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

NAME

     seteuid, setegid - set effective user or group ID

SYNOPSIS

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

     int seteuid(uid_t euid);
     int setegid(gid_t egid);

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     seteuid(), setegid():
         _POSIX_C_SOURCE >= 200112L
             || /* Glibc versions <= 2.19: */ _BSD_SOURCE

DESCRIPTION

     seteuid()  sets the effective user ID of the calling process.  Unprivi-
     leged processes may only set the effective user ID to the real user ID,
     the effective user ID or the saved set-user-ID.

     Precisely  the same holds for setegid() with "group" instead of "user".

RETURN VALUE

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

     Note:  there are cases where seteuid() can fail even when the caller is
     UID 0; it is a grave security error to  omit  checking  for  a  failure
     return from seteuid().

ERRORS

     EINVAL The target user or group ID is not valid in this user namespace.

     EPERM  In the case of seteuid(): the calling process is not  privileged
            (does  not have the CAP_SETUID capability in its user namespace)
            and euid does not match the current real user ID, current effec-
            tive user ID, or current saved set-user-ID.

            In  the case of setegid(): the calling process is not privileged
            (does not have the CAP_SETGID capability in its user  namespace)
            and  egid  does  not  match  the  current real group ID, current
            effective group ID, or current saved set-group-ID.

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008, 4.3BSD.

NOTES

     Setting the effective user (group) ID to the saved  set-user-ID  (saved
     set-group-ID) is possible since Linux 1.1.37 (1.1.38).  On an arbitrary
     system one should check _POSIX_SAVED_IDS.

     Under glibc 2.0, seteuid(euid) is equivalent to setreuid(-1, euid)  and
     hence  may change the saved set-user-ID.  Under glibc 2.1 and later, it
     is equivalent to setresuid(-1, euid, -1) and hence does not change  the
     saved set-user-ID.  Analogous remarks hold for setegid(), with the dif-
     ference that the change in implementation from  setregid(-1,  egid)  to
     setresgid(-1,  egid, -1) occurred in glibc 2.2 or 2.3 (depending on the
     hardware architecture).

     According to POSIX.1, seteuid() (setegid()) need not permit euid (egid)
     to be the same value as the current effective user (group) ID, and some
     implementations do not permit this.

 C library/kernel differences
     On Linux, seteuid() and setegid() are implemented as library  functions
     that call, respectively, setreuid(2) and setregid(2).