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FreeBSD 11.0 - man page for setreuid (freebsd section 2)

SETREUID(2)						      BSD System Calls Manual						       SETREUID(2)

NAME
setreuid -- set real and effective user ID's
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h> int setreuid(uid_t ruid, uid_t euid);
DESCRIPTION
The real and effective user IDs of the current process are set according to the arguments. If ruid or euid is -1, the current uid is filled in by the system. Unprivileged users may change the real user ID to the effective user ID and vice-versa; only the super-user may make other changes. If the real user ID is changed (i.e. ruid is not -1) or the effective user ID is changed to something other than the real user ID, then the saved user ID will be set to the effective user ID. The setreuid() system call has been used to swap the real and effective user IDs in set-user-ID programs to temporarily relinquish the set- user-ID value. This purpose is now better served by the use of the seteuid(2) system call. When setting the real and effective user IDs to the same value, the standard setuid() system call is preferred.
RETURN VALUES
The setreuid() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indi- cate the error.
ERRORS
[EPERM] The current process is not the super-user and a change other than changing the effective user-id to the real user-id was specified.
SEE ALSO
getuid(2), issetugid(2), seteuid(2), setuid(2)
HISTORY
The setreuid() system call appeared in 4.2BSD.
BSD
February 8, 2001 BSD