Why is there no WORD_BIT in limits.h on Linux?

Question

From the manpage man limits.h:

       {WORD_BIT}
             Number of bits in an object of type int.
             Minimum Acceptable Value: 32

However if I run a simple program:

#include <limits.h>
#include <stdio.h>

int main() {
  printf("%d\n", WORD_BIT);
  return 0;
}

However when trying to compile with gcc (gcc file.c -o file) I get the following:

error: ‘WORD_BIT’ undeclared (first use in this function)

How come this is not defined on my system, and where else can I find this information (in a C program)?

My system:

• Fedora 36 (Silverblue)
• gcc version 12.2.1 20220819 (Red Hat 12.2.1-1) (GCC)
• ldd (GNU libc) 2.35

Answer 1

I’m not sure this is documented, but with the GNU C library you need to set _GNU_SOURCE to get WORD_BIT:

$ gcc -include limits.h -E - <<<"WORD_BIT" | tail -n1
WORD_BIT

$ gcc -D_GNU_SOURCE -include limits.h -E - <<<"WORD_BIT" | tail -n1
32

You should really use sysconf:

#include <unistd.h>
#include <stdio.h>

int main(int argc, char **argv) {
  printf("%ld\n", sysconf(_SC_WORD_BIT));
}

or, as recommended in the GNU C library documentation:

#include <limits.h>
#include <unistd.h>
#include <stdio.h>

int main(int argc, char **argv) {
#ifdef WORD_BIT
  printf("%d\n", WORD_BIT);
#else
  printf("%ld\n", sysconf(_SC_WORD_BIT));
#endif
}

(As for why this appears in man limits.h, this man page is the POSIX reference for limits.h, and doesn’t necessarily document the C library on your system exactly. You can see this by looking at the section of the man page — the “P” suffix indicates that it’s POSIX documentation.)

You could use sizeof(int) * CHAR_BIT instead; CHAR_BIT is always defined. What’s more, POSIX extends the C standard and specifies that CHAR_BIT is 8, so if you assume POSIX, you could use sizeof(int) * 8.

Answer 2

The gcc has a set of builtin defines. The one you looking for is __SIZEOF_INT__

Full list of defines is in the documentation:
https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html

The pro-con of using these instead of those defined in limits.h is that the defines built into compiler are built into compiler - they are absolutely correct for this particular platform but are tied to this particular brand of compilers.

But for size of the int it is much easier to use sizeof(int).