1SETGID(2) Linux Programmer's Manual SETGID(2)
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6 setgid - set group identity
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9 #include <sys/types.h>
10 #include <unistd.h>
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12 int setgid(gid_t gid);
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15 setgid() sets the effective group ID of the calling process. If the
16 calling process is privileged (more precisely: has the CAP_SETGID capa‐
17 bility in its user namespace), the real GID and saved set-group-ID are
18 also set.
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20 Under Linux, setgid() is implemented like the POSIX version with the
21 _POSIX_SAVED_IDS feature. This allows a set-group-ID program that is
22 not set-user-ID-root to drop all of its group privileges, do some un-
23 privileged work, and then reengage the original effective group ID in a
24 secure manner.
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27 On success, zero is returned. On error, -1 is returned, and errno is
28 set appropriately.
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31 EINVAL The group ID specified in gid is not valid in this user names‐
32 pace.
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34 EPERM The calling process is not privileged (does not have the
35 CAP_SETGID capability in its user namespace), and gid does not
36 match the real group ID or saved set-group-ID of the calling
37 process.
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40 POSIX.1-2001, POSIX.1-2008, SVr4.
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43 The original Linux setgid() system call supported only 16-bit group
44 IDs. Subsequently, Linux 2.4 added setgid32() supporting 32-bit IDs.
45 The glibc setgid() wrapper function transparently deals with the varia‐
46 tion across kernel versions.
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48 C library/kernel differences
49 At the kernel level, user IDs and group IDs are a per-thread attribute.
50 However, POSIX requires that all threads in a process share the same
51 credentials. The NPTL threading implementation handles the POSIX
52 requirements by providing wrapper functions for the various system
53 calls that change process UIDs and GIDs. These wrapper functions
54 (including the one for setgid()) employ a signal-based technique to
55 ensure that when one thread changes credentials, all of the other
56 threads in the process also change their credentials. For details, see
57 nptl(7).
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60 getgid(2), setegid(2), setregid(2), capabilities(7), credentials(7),
61 user_namespaces(7)
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64 This page is part of release 5.07 of the Linux man-pages project. A
65 description of the project, information about reporting bugs, and the
66 latest version of this page, can be found at
67 https://www.kernel.org/doc/man-pages/.
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71Linux 2019-03-06 SETGID(2)