Size of pid_t, uid_t, gid_t on Linux

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

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

EDIT: Per popular request (and because, realistically, 99% of the people coming to this question are going to be running x86 or x86_64)…

On an i686 and x86_64 (so, 32-bit and 64-bit) processor running Linux >= 3.0.0, the answer is:

pid_t: 4
uid_t: 4
gid_t: 4

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