When to define _GNU_SOURCE [Solved]

[Akkara]

I was writing some C code that needed the pow10 function. I got a "incompatible implicit declaration" error despite #including <math.h>.

From the man page I learned it is a gnu extension, and that _GNU_SOURCE needs to be defined. So I define it:

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#include <stdio.h>
#include <stdlib.h>
#define _GNU_SOURCE     1
#include <math.h>

But I still get that error. I search and search, and nothing. As far as I can tell I was doing everything correctly.

On a whim I massage the code some:

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#define _GNU_SOURCE     1
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

The error goes away and the code works!

So I think, "that's strange, maybe pow10 is defined in one of the other header files?" I try putting _GNU_SOURCE between stdio and stdlib, but the error returns. It only works when _GNU_SOURCE is defined first, before *any* include files have been included.

My problem is solved, but I'm still curious: how does defining a macro before or after including <stdio.h> have any bearing on what happens when <math.h> is included? There seems to be a strange side-effect going on here. Maybe it is a bug? Maybe I don't understand how _GNU_SOURCE is supposed to operate?

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$ equery list glibc
[ Searching for package 'glibc' in all categories among: ]
 * installed packages
[I--] [ ~] sys-libs/glibc-2.8_p20080602-r1 (2.2)

$ gcc --version
gcc (Gentoo 4.3.2 p1.1) 4.3.2

[Hu]

A quick glance through /usr/include/features.h shows that defining _GNU_SOURCE causes various other preprocessor values to be modified. In particular, it enables __USE_GNU. pow10 is guarded by __USE_GNU, rather than _GNU_SOURCE. Thus, you need the headers to enable __USE_GNU to grant you access to pow10. Since stdio.h includes features.h, and features.h has an include guard, the preprocessor values get configured as soon as you include stdio.h and stay that way for the rest of the preprocessing phase.

It should be possible to include headers before you define _GNU_SOURCE. However, the libc headers that I have checked include features.h, so you can only safely include your own headers, and then only if they do not in turn use libc headers.

[Akkara]

Thanks!, Hu, for a very clear explanation.

[Captain Newbie]

Thanks!, Hu, for a very clear explanation.

I think I've seen that bad boy defined on the command line to gcc, i.e.:

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gcc -O2 -march=nocona -pipe -D_GNU_SOURCE foo.c -o ...

which might let you set it and forget it in your Makefile.