14

This is some code I'm compiling on Linux:

#include <net/if.h>

int main() {
  struct ifreq ifr;
}

gcc test.c is fine.

gcc -std=gnu99 test.c is fine.

gcc -std=c99 test.c fails with the following error:

test.c: In function ‘main’:
test.c:4:16: error: storage size of ‘ifr’ isn’t known

What's different about C99 that it doesn't like the definition of struct ifreq in Linux?

ArjunShankar
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Jim Hunziker
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1 Answers1

22

It's a chain of consequences of preprocessing and GNU C vs C99.

First up, net/if.h:

  1. net/if.h includes features.h
  2. Later on, it defines struct ifreq inside a #ifdef __USE_MISC block.

So:

  1. What is __USE_MISC? -- it is stuff common to BSD and System V
  2. Is it defined at this point? -- We need to check that out in features.h

So now, features.h:

  1. When you use --std=c99 GCC by default defines __STRICT_ANSI__ (since thats what C99 is)
  2. While preprocessing features.h, when __STRICT_ANSI__ is on, the BSD and System V features don't kick in. i.e. __USE_MISC is left undefined.

Back up to net/if.h: struct ifreq does not even exist after preprocessing! Therefore, the complaint about storage size.

You can catch the whole story by doing:

vimdiff <(cpp test.c --std=c99 -dD) <(cpp test.c --std=gnu99 -dD)

or diff'ing them in any other way (like diff --side-by-side) instead of vimdiff.

If you want this to cleanly compile with -std=c99, you must consider the inclusion of the _DEFAULT_SOURCE feature test macro (for glibc versions >= 2.19; for older glibc versions, use either _BSD_SOURCE or _SVID_SOURCE) so that the required functionality is enabled on top of what is offered by C99.

ArjunShankar
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  • Thanks for the explanation. I just stumbled over exactly the same issue. Is there a way to successfully compile such code with `--std=c99`? – michas Jul 16 '14 at 07:27
  • @michas Yes. You could define either the `_BSD_SOURCE` or the `_SVID_SOURCE` [feature test macro](http://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html) in your C sources. – ArjunShankar Jul 16 '14 at 10:09