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RedHat 9 (Linux i386) - man page for setresuid (redhat section 2)

SETRESUID(2)						     Linux Programmer's Manual						      SETRESUID(2)

NAME
setresuid, setresgid - set real, effective and saved user or group ID
SYNOPSIS
#include <unistd.h> int setresuid(uid_t ruid, uid_t euid, uid_t suid); int setresgid(gid_t rgid, gid_t egid, gid_t sgid);
DESCRIPTION
setresuid sets the real user ID, the effective user ID, and the saved set-user-ID of the current process. Unprivileged user processes (i.e., processes with each of real, effective and saved user ID nonzero) may change the real, effective and saved user ID, each to one of: the current uid, the current effective uid or the current saved uid. The super-user may set real, effective and saved user ID to arbitrary values. If one of the parameters equals -1, the corresponding value is not changed. Completely analogously, setresgid sets the real, effective and saved group ID's of the current process, with the same restrictions for pro- cesses with each of real, effective and saved user ID nonzero.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EPERM The current process was not privileged and tried to change the IDs is a not allowed way.
CONFORMING TO
This call is nonstandard.
HISTORY
This system call was first introduced in HP-UX. It is available under Linux since Linux 2.1.44. These days it is also found in FreeBSD (for emulation of Linux binaries).
NOTES
Under HP-UX and FreeBSD the prototype is found in <unistd.h>. Under Linux there is so far no include file giving the prototype - this is a glibc bug. Programs using this system call must add the prototype themselves.
SEE ALSO
getuid(2), setuid(2), setreuid(2), getresuid(2) Linux 2.1.44 2001-11-15 SETRESUID(2)