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       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)):

       setgroups(): _BSD_SOURCE


       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  (Linux:  the CAP_SETGID capability) are required.  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


       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 (it does  not  have  the  CAP_SETGID

       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).


       SVr4, 4.3BSD.  The getgroups()  function  is  in  POSIX.1-2001  and  POSIX.1-2008.   Since
       setgroups() requires privilege, it is not covered by POSIX.1.


       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).


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


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