setresuid() sets the real user ID, the effective user ID, and the saved set-user-ID of the calling process.
An unprivileged process may change its real UID, effective UID, and saved set-user-ID, each to one of: the current real UID, the current effective UID or the current saved set-user-ID.
A privileged process (on Linux, one having the CAP_SETUID capability) may set its real UID, effective UID, and saved set-user-ID to arbitrary values.
If one of the arguments equals -1, the corresponding value is not changed.
Regardless of what changes are made to the real UID, effective UID, and saved set-user-ID, the filesystem UID is always set to the same value as the (possibly new) effective UID.
Completely analogously, setresgid() sets the real GID, effective GID, and saved set-group-ID of the calling process (and always modifies the filesystem GID to be the same as the effective GID), with the same restrictions for unprivileged processes.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
Note: there are cases where setresuid() can fail even when the caller is UID 0; it is a grave security error to omit checking for a failure return from setresuid().
The call would change the caller's real UID (i.e., ruid does not match the caller's real UID), but there was a temporary failure allocating the necessary kernel data structures.
ruid does not match the caller's real UID and this call would bring the number of processes belonging to the real user ID ruid 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).
One or more of the target user or group IDs is not valid in this user namespace.
The calling process is not privileged (did not have the necessary capability in its user namespace) and tried to change the IDs to values that are not permitted. For setresuid(), the necessary capability is CAP_SETUID; for setresgid(), it is CAP_SETGID.
Under HP-UX and FreeBSD, the prototype is found in <unistd.h>. Under Linux, the prototype is provided by glibc since version 2.3.2.
The original Linux setresuid() and setresgid() system calls supported only 16-bit user and group IDs. Subsequently, Linux 2.4 added setresuid32() and setresgid32(), supporting 32-bit IDs. The glibc setresuid() and setresgid() wrapper functions transparently deal with the variations 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 those for setresuid() and setresgid()) 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).