getresuid() returns the real UID, the effective UID, and the saved set-user-ID of the calling process, in the arguments ruid, euid, and suid, respectively. getresgid() performs the analogous task for the process's group IDs.
These system calls appeared on Linux starting with kernel 2.1.44.
The prototypes are given by glibc since version 2.3.2, provided _GNU_SOURCE is defined.
The original Linux getresuid() and getresgid() system calls supported only 16-bit user and group IDs. Subsequently, Linux 2.4 added getresuid32() and getresgid32(), supporting 32-bit IDs. The glibc getresuid() and getresgid() wrapper functions transparently deal with the variations across kernel versions.