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

```
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.