SYNC_GID GID )
Description
Reset the number of outstanding requests within the specified synchronous
group to zero so that ca_sg_test() will return ECA_IODONE and ca_sg_block()
will not block unless additional subsequent requests are made.
Arguments
GID
- Identifier of the synchronous group.
Examples
CA_SYNC_GID gid;
status = ca_sg_reset(gid);
Returns
ECA_NORMAL - Normal successful completion
ECA_BADSYNCGRP - Invalid synchronous group
#include <cadef.h>
int ca_sg_array_put ( CA_SYNC_GID GID, chtype TYPE,
unsigned long COUNT, chid CHID, void *PVALUE );
Write a value, or array of values, to a channel and increment the
outstanding request count of a synchronous group.
All remote operation requests such as the above are accumulated (buffered)
and not forwarded to the server until one of ca_flush_io(), ca_pend_io(),
ca_pend_event(), or ca_sg_pend() are called. This allows several requests to be
efficiently sent in one message.
If a connection is lost and then resumed outstanding puts are not
reissued.
Arguments
GID
- synchronous group identifier
TYPE
- The type of supplied value. Conversion will occur if it does not match
the native type. Specify one from the set of DBR_XXXX in db_access.h.
COUNT
- element count to be written to the specified channel - must match the
array pointed to by PVALUE
CHID
- channel identifier
PVALUE
- A pointer to an application supplied buffer containing the value or
array of values returned
Returns
ECA_NORMAL - Normal successful completion
ECA_BADSYNCGRP - Invalid synchronous group
ECA_BADCHID - Corrupted CHID
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_BADCOUNT - Requested count larger than native element count
ECA_STRTOBIG - Unusually large string supplied
ECA_PUTFAIL - A local database put failed
See Also
ca_flush_io()
#include <cadef.h>
int ca_sg_array_get ( CA_SYNC_GID GID,
chtype TYPE, unsigned long COUNT,
chid CHID, void *PVALUE );
Description
Read a value from a channel and increment the outstanding request count of a
synchronous group.
The values written into your program's variables by ca_sg_get should not be
referenced by your program until ECA_NORMAL has been received from ca_sg_block
, or until ca_sg_test returns ECA_IODONE.
All remote operation requests such as the above are accumulated (buffered)
and not forwarded to the server until one of ca_flush_io, ca_pend_io,
ca_pend_event, or ca_sg_pend are called. This allows several requests to be
efficiently sent in one message.
If a connection is lost and then resumed outstanding gets are not
reissued.
Arguments
GID
- Identifier of the synchronous group.
TYPE
- External type of returned value. Conversion will occur if this does not
match native type. Specify one from the set of DBR_XXXX in
db_access.h
COUNT
- Element count to be read from the specified channel. It must match the
array pointed to by PVALUE.
CHID
- channel identifier
PVALUE
- Pointer to application supplied buffer that is to contain the value or
array of values to be returned
Returns
ECA_NORMAL - Normal successful completion
ECA_BADSYNCGRP - Invalid synchronous group
ECA_BADCHID - Corrupted CHID
ECA_BADCOUNT - Requested count larger than native element count
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_GETFAIL - A local database get failed
See Also
ca_pend_io()
ca_flush_io()
ca_get_callback()
int ca_client_status ( unsigned level );
int ca_context_status ( struct ca_client_context *,
unsigned level );
Description
Prints information about the client context including, at higher interest
levels, status for each channel. Lacking a CA context pointer,
ca_client_status() prints information about the calling threads CA context.
Arguments
CONTEXT
- A pointer to the CA context to join with.
LEVEL
- The interest level. Increasing level produces increasing detail.
struct ca_client_context * ca_current_context ();
Description
Returns a pointer to the current thread's CA context. If none then nil is
returned.
See Also
ca_attach_context()
ca_detach_context()
ca_context_create()
ca_context_destroy()
int ca_attach_context (struct ca_client_context *CONTEXT);
Description
The calling thread becomes a member of the specified CA context. If
ca_disable_preemptive_callback is specified when
ca_context_create() is called (or if ca_task_initialize() is called) then
additional threads are not allowed to join the CA context because
allowing other threads to join implies that CA callbacks will be called
preemptively from more than one thread.
Arguments
CONTEXT
- A pointer to the CA context to join with.
Returns
ECA_ISATTACHED - already attached to a CA context
ECA_NOTTHREADED - the specified context is non-preemptive and therefore does
not allow other threads to join
ECA_ISATTACHED - the current thread is already attached to a CA context
See Also
ca_current_context()
ca_detach_context()
ca_context_create()
ca_context_destroy()
void ca_detach_context();
Description
Detach from any CA context currently attached to the calling thread. This
does not cleanup or shutdown any currently attached CA context (for
that use ca_context_destroy).
See Also
ca_current_context()
ca_attach_context()
ca_context_create()
ca_context_destroy()
void ca_dump_dbr ( chtype TYPE, unsigned COUNT, const
void * PDBR );
Description
Dumps the specified dbr data type to standard out.
Arguments
TYPE
- The data type (from the DBR_XXX set described in db_access.h).
COUNT
- The array element count
PDBR
- A pointer to data of the specified count and number.
- ECA_NORMAL
- Normal successful completion
- ECA_ALLOCMEM
- Unable to allocate additional dynamic memory
- ECA_TOLARGE
- The requested data transfer is greater than available memory or
EPICS_CA_MAX_ARRAY_BYTES
- ECA_BADTYPE
- The data type specified is invalid
- ECA_BADSTR
- Invalid string
- ECA_BADCHID
- Invalid channel identifier
- ECA_BADCOUNT
- Invalid element count requested
- ECA_PUTFAIL
- Channel write request failed
- ECA_GETFAIL
- Channel read request failed
- ECA_ADDFAIL
- unable to install subscription request
- ECA_TIMEOUT
- User specified timeout on IO operation expired
- ECA_EVDISALLOW
- function called was inappropriate for use within a callback
function
- ECA_IODONE
- IO operations have completed
- ECA_IOINPROGRESS
- IO operations are in progress
- ECA_BADSYNCGRP
- Invalid synchronous group identifier
- ECA_NORDACCESS
- Read access denied
- ECA_NOWTACCESS
- Write access denied
- ECA_DISCONN
- Virtual circuit disconnect"
- ECA_DBLCHNL
- Identical process variable name on multiple servers
- ECA_EVDISALLOW
- Request inappropriate within subscription (monitor) update callback
- ECA_BADMONID
- Bad event subscription (monitor) identifier
- ECA_BADMASK
- Invalid event selection mask
- ECA_PUTCBINPROG
- Put callback timed out
- ECA_PUTCBINPROG
- Put callback timed out
- ECA_ANACHRONISM
- Requested feature is no longer supported
- ECA_NOSEARCHADDR
- Empty PV search address list
- ECA_NOCONVERT
- No reasonable data conversion between client and server types
- ECA_BADFUNCPTR
- Invalid function pointer
- ECA_ISATTACHED
- Thread is already attached to a client context
- ECA_UNAVAILINSERV
- Not supported by attached service
- ECA_CHANDESTROY
- User destroyed channel
- ECA_BADPRIORITY
- Invalid channel priority
- ECA_NOTTHREADED
- Preemptive callback not enabled - additional threads may not join
context
- ECA_16KARRAYCLIENT
- Client's protocol revision does not support transfers exceeding 16k
bytes
$Id: CAref.html,v 1.58.2.43 2008/11/14 13:12:26 lange Exp $
.
|