---
name: "setreuid"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/setreuid"
---

# setreuid(2)

set real and effective user IDs

## NAME

**setreuid** – set real and effective user IDs

## 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.

The **setreuid**() function 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**() function (see [setuid(2)](/setuid/2.md)).

When setting the real and effective user IDs to the same value, the standard **setuid**() function 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 indicate 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)](/getuid/2.md), [issetugid(2)](/issetugid/2.md), [seteuid(2)](/seteuid/2.md), [setuid(2)](/setuid/2.md)

## HISTORY

The **setreuid**() system call appeared in 4.2BSD.