Experimental Physics and
Industrial Control System

Jeff Hill <[email protected]> · Mon, 28 Feb 2005 18:10:01 -0700

> Jeff, could you please explain why the interface of 
> std::string is not 
> compatible with such implementations?
> 

One of the OO convential wisdoms is to never implement an
interface that passes off handles to an internal data structure.
See Item 29 in "Effective C++ 2nd edition", Scott Meyers. To my
continued amazement there is in std::basic_string a function
called c_str() which all implementations I have looked at so far
implement by providing a pointer to the raw internal storage for
the string. Here is the latest GNU source code. This
implementation appears to prevent strings stored in
non-contiguous, fixed sized blocks.

const value_type *c_str( ) const;

      // String operations:
      /**
       *  @brief  Return const pointer to null-terminated
contents.
       *
       *  This is a handle to internal data.  Do not modify or
dire things may
       *  happen.
      */
      const _CharT*
      c_str() const { return _M_data(); }

> BTW, "free-list" is a strange term for allocation in 
> fixed-sized blocks. 
> AFAIK, many different memory allocation schemes keep a list 
> of free memory 
> blocks.

Sorry, this is probably yet another name that has grown to become
universally meaningful to EPICS base persons, but perhaps not in
the wider community.

> 
> > We did experience memory fragmentation issues on vxWorks in
the
> > 90's. I vaguely recall that at the time, after someone had a
> > quick look at the WRS source code, it was stated that the WRS
> > memory allocation algorithms wasn't main stream.
> 
> I've been told things were even worse with the original 
> VxWorks semaphore implementation ;-)
> 

Some of my favorites old WRS bugs...

o crashing every vxWorks when any SUN system booted using bootp
o crashing vxWorks if a person typed "logout" instead of using
"<ctrl>]",
and for some reason they could never log in again!

I dont remember the semaphore bug despite using vxWorks even when
it was still based on the VRTX kernel, but perhaps we jumped over
that version.

Jeff

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System