AUTOCONF macro + gcc -Wunreachable-code = NOWORK

hi,
sry for the strange subject but i don't know where to ask this
(if someone knows a good place PLZ let me know)

-i work on gentoo linux on x86 (nothing speciall)

-in my configure.ac i use AC_FUNC_MALLOC
to check for a system malloc function that is gnu compilant.
if i compile with the standard options (i think just -02) it
works fine:
">checking for GNU libc compatible malloc... yes"

-BUT if i use "-Dunreachable-code -W -Werror"
">checking for GNU libc compatible malloc... no"
so i guess there is sth. strange in malloc.h that make the test
bail out with the options above. but this is a system header it should
compile without any warnings RIGHT!?


PLZ if someone has ever had a prob. like this or knows there to
post this question. let me know

thomas zauner

Dunno, but malloc isn't in malloc.h, it's in stdlib.h

> -Dunreachable-code
-W
-D
Are you making this up as you go?

Assuming that you mean "-Wunreachable-code" rather than "-Dunreachable-code", the answer is fairly simple (though hard to track down.)

The problem is that the configure script is generating something like:

Code:

---

#include <stdlib.h>
int main()
{
exit (malloc (0) ? 0 : 1);
return 0;
}

---

The combination of "-Werror" and "-Wunreachable-code" to GCC will cause the "return 0" in the generated code to be unreachable (GCC recognizes exit() as not returning), thus produces a warning, which (due to "-Werror") is an error, hence the test fails because the compile fails.

If you're feeling ambitious, and insist on using the combination of "-Werror" and "-Wunreachable-code", you could go to /usr/lib/share/autoconf/autoconf (or wherever you have the autoconf M4 files), and edit the file autoconf.m4f; search for the line that reads:

Code:

---

      [exit (malloc (0) ? 0 : 1);])],

---

and replace it with something like:

Code:

---

      [if (malloc(0) == 0) exit(1);])],

---

This should provide an execution path to the "return 0;", thus get rid of the unreachable-code warning, which should then produce the correct result. Test it, of course, as I may have screwed the code fragment up. This only fixes this instance of unreachable code in autoconf's generated tests, so my recommendation is that you don't use "-Wunreachable-code" in distributed apps, just in development.

I hope this helps.