PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

NAME

       shmctl — XSI shared memory control operations

SYNOPSIS

       #include <sys/shm.h>

       int shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION

       The  shmctl()  function  operates on XSI shared memory (see the Base Definitions volume of
       POSIX.1‐2017, Section 3.346, Shared  Memory  Object).   It  is  unspecified  whether  this
       function  interoperates with the realtime interprocess communication facilities defined in
       Section 2.8, Realtime.

       The shmctl() function provides a variety of shared memory control operations as  specified
       by cmd.  The following values for cmd are available:

       IPC_STAT    Place  the  current  value  of  each  member  of  the  shmid_ds data structure
                   associated with shmid into the structure pointed to by buf.  The  contents  of
                   the structure are defined in <sys/shm.h>.

       IPC_SET     Set  the  value  of  the  following  members  of  the  shmid_ds data structure
                   associated with shmid to  the  corresponding  value  found  in  the  structure
                   pointed to by buf:

                       shm_perm.uid
                       shm_perm.gid
                       shm_perm.mode    Low-order nine bits.

                   Also,  the  shm_ctime timestamp shall be set to the current time, as described
                   in Section 2.7.1, IPC General Description.

                   IPC_SET can only be executed by a process that has an effective user ID  equal
                   to  either  that  of  a process with appropriate privileges or to the value of
                   shm_perm.cuid or shm_perm.uid in the shmid_ds data structure  associated  with
                   shmid.

       IPC_RMID    Remove  the  shared  memory  identifier specified by shmid from the system and
                   destroy the shared memory segment and shmid_ds data structure associated  with
                   it.  IPC_RMID  can only be executed by a process that has an effective user ID
                   equal to either that of a process with appropriate privileges or to the  value
                   of  shm_perm.cuid  or  shm_perm.uid  in the shmid_ds data structure associated
                   with shmid.

RETURN VALUE

       Upon successful completion, shmctl() shall return 0; otherwise, it shall return -1 and set
       errno to indicate the error.

ERRORS

       The shmctl() function shall fail if:

       EACCES The  argument  cmd  is equal to IPC_STAT and the calling process does not have read
              permission; see Section 2.7, XSI Interprocess Communication.

       EINVAL The value of shmid is not a valid shared memory identifier, or the value of cmd  is
              not a valid command.

       EPERM  The  argument  cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the
              calling process is not equal to that of a process with appropriate  privileges  and
              it is not equal to the value of shm_perm.cuid or shm_perm.uid in the data structure
              associated with shmid.

       The shmctl() function may fail if:

       EOVERFLOW
              The cmd argument is IPC_STAT and the gid or uid value is too large to be stored  in
              the structure pointed to by the buf argument.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       The   POSIX   Realtime   Extension   defines   alternative   interfaces  for  interprocess
       communication. Application developers who need to use IPC should design their applications
       so  that  modules  using  the  IPC  routines  described  in  Section 2.7, XSI Interprocess
       Communication can be easily modified to use the alternative interfaces.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       Section 2.7, XSI Interprocess Communication,  Section  2.8,  Realtime,  shmat(),  shmdt(),
       shmget(), shm_open(), shm_unlink()

       The  Base  Definitions  volume  of  POSIX.1‐2017,  Section  3.346,  Shared  Memory Object,
       <sys_shm.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1-2017,  Standard  for  Information Technology -- Portable Operating System Interface
       (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C)  2018  by
       the  Institute  of  Electrical  and Electronics Engineers, Inc and The Open Group.  In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most  likely  to  have
       been  introduced  during  the conversion of the source files to man page format. To report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .