group_member - test whether a process is in a group
int group_member(gid_t gid);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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.
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