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SETRESUID(2) System Calls Manual SETRESUID(2)

getresgid, getresuid, setresgid, setresuid
get or set real, effective and saved user or group ID

#include <sys/types.h>
#include <unistd.h>
getresgid(gid_t *rgid, gid_t *egid, gid_t *sgid);
getresuid(uid_t *ruid, uid_t *euid, uid_t *suid);
setresgid(gid_t rgid, gid_t egid, gid_t sgid);
setresuid(uid_t ruid, uid_t euid, uid_t suid);

The setresuid() function sets the real, effective and saved user IDs of the current process. The analogous setresgid() sets the real, effective and saved group IDs.
Privileged processes may set these IDs to arbitrary values. Unprivileged processes are restricted in that each of the new IDs must match one of the current IDs.
Passing -1 as an argument causes the corresponding value to remain unchanged.
The getresgid() and getresuid() calls retrieve the real, effective, and saved group and user IDs of the current process, respectively.

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The calling process was not privileged and tried to change one or more IDs to a value which was not the current real ID, the current effective ID nor the current saved ID.
An address passed to getresgid() or getresuid() was invalid.

getegid(2), geteuid(2), getgid(2), getuid(2), issetugid(2), setgid(2), setregid(2), setreuid(2), setuid(2)

These functions are not part of the IEEE Std 1003.1 (“POSIX.1”) specification. While they are not completely portable, they are the least ambiguous way to manage user and group IDs.

These functions first appeared in HP-UX.
September 10, 2015 OpenBSD-current