setgid - set group identity


   #include <sys/types.h>
   #include <unistd.h>

   int setgid(gid_t gid);


   setgid()  sets  the  effective group ID of the calling process.  If the
   calling process is privileged (has the  CAP_SETGID  capability  in  its
   user namespace), the real GID and saved set-group-ID are also set.

   Under  Linux,  setgid()  is implemented like the POSIX version with the
   _POSIX_SAVED_IDS feature.  This allows a set-group-ID program  that  is
   not  set-user-ID-root  to drop all of its group privileges, do some un-
   privileged work, and then reengage the original effective group ID in a
   secure manner.


   On  success,  zero is returned.  On error, -1 is returned, and errno is
   set appropriately.


   EINVAL The group ID  specified  in  gid  is  not  valid  in  this  user

   EPERM  The  calling  process  is  not  privileged  (does  not  have the
          CAP_SETGID capability), and gid does not match the real group ID
          or saved set-group-ID of the calling process.


   POSIX.1-2001, POSIX.1-2008, SVr4.


   The  original  Linux  setgid()  system call supported only 16-bit group
   IDs.  Subsequently, Linux 2.4 added setgid32() supporting  32-bit  IDs.
   The  glibc  setgid()  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  setgid()) 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), setegid(2),  setregid(2),  capabilities(7),  credentials(7),


   This  page  is  part of release 4.09 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


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