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RedHat 9 (Linux i386) - man page for getgroups (redhat section 2)

GETGROUPS(2)				      Linux Programmer's Manual 				 GETGROUPS(2)

getgroups, setgroups - get/set list of supplementary group IDs
#include <sys/types.h> #include <unistd.h> int getgroups(int size, gid_t list[]); #include <grp.h> int setgroups(size_t size, const gid_t *list);
getgroups Up to size supplementary group IDs are returned in list. It is unspecified whether the effective group ID of the calling process is included in the returned list. (Thus, an application should also call getegid(2) and add or remove the resulting value.) If size is zero, list is not modified, but the total number of supplementary group IDs for the process is returned. setgroups Sets the supplementary group IDs for the process. Only the super-user may use this function.
getgroups On success, the number of supplementary group IDs is returned. On error, -1 is returned, and errno is set appropriately. setgroups On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EFAULT list has an invalid address. EPERM For setgroups, the user is not the super-user. EINVAL For setgroups, size is greater than NGROUPS (32 for Linux 2.0.32). For getgroups, size is less than the number of supplementary group IDs, but is not zero.
A process can have up to at least NGROUPS_MAX supplementary group IDs in addition to the effective group ID. The set of supplementary group IDs is inherited from the parent process and may be changed using setgroups. The maximum number of supplementary group IDs can be found using sysconf(3): long ngroups_max; ngroups_max = sysconf(_SC_NGROUPS_MAX); The maximal return value of getgroups cannot be larger than one more than the value obtained this way. The prototype for setgroups is only available if _BSD_SOURCE is defined (either explicitly, or implicitly, by not defining _POSIX_SOURCE or compiling with the -ansi flag).
SVr4, SVID (issue 4 only; these calls were not present in SVr3), X/OPEN, 4.3BSD. The getgroups function is in POSIX.1. Since setgroups requires privilege, it is not covered by POSIX.1.
initgroups(3), getgid(2), setgid(2) Linux 2.0.32 1997-12-10 GETGROUPS(2)

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