Size of pid_t, uid_t, gid_t on Linux

Question

On Linux systems (either 32- or 64-bit), what is the size of pid_t, uid_t, and gid_t?

Answer 1

On intel architectures, sizes are defined in /usr/include/bits/typesizes.h:

#define __UID_T_TYPE            __U32_TYPE
#define __GID_T_TYPE            __U32_TYPE
#define __PID_T_TYPE            __S32_TYPE

In other words, uid_t and gid_t are unsigned 32-bit integers and pid_t is a signed 32-bit integer. This applies for both 32- and 64-bits.

I am not sure what they are on other architectures offhand as I don't have any available at the moment, but the definitive way is to compile a program which prints the output of sizeof(uid_t), etc.

Answer 2

#include <stdio.h>
#include <sys/types.h>

int main()
{
    printf("pid_t: %d\n", sizeof(pid_t));
    printf("uid_t: %d\n", sizeof(uid_t));
    printf("gid_t: %d\n", sizeof(gid_t));
}

Answer 3

The standard defines those types as "unsigned integer types", so portable code shouldn't assume any particular type.