How to Declare a 32-bit Integer in C

#include <stdint.h>

int32_t my_32bit_int;

C doesn't concern itself very much with exact sizes of integer types, C99 introduces the header stdint.h , which is probably your best bet. Include that and you can use e.g. int32_t. Of course not all platforms might support that.

Corey's answer is correct for "best", in my opinion, but a simple "int" will also work in practice (given that you're ignoring systems with 16-bit int). At this point, so much code depends on int being 32-bit that system vendors aren't going to change it.

(See also why long is 32-bit on lots of 64-bit systems and why we have "long long".)

One of the benefits of using int32_t, though, is that you're not perpetuating this problem!

You could hunt down a copy of Brian Gladman's brg_types.h if you don't have stdint.h.

brg_types.h will discover the sizes of the various integers on your platform and will create typedefs for the common sizes: 8, 16, 32 and 64 bits.

C99 or later

Use <stdint.h>.

If your implementation supports 2's complement 32-bit integers then it must define int32_t.

If not then the next best thing is int_least32_t which is an integer type supported by the implementation that is at least 32 bits, regardless of representation (two's complement, one's complement, etc.).

There is also int_fast32_t which is an integer type at least 32-bits wide, chosen with the intention of allowing the fastest operations for that size requirement.

ANSI C

You can use long, which is guaranteed to be at least 32-bits wide as a result of the minimum range requirements specified by the standard.

If you would rather use the smallest integer type to fit a 32-bit number, then you can use preprocessor statements like the following with the macros defined in <limits.h>:

#define TARGET_MAX 2147483647L

#if   SCHAR_MAX >= TARGET_MAX
  typedef signed char int32;
#elif SHORT_MAX >= TARGET_MAX
  typedef short int32;
#elif INT_MAX   >= TARGET_MAX
  typedef int int32;
#else
  typedef long int32;
#endif

#undef TARGET_MAX

If stdint.h is not available for your system, make your own. I always have a file called "types.h" that have typedefs for all the signed/unsigned 8, 16, and 32 bit values.

also depending on your target platforms you can use autotools for your build system

it will see if stdint.h/inttypes.h exist and if they don't will create appropriate typedefs in a "config.h"

stdint.h is the obvious choice, but it's not necessarily available.

If you're using a portable library, it's possible that it already provides portable fixed-width integers. For example, SDL has Sint32 (S is for “signed”), and GLib has gint32.

You can declare 32 bits with signed or unsigned long.

int32_t variable_name;
uint32_t variable_name;