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       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 caller is privileged
       (has the CAP_SETGID capability), 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 namespace.

       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


       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 nptl(7).


       getgid(2), setegid(2), setregid(2), capabilities(7), credentials(7), user_namespaces(7)


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