Experimental Physics and
Industrial Control System

Bill Lavender <[email protected]> · Mon, 23 Mar 2009 15:05:20 -0500

I have encountered a minor problem with EPICS base 3.14.10 on MacOS X
systems that I would like to report.  In my situation, I have a Python
module coded in C that invokes a library of mine, which in turn links
to the EPICS libCom and libca libraries.  The symptom of the problem is
that when I try to import the Python module, I get an error like this:

ImportError: dlopen(./MpPrivate.so, 2): Symbol not found: _environ
  Referenced from: /opt/epics/base-3.14.10/lib/darwin-x86/libCom.3.14.10.dylib
  Expected in: flat namespace

This is on a computer running MacOS X 10.5 (Leopard).

The source of the problem is that MacOS X defines the symbol 'environ'
in some situations, but not all situations.  I have found a variety of
forum posts and mailing list entries on the Internet that describe the
problem and how to fix it.  The message

  http://lists.apple.com/archives/xcode-users/2004/jan/msg00230.html

gives a particularly clear description of the issue and I quote part of
it here:

> _environ is defined in /usr/lib/crt1.o, which is only linked into
> executables. The corresponding file for dylibs (which frameworks use) is
> /usr/lib/dylib1.o and doesn't define _environ. Annoying as that is, it's
> correct because otherwise your library's _environ would be different from
> the _environ of the executable that loaded it.
>
> If you want to use _environ in a library on Mac OS X, you should use
> *_NSGetEnviron() instead. It's defined in /usr/include/crt_externs.h.
>
> Hope this helps,
> Eric

As far as I can tell, the only place in EPICS base that uses the 'environ'
symbol is the function epicsEnvShow() in "src/libCom/osi/os/default/osdEnv.c"
I have created a modified version of this function that looks like this

  #if defined(__APPLE__)
  #  include <crt_externs.h>
  #  define environ (*_NSGetEnviron())
  #endif

  /*
   * Show the value of the specified, or all, environment variables
   */
  epicsShareFunc void epicsShareAPI epicsEnvShow (const char *name)
  {
      if (name == NULL) {

  #if !defined(__APPLE__)
          extern char **environ;
  #endif

          char **sp;

          for (sp = environ ; (sp != NULL) && (*sp != NULL) ; sp++)
              printf ("%s\n", *sp);
      }
      else {
          const char *cp = getenv (name);
          if (cp == NULL)
              printf ("%s is not an environment variable.\n", name);
          else
              printf ("%s=%s\n", name, cp);
      }
  }

I haven't actually tested the modified version of the function, since I
do not use epicsEnvShow() for anything, but this change is enough to get
my Python module to load successfully without any missing symbols.

Hope this helps,

Bill Lavender

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System