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

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 feature. This allows a setuid (other than root) pro- gram 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 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 program to regain root privileges. Thus, a setuid-root program wishing to temporarily drop root privileges, assume the identity of a non-root user, and then regain root priv- ileges 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-SPECIFIC REMARKS 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)