Real, effective and saved user id in C program

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Real, effective and saved user id in C program

I figured it out by now.

Last edited by Ralph; 04-02-2019 at 02:26 AM..
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SETREUID(2)						     Linux Programmer's Manual						       SETREUID(2)

setreuid, setregid - set real and/or effective user or group ID SYNOPSIS
#include <sys/types.h> #include <unistd.h> int setreuid(uid_t ruid, uid_t euid); int setregid(gid_t rgid, gid_t egid); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): setreuid(), setregid(): _BSD_SOURCE || _XOPEN_SOURCE >= 500 DESCRIPTION
setreuid() sets real and effective user IDs of the calling process. Supplying a value of -1 for either the real or effective user ID forces the system to leave that ID unchanged. Unprivileged processes may only set the effective user ID to the real user ID, the effective user ID, or the saved set-user-ID. Unprivileged users may only set the real user ID to the real user ID or the effective user ID. If the real user ID is set or the effective user ID is set to a value not equal to the previous real user ID, the saved set-user-ID will be set to the new effective user ID. Completely analogously, setregid() sets real and effective group ID's of the calling process, and all of the above holds with "group" instead of "user". RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS
EPERM The calling process is not privileged (Linux: does not have the CAP_SETUID capability in the case of setreuid(), or the CAP_SETGID capability in the case of setregid()) and a change other than (i) swapping the effective user (group) ID with the real user (group) ID, or (ii) setting one to the value of the other or (iii) setting the effective user (group) ID to the value of the saved set-user- ID (saved set-group-ID) was specified. CONFORMING TO
POSIX.1-2001, 4.3BSD (the setreuid() and setregid() function calls first appeared in 4.2BSD). 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). POSIX.1 does not specify all of possible ID changes that are permitted on Linux for an unprivileged process. For setreuid(), the effective user ID can be made the same as the real user ID or the save set-user-ID, and it is unspecified whether unprivileged processes may set the real user ID to the real user ID, the effective user ID, or the saved set-user-ID. For setregid(), the real group ID can be changed to the value of the saved set-group-ID, and the effective group ID can be changed to the value of the real group ID or the saved set-group-ID. The precise details of what ID changes are permitted vary across implementations. POSIX.1 makes no specification about the effect of these calls on the saved set-user-ID and saved set-group-ID. SEE ALSO
getgid(2), getuid(2), seteuid(2), setgid(2), setresuid(2), setuid(2), capabilities(7) COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at Linux 2009-10-17 SETREUID(2)

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