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GL_LINK_WARNING (was: fcntl for mingw)

From: Eric Blake
Subject: GL_LINK_WARNING (was: fcntl for mingw)
Date: Tue, 15 Dec 2009 19:29:28 +0000 (UTC)
User-agent: Loom/3.14 (http://gmane.org/)

Eric Blake <ebb9 <at> byu.net> writes:

> So, what should I do?  Options:
> 1. Check in the patches below as-is
> 2. Ditch this series, and instead go and change all uses of GNULIB_POSIXCHECK 
> that used
>  #define func(args) (GL_LINK_WARNING("..."),func(args))
> to instead use
>  #define func (GL_LINK_WARNING("..."),func)
> 3. Like 2, but also check in the new va-args module (others might have a use 
> for it, even though it would be unused in gnulib at this point)

Another thing to consider.  Why are we even bothering with a link warning, 
which only works for ELF, when gcc provides a more generic solution that will 
also work for Cygwin and other non-ELF platforms?

For example, on cygwin:

$ cat foo.c
#include <stdlib.h>
main (int argc, char **argv)
  char x[] = "aaaXXXXXX";
  char * (*func) (char *) = mktemp;
  func (x);
  mktemp (x);
  return 0;
$ gcc -E foo.c | grep mktemp
char * __attribute__((__cdecl__)) mktemp (char *) __attribute__ ((warning ("the 
use of `mktemp' is dangerous; use `mkstemp' instead")));
char * __attribute__((__cdecl__)) _mktemp_r (struct _reent *, char *) 
__attribute__ ((warning ("the use of `mktemp' is dangerous; use `mkstemp' 
  char * (*func) (char *) = mktemp;
  mktemp (x);
$ gcc -Wall -o foo foo.c
foo.c: In function ‘main’:
foo.c:8: warning: call to ‘mktemp’ declared with attribute warning: the use of 
`mktemp' is dangerous; use `mkstemp' instead

OK, so gcc didn't warn about the suspicious use of the address of mktemp in 
line 6, such that the call to mktemp in line 7 didn't warn.  I guess we should 
report that as a bug to the gcc folks.  But as indirection is less common, the 
use of __attribute__((__warning__(""))) would allow even more platforms to 
detect GNULIB_POSIXCHECK holes, without resorting to ELF-only magic that 
doesn't kick in until the link phase.

In other words, maybe I should go with option:
4. Redefine GL_LINK_WARNING to instead be a way to apply __attribute__
((__warning__)) as a function attribute when gcc supports it (and probably 
rename the module/macro from link-warning/GL_LINK_WARNING to usage-
warning/GL_USAGE_WARNING, since it would now be a compiler rather than a linker 
warning), and adjust all GNULIB_POSIXCHECK call points to use the new semantics.

Eric Blake

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