1GETGROUPS(2) Linux Programmer's Manual GETGROUPS(2)
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6 getgroups, setgroups - get/set list of supplementary group IDs
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9 #include <unistd.h>
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11 int getgroups(int size, gid_t list[]);
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13 #include <grp.h>
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15 int setgroups(size_t size, const gid_t *list);
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17 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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19 setgroups():
20 Since glibc 2.19:
21 _DEFAULT_SOURCE
22 Glibc 2.19 and earlier:
23 _BSD_SOURCE
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26 getgroups() returns the supplementary group IDs of the calling process
27 in list. The argument size should be set to the maximum number of
28 items that can be stored in the buffer pointed to by list. If the
29 calling process is a member of more than size supplementary groups,
30 then an error results.
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32 It is unspecified whether the effective group ID of the calling process
33 is included in the returned list. (Thus, an application should also
34 call getegid(2) and add or remove the resulting value.)
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36 If size is zero, list is not modified, but the total number of supple‐
37 mentary group IDs for the process is returned. This allows the caller
38 to determine the size of a dynamically allocated list to be used in a
39 further call to getgroups().
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41 setgroups() sets the supplementary group IDs for the calling process.
42 Appropriate privileges are required (see the description of the EPERM
43 error, below). The size argument specifies the number of supplementary
44 group IDs in the buffer pointed to by list. A process can drop all of
45 its supplementary groups with the call:
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47 setgroups(0, NULL);
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50 On success, getgroups() returns the number of supplementary group IDs.
51 On error, -1 is returned, and errno is set to indicate the error.
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53 On success, setgroups() returns 0. On error, -1 is returned, and errno
54 is set to indicate the error.
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57 EFAULT list has an invalid address.
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59 getgroups() can additionally fail with the following error:
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61 EINVAL size is less than the number of supplementary group IDs, but is
62 not zero.
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64 setgroups() can additionally fail with the following errors:
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66 EINVAL size is greater than NGROUPS_MAX (32 before Linux 2.6.4; 65536
67 since Linux 2.6.4).
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69 ENOMEM Out of memory.
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71 EPERM The calling process has insufficient privilege (the caller does
72 not have the CAP_SETGID capability in the user namespace in
73 which it resides).
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75 EPERM (since Linux 3.19)
76 The use of setgroups() is denied in this user namespace. See
77 the description of /proc/[pid]/setgroups in user_namespaces(7).
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80 getgroups(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
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82 setgroups(): SVr4, 4.3BSD. Since setgroups() requires privilege, it is
83 not covered by POSIX.1.
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86 A process can have up to NGROUPS_MAX supplementary group IDs in addi‐
87 tion to the effective group ID. The constant NGROUPS_MAX is defined in
88 <limits.h>. The set of supplementary group IDs is inherited from the
89 parent process, and preserved across an execve(2).
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91 The maximum number of supplementary group IDs can be found at run time
92 using sysconf(3):
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94 long ngroups_max;
95 ngroups_max = sysconf(_SC_NGROUPS_MAX);
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97 The maximum return value of getgroups() cannot be larger than one more
98 than this value. Since Linux 2.6.4, the maximum number of supplemen‐
99 tary group IDs is also exposed via the Linux-specific read-only file,
100 /proc/sys/kernel/ngroups_max.
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102 The original Linux getgroups() system call supported only 16-bit group
103 IDs. Subsequently, Linux 2.4 added getgroups32(), supporting 32-bit
104 IDs. The glibc getgroups() wrapper function transparently deals with
105 the variation across kernel versions.
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107 C library/kernel differences
108 At the kernel level, user IDs and group IDs are a per-thread attribute.
109 However, POSIX requires that all threads in a process share the same
110 credentials. The NPTL threading implementation handles the POSIX re‐
111 quirements by providing wrapper functions for the various system calls
112 that change process UIDs and GIDs. These wrapper functions (including
113 the one for setgroups()) employ a signal-based technique to ensure that
114 when one thread changes credentials, all of the other threads in the
115 process also change their credentials. For details, see nptl(7).
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118 getgid(2), setgid(2), getgrouplist(3), group_member(3), initgroups(3),
119 capabilities(7), credentials(7)
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122 This page is part of release 5.13 of the Linux man-pages project. A
123 description of the project, information about reporting bugs, and the
124 latest version of this page, can be found at
125 https://www.kernel.org/doc/man-pages/.
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129Linux 2021-03-22 GETGROUPS(2)