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

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

       setuid - set user identity

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

       int setuid(uid_t uid);

       setuid  sets the effective user ID of the current process.  If the effective userid of the
       caller is root, the real and saved user ID's are also set.

       Under Linux, setuid is implemented like the POSIX version with the  _POSIX_SAVED_IDS  fea-
       ture.   This allows a setuid (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

       If  the user is root or the program is setuid root, special care must be taken. The setuid
       function checks the effective uid 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 pro-
       gram to regain root privileges.

       Thus, a setuid-root program wishing to temporarily drop root privileges, assume the  iden-
       tity  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.

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

       EPERM  The user is not the super-user, and uid does not match the real or saved user ID of
	      the calling process.

       SVr4,  SVID,  POSIX.1.	Not  quite compatible with the 4.4BSD call, which sets all of the
       real, saved, and effective user IDs.  SVr4 documents an additional EINVAL error condition.

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

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

       getuid(2), setreuid(2), seteuid(2), setfsuid(2)

Linux 1.1.36				    1994-07-29					SETUID(2)
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