setuid() sets the effective user ID of the calling process. If the calling process is privileged (more precisely: if the process has the CAP_SETUID capability in its user namespace), 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 reengage 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: setuid() 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 an unprivileged user, and then regain root privileges afterward cannot use setuid(). You can accomplish this with seteuid(2).
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
Note: there are cases where setuid() can fail even when the caller is UID 0; it is a grave security error to omit checking for a failure return from setuid().
The call would change the caller's real UID (i.e., uid does not match the caller's real UID), but there was a temporary failure allocating the necessary kernel data structures.
uid does not match the real user ID of the caller and this call would bring the number of processes belonging to the real user ID uid over the caller's RLIMIT_NPROC resource limit. Since Linux 3.1, this error case no longer occurs (but robust applications should check for this error); see the description of EAGAIN in execve(2).
The user ID specified in uid is not valid in this user namespace.
The user is not privileged (Linux: does not have the CAP_SETUID capability in its user namespace) and uid does not match the real UID or saved set-user-ID of the calling process.
Linux has the concept of the filesystem user ID, normally equal to the effective user ID. The setuid() call also sets the filesystem 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.
The original Linux setuid() system call supported only 16-bit user IDs. Subsequently, Linux 2.4 added setuid32() supporting 32-bit IDs. The glibc setuid() wrapper function transparently deals with the variation across kernel versions.
At the kernel level, user IDs and group IDs are a per-thread attribute. However, POSIX requires that all threads in a process share the same credentials. The NPTL threading implementation handles the POSIX requirements by providing wrapper functions for the various system calls that change process UIDs and GIDs. These wrapper functions (including the one for setuid()) employ a signal-based technique to ensure that when one thread changes credentials, all of the other threads in the process also change their credentials. For details, see nptl(7).