setreuid - Man Page

set real and effective user IDs


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <unistd.h>

int setreuid(uid_t ruid, uid_t euid);


The setreuid() function shall set the real and effective user IDs of the current process to the values specified by the ruid and euid arguments. If ruid or euid is -1, the corresponding effective or real user ID of the current process shall be left unchanged.

A process with appropriate privileges can set either ID to any value. An unprivileged process can only set the effective user ID if the euid argument is equal to either the real, effective, or saved user ID of the process.

If the real user ID is being set (ruid is not -1), or the effective user ID is being set to a value not equal to the real user ID, then the saved set-user-ID of the current process shall be set equal to the new effective user ID.

It is unspecified whether a process without appropriate privileges is permitted to change the real user ID to match the current effective user ID or saved set-user-ID of the process.

Return Value

Upon successful completion, 0 shall be returned. Otherwise, -1 shall be returned and errno set to indicate the error.


The setreuid() function shall fail if:


The value of the ruid or euid argument is invalid or out-of-range.


The current process does not have appropriate privileges, and either an attempt was made to change the effective user ID to a value other than the real user ID or the saved set-user-ID or an attempt was made to change the real user ID to a value not permitted by the implementation.

The following sections are informative.


Setting the Effective User ID to the Real User ID

The following example sets the effective user ID of the calling process to the real user ID, so that files created later will be owned by the current user. It also sets the saved set-user-ID to the real user ID, so any future attempt to set the effective user ID back to its previous value will fail.

#include <unistd.h>
#include <sys/types.h>
setreuid(getuid(), getuid());

Application Usage



Earlier versions of this standard did not specify whether the saved set-user-ID was affected by setreuid() calls. This version specifies common existing practice that constitutes an important security feature. The ability to set both the effective user ID and saved set-user-ID to be the same as the real user ID means that any security weakness in code that is executed after that point cannot result in malicious code being executed with the previous effective user ID. Privileged applications could already do this using just setuid(), but for non-privileged applications the only standard method available is to use this feature of setreuid().

Future Directions


See Also

getegid(), geteuid(), getgid(), getuid(), setegid(), seteuid(), setgid(), setregid(), setuid()

The Base Definitions volume of POSIX.1-2017, <unistd.h>

Referenced By

getegid(3p), geteuid(3p), getgid(3p), getuid(3p), setegid(3p), seteuid(3p), setgid(3p), setregid(3p), setuid(3p), unistd.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual