GROUP_MEMBER(3) Linux Programmer's Manual GROUP_MEMBER(3)

group_member - test whether a process is in a group 

#include <unistd.h>
int group_member(gid_t gid);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

group_member():

_GNU_SOURCE

The group_member() function tests whether any of the caller's supplementary group IDs (as returned by getgroups(2)) matches gid.

The group_member() function returns nonzero if any of the caller's supplementary group IDs matches gid, and zero otherwise.

This function is a nonstandard GNU extension.

getgid(2), getgroups(2), getgrouplist(3), group(5)

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2021-03-22 GNU