Experimental Physics and
Industrial Control System

Mark Rivers <[email protected]> · Thu, 2 Nov 2017 18:33:55 +0000

Hi Dirk,

> * Add new fields only ever at the end of structure passed to API  functions by reference 
> (and then handle cases gracefully where the  fields don't exist).

Consider this change that was made 2 years ago to the asynUser structure in asynDriver.h.

corvette:asyn/asyn/asynDriver>git diff 325410dfaac1c0b000c32f515b8d9d96be15fefa ee6839f7c10bcc36454fec21ca0164c3579c9e55 asynDriver.h

diff --git a/asyn/asynDriver/asynDriver.h b/asyn/asynDriver/asynDriver.h
index 2d698ad..71ca083 100644
--- a/asyn/asynDriver/asynDriver.h
+++ b/asyn/asynDriver/asynDriver.h
@@ -56,16 +56,18 @@ typedef struct asynUser {
     char          *errorMessage;
     int            errorMessageSize;
     /* timeout must be set by the user */
...
+    int            auxStatus;     /* For auxillary status*/
+    int            alarmStatus;   /* Typically for EPICS record alarm status */
+    int            alarmSeverity; /* Typically for EPICS record alarm severity */
 }asynUser;

So I have followed your rule above, adding new fields to the end of the asyn user structure.  The implication of your message is that does not change the ABI, i.e. I don't need to make a new major version of asyn.

However, if code which was compiled against the old version of the header file calls this new library version how can the library know whether the asynUser structure that was passed contains these fields or not?

Mark

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Dirk Zimoch
Sent: Thursday, November 02, 2017 7:43 AM
To: [email protected]
Subject: Re: exporting module versions


On 02.11.2017 11:18, Ralph Lange wrote:
> Hi Dirk,
>
> On Thu, Nov 2, 2017 at 10:23 AM, Dirk Zimoch <[email protected]
> <mailto:[email protected]>> wrote:
>
>     [...]  I also opt for binary backward compatibility, so that it is
>     always possible to replace a dynamic library with a newer version
>     without needing to re-build all programs. Forcing a program to link
>     only with a very specific library version is, in my opinion, not
>     very maintenance friendly.
>
>
> As you seem to have experience with that: which tools / methodology do
> you suggest to detect and track ABI changes in libraries, especially
> libraries created from C++ sources?
>
> Thanks,
> ~Ralph
>

I don't know any tools to support compatibility checks, but here is what 
I try to do:

* Never remove API functions (declaring them depreciated is OK)
* Never change the signature of an existing (extern C) function (...in 
an incompatible way. Signedness change is often OK. Adding const or 
volatile where appropriate is also often OK. Changing 32 bit args (like 
int) to potentially 64 bit args (e.g. size_t) is only OK if no 64 bit 
was supported previously, but that is already ancient history.)
* Never change the semantics of an existing function (e.g. swap src and 
dest parameters in some copy function)
* Never remove, re-order, or change size of the fields of a structure 
that is used in an API.
* Add new fields only ever at the end of structure passed to API 
functions by reference (and then handle cases gracefully where the 
fields don't exist).
* Never remove or re-order virtual methods (the same for non-C++ 
function tables like in asyn).
* Expose as little as possible in the API. Not all functions are API 
functions, not all structures/classes are used in the API. Not all 
Macros are part of the API. Keep public and private header files 
separate. Do not install private headers. This allows to change any non 
API function, class, etc. at any time without breaking the API.
* Do not put private fields/methods in API classes. If private members 
are needed, inherit from a API base class without private members. APIs 
are not private.

As it is often not feasible to be so strictly backward compatible, I 
suggest (any use in my software) the following rules:

* A version consists of 3 numbers: major.minor.patch
* Whenever a change is not binary backward compatible, the major number 
increases.
* Whenever there are new features, the minor number increases.
* Whenever a bug is fixed without a new feature, the patch number 
increases. (A bugfix may be incompatible in so far that the 
incompatibility was the actual bug that has been fixed.)
* In linking use the major number in the file name to ensure no 
incompatible version can be used.
* Do not use the minor number or patch number in linking in order to 
allow upgrading the library.


See also how Linux (or GNU) does it: /bin/bash on my computer is linked 
to libtinfo.so.5 which is a symbolic link to libtinfo.so.5.7. Note that 
is is not linked to version 5.7 but only to version 5. This allows to 
upgrade the library to 5.8 but not to 6.0 without having to rebuild the 
executable.

Or: softIoc on my computer is linked to libstdc++.so.6 which is a 
symbolic link to libstdc++.so.6.0.13.


I have no idea how to automatically check for backward compatibility 
when releasing a new version. I can imagine checking function and 
structure/class signatures automatically. But how to check for semantic 
changes?


Dirk



Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System