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setuid.2

SETUID

Section: Linux Programmer's Manual (2)
Updated: 2004-05-27
Index Return to Main Contents
 

NAME

setuid - set user identity  

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

int setuid(uid_t uid);  

DESCRIPTION

setuid() sets the effective user ID of the calling process. If the effective UID of the caller is root, the real UID and saved set-user-ID are also set.

Under Linux, setuid() is implemented like the POSIX version with the _POSIX_SAVED_IDS feature. This allows a set-user-ID (other than root) program to drop all of its user privileges, do some un-privileged work, and then re-engage the original effective user ID in a secure manner.

If the user is root or the program is set-user-ID-root, special care must be taken. The setuid() function checks the effective user ID of the caller and if it is the superuser, all process-related user ID's are set to uid. After this has occurred, it is impossible for the program to regain root privileges.

Thus, a set-user-ID-root program wishing to temporarily drop root privileges, assume the identity of a non-root user, and then regain root privileges afterwards cannot use setuid(). You can accomplish this with the (non-POSIX, BSD) call seteuid(2).  

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.  

ERRORS

EAGAIN
The uid does not match the current uid and uid brings process over its RLIMIT_NPROC resource limit.
EPERM
The user is not privileged (Linux: does not have the CAP_SETUID capability) and uid does not match the real UID or saved set-user-ID of the calling process.
 

CONFORMING TO

SVr4, POSIX.1-2001. Not quite compatible with the 4.4BSD call, which sets all of the real, saved, and effective user IDs.  

NOTES

 

Linux Notes

Linux has the concept of file system user ID, normally equal to the effective user ID. The setuid() call also sets the file system user ID of the calling process. See setfsuid(2).

If uid is different from the old effective uid, the process will be forbidden from leaving core dumps.  

SEE ALSO

getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7), credentials(7)  

COLOPHON

This page is part of release 3.21 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
Linux Notes
SEE ALSO
COLOPHON

 

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