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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);
 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
         Since glibc 2.19:
         Glibc 2.19 and earlier:


     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
     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.


     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.


     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).


     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.


     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,
     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


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


     This page is part of release 4.16 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

Linux 2017-09-15 GETGROUPS(2)

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