int initgroups(const char *user, gid_t group);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
The initgroups() function initializes the group access list by reading
the group database /etc/group and using all groups of which user is a
member. The additional group group is also added to the list.
The user argument must be non-NULL.
The initgroups() function returns 0 on success. On error, -1 is
returned, and errno is set appropriately.
ENOMEM Insufficient memory to allocate group information structure.
EPERM The calling process has insufficient privilege. See the under-
lying system call setgroups(2).
/etc/group group database file
getgroups(2), setgroups(2), credentials(7)
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