Get real, effective and saved user/group ids
#define _GNU_SOURCE /* See feature_test_macros(7) */
int getresuid(uid_t *ruid, uid_t *euid, uid_t *suid);
int getresgid(gid_t *rgid, gid_t *egid, gid_t *sgid);
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.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
One of the arguments specified an address outside the calling program's address space.
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.
These calls are nonstandard; they also appear on HP-UX and some of the BSDs.
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.
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