Re: how to test for a prototype difference

[Thomas Klausner]

Hi Eric!

On Wed, Apr 30, 2014 at 06:51:23PM -0600, Eric Blake wrote:
> On 04/30/2014 06:12 PM, Thomas Klausner wrote:
> > Hi!
> > 
> > I found out that the prototypes for backtrace() and backtrace_symbol()
> > differ on some operating systems.
> > 
> 
> > 
> > The main difference is that on NetBSD, size_t is used for most
> > arguments that are int on the other systems; also,
> > backtrace_symbols_fd returns an int.
> > 
> > I'd like to test for the difference int <-> size_t in a configure
> > script so I can get integer type/sign conversion-warnings free code on
> > all these operating systems.
> > 
> > I'm however not sure how to do this. Does anyone have a suggestion?
> 
> Yep - use function pointer assignment or redeclaration to force the
> compiler to complain if the redeclaration used the wrong type.  Gnulib
> has some examples; here's one with ioctl(), which on Linux takes an
> 'unsigned long' instead of the POSIX 'int' for the second parameter:

Thanks for the idea and the examples!

Here's what I came up with:
AC_CACHE_CHECK([parameter type for backtrace()],
  [am_cv_proto_backtrace_type],
  [AC_COMPILE_IFELSE(
    [AC_LANG_PROGRAM(
      [[
#include <execinfo.h>
extern
#ifdef __cplusplus
"C"
#endif
size_t backtrace(void **addrlist, size_t len);
      ]])
    ],
    [am_cv_proto_backtrace_type=size_t],
    [am_cv_proto_backtrace_type=int])
  ])
AC_DEFINE_UNQUOTED([backtrace_t], [$am_cv_proto_backtrace_type],
  [Define to return type for backtrace().])


It seems to work fine on NetBSD, OS X, and Linux.

This way I can define my variables

backtrace_t len;

and it'll work even if I don't have any backtrace() functionality.

Cheers,
 Thomas