focal (2) getgroups.2.gz

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 supplementary 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.  A process can drop all  of  its  supplementary
       groups with the call:

           setgroups(0, NULL);

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 addition 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 supplementary 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

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