-std=c99 wtf on Linux

Question

When I try to compile this on Linux with -std=c99 gcc complains about not knowing struct timespec. However if I compile this w/o -std=c99 everything works fine.

#include <time.h>

void blah(struct timespec asdf)
{}

int main()
{
  struct timespec asdf;
  return 0;
}

Why is this and is there a way to still get it to work with -std=c99?

Answer 1

I would recommend compiling with -std=gnu99.

To elaborate on this. By default, gcc compiles with -std=gnu89. Here are the results for the following source code.

#include <time.h>

int main() {
    struct timespec asdf;
    return 0;
}

---

[1:25pm][wlynch@cardiff /tmp] gcc -std=gnu89 foo.c
[1:26pm][wlynch@cardiff /tmp] gcc -std=gnu99 foo.c

[1:25pm][wlynch@cardiff /tmp] gcc -std=c89 foo.c
foo.c: In function ‘main’:
foo.c:4: error: storage size of ‘asdf’ isn’t known

[1:26pm][wlynch@cardiff /tmp] gcc -std=c99 foo.c
foo.c: In function ‘main’:
foo.c:4: error: storage size of ‘asdf’ isn’t known

Answer 2

The timespec comes from POSIX, so you have to 'enable' POSIX definitions:

#if __STDC_VERSION__ >= 199901L
#define _XOPEN_SOURCE 600
#else
#define _XOPEN_SOURCE 500
#endif /* __STDC_VERSION__ */

#include <time.h>

void blah(struct timespec asdf)
{
}

int main()
{
    struct timespec asdf;
    return 0;
}

The stanza at the top is what I currently use - it triggers the definitions from Single UNIX Specification (SUS) based on whether you're using a C99 or C89 compiler.

• If you want the POSIX 2008 (SUS v4) material, use _XOPEN_SOURCE 700
• If you want the POSIX 2004 (SUS v3) material, use _XOPEN_SOURCE 600
• If you want the POSIX 1995 (SUS v2, 1997) material, use _XOPEN_SOURCE 500

For my systems, POSIX 2008 is not as widely available as 2004, so that's what I use - but YMMV. Note that SUS v3 and v4 both require C99 compilation. On Solaris, at least, using C89 will fail.