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

       seteuid, setegid - set effective user or group ID

       #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

       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".

       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().

       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.

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

       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).

       geteuid(2), setresuid(2), setreuid(2), setuid(2), capabilities(7), cre-
       dentials(7), user_namespaces(7)

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Linux				  2016-10-08			    SETEUID(2)