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

NAME
       getgroups, setgroups - get/set list of supplementary group IDs

SYNOPSIS
       #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);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       setgroups():
	   Since glibc 2.19:
	       _DEFAULT_SOURCE
	   Glibc 2.19 and earlier:
	       _BSD_SOURCE

DESCRIPTION
       getgroups()  returns the supplementary group IDs of the calling process
       in list.	 The argument size should be set  to  the  maximum  number  of
       items  that  can	 be  stored  in the buffer pointed to by list.	If the
       calling process is a member of more  than  size	supplementary  groups,
       then  an	 error results.	 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  supple‐
       mentary	group IDs for the process is returned.	This allows the caller
       to determine the size of a dynamically allocated list to be used	 in  a
       further call to getgroups().

       setgroups()  sets  the supplementary group IDs for the calling process.
       Appropriate privileges are required (see the description of  the	 EPERM
       error, below).  The size argument specifies the number of supplementary
       group IDs in the buffer pointed to by list.

RETURN VALUE
       On success, getgroups() returns the number of supplementary group  IDs.
       On error, -1 is returned, and errno is set appropriately.

       On success, setgroups() returns 0.  On error, -1 is returned, and errno
       is set appropriately.

ERRORS
       EFAULT list has an invalid address.

       getgroups() can additionally fail with the following error:

       EINVAL size is less than the number of supplementary group IDs, but  is
	      not zero.

       setgroups() can additionally fail with the following errors:

       EINVAL size  is	greater than NGROUPS_MAX (32 before Linux 2.6.4; 65536
	      since Linux 2.6.4).

       ENOMEM Out of memory.

       EPERM  The calling process has insufficient privilege (the caller  does
	      not  have	 the  CAP_SETGID  capability  in the user namespace in
	      which it resides).

       EPERM (since Linux 3.19)
	      The use of setgroups() is denied in this	user  namespace.   See
	      the description of /proc/[pid]/setgroups in user_namespaces(7).

CONFORMING TO
       getgroups(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

       setgroups(): SVr4, 4.3BSD.  Since setgroups() requires privilege, it is
       not covered by POSIX.1.

NOTES
       A process can have up to NGROUPS_MAX supplementary group IDs  in	 addi‐
       tion to the effective group ID.	The constant NGROUPS_MAX is defined in
       <limits.h>.  The set of supplementary group IDs is inherited  from  the
       parent process, and preserved across an execve(2).

       The  maximum number of supplementary group IDs can be found at run time
       using sysconf(3):

	   long ngroups_max;
	   ngroups_max = sysconf(_SC_NGROUPS_MAX);

       The maximum return value of getgroups() cannot be larger than one  more
       than  this  value.  Since Linux 2.6.4, the maximum number of supplemen‐
       tary group IDs is also exposed via the Linux-specific  read-only	 file,
       /proc/sys/kernel/ngroups_max.

       The  original Linux getgroups() system call supported only 16-bit group
       IDs.  Subsequently, Linux 2.4 added  getgroups32(),  supporting	32-bit
       IDs.   The  glibc getgroups() wrapper function transparently deals with
       the variation across kernel versions.

   C library/kernel differences
       At the kernel level, user IDs and group IDs are a per-thread attribute.
       However,	 POSIX	requires  that all threads in a process share the same
       credentials.  The  NPTL	threading  implementation  handles  the	 POSIX
       requirements  by	 providing  wrapper  functions	for the various system
       calls that change process  UIDs	and  GIDs.   These  wrapper  functions
       (including  the one for setgroups()) employ a signal-based technique to
       ensure that when one thread  changes  credentials,  all	of  the	 other
       threads in the process also change their credentials.  For details, see
       nptl(7).

SEE ALSO
       getgid(2), setgid(2), getgrouplist(3), group_member(3),	initgroups(3),
       capabilities(7), credentials(7)

COLOPHON
       This  page  is  part of release 4.14 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.

Linux				  2017-09-15			  GETGROUPS(2)
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