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

Answer: 
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.
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).