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

Answer: 
```
#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

```
The [standard](http://pubs.opengroup.org/onlinepubs/007904975/toc.htm) defines `pid_t` as a "signed integer type" and `uid_t` and `gid_t` merely as "integer types" (so portable code shouldn't assume any particular type for them).