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To: cygwin AT cygwin DOT com
From: Andrew Schulman <schulman DOT andrew AT epa DOT gov>
Subject: Re: libargp: argp_program_version does not work
Date: Thu, 11 Jul 2013 16:28:26 -0400
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> >  Hello!
> > 
> >  I have found a problem: argp_program_version string is ignored by libargp.
> 
> Hi Pavel.  Thanks for reporting this.  I confirm your observation.  For
> example, when I compile argp's example #2
> (http://www.gnu.org/software/libc/manual/html_node/Argp-Example-2.html#Argp-Example-2),
> the --version option doesn't work.
> 
> >  I guess the problem happens because of DLL's nature. DLLs cannot contain
> > unresolved symbols, so the DLL has own version of argp_program_version which
> > is always initialized to NULL. There's no way to override it.
> 
> Also confirmed.  argp.h declares argp_program_version as
> 
> extern const char *argp_program_version;
> 
> But gllib/argp-pv.c initializes it as
> 
> const char *argp_program_version = (const char *) 0
> 
> Whether there's a way to override that value by one declared in the user's
> code, I don't know.  Maybe a linker switch?

OK, I remembered how this works.  To override the initial values in your
program, you just have to reassign them at run time.

Here's how it's done with argp's example #2.  Note the first two lines of
main(), which aren't present in the upstream example.

/*** Begin modified argp example #2 ***/

#include <argp.h>

const char version[]     = "argp-ex2 1.0";
const char bug_address[] = "<bug-gnu-utils AT gnu DOT org>";

/* Program documentation. */
static char doc[] = "Argp example #2 -- a pretty minimal program using
argp";
static struct argp argp = { 0, 0, 0, doc };

int main (int argc, char **argv)
{
  argp_program_version = version;
  argp_program_bug_address = bug_address;

  argp_parse (&argp, argc, argv, 0, 0, 0);
}

/*** End modified argp example #2 ***/

If you compile the above version, you'll see that the --version option and
bug tracker text are available.

I'm sure this problem is familiar to developers and users of shared
libraries, but I had forgotten it.  I'll add a note to the Cygwin README
file, to help all of us remember it next time.

Andrew


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