shmctl - System V shared memory control


   #include <sys/ipc.h>
   #include <sys/shm.h>

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


   shmctl()  performs  the  control  operation  specified  by  cmd  on the
   System V shared memory segment whose identifier is given in shmid.

   The buf argument is a pointer  to  a  shmid_ds  structure,  defined  in
   <sys/shm.h> as follows:

       struct shmid_ds {
           struct ipc_perm shm_perm;    /* Ownership and permissions */
           size_t          shm_segsz;   /* Size of segment (bytes) */
           time_t          shm_atime;   /* Last attach time */
           time_t          shm_dtime;   /* Last detach time */
           time_t          shm_ctime;   /* Last change time */
           pid_t           shm_cpid;    /* PID of creator */
           pid_t           shm_lpid;    /* PID of last shmat(2)/shmdt(2) */
           shmatt_t        shm_nattch;  /* No. of current attaches */

   The  ipc_perm  structure  is defined as follows (the highlighted fields
   are settable using IPC_SET):

       struct ipc_perm {
           key_t          __key;    /* Key supplied to shmget(2) */
           uid_t          uid;      /* Effective UID of owner */
           gid_t          gid;      /* Effective GID of owner */
           uid_t          cuid;     /* Effective UID of creator */
           gid_t          cgid;     /* Effective GID of creator */
           unsigned short mode;     /* Permissions + SHM_DEST and
                                       SHM_LOCKED flags */
           unsigned short __seq;    /* Sequence number */

   Valid values for cmd are:

   IPC_STAT  Copy information from the kernel  data  structure  associated
             with  shmid  into  the  shmid_ds structure pointed to by buf.
             The caller must have read permission  on  the  shared  memory

   IPC_SET   Write  the  values  of some members of the shmid_ds structure
             pointed to by buf to the  kernel  data  structure  associated
             with  this shared memory segment, updating also its shm_ctime
             member.  The following fields can be  changed:  shm_perm.uid,
             shm_perm.gid,   and   (the   least  significant  9  bits  of)
             shm_perm.mode.  The effective UID of the calling process must
             match  the owner (shm_perm.uid) or creator (shm_perm.cuid) of
             the shared memory segment, or the caller must be privileged.

   IPC_RMID  Mark the segment to be destroyed.  The segment will  actually
             be  destroyed  only after the last process detaches it (i.e.,
             when  the  shm_nattch  member  of  the  associated  structure
             shmid_ds  is  zero).  The caller must be the owner or creator
             of the segment,  or  be  privileged.   The  buf  argument  is

             If  a  segment  has  been  marked  for  destruction, then the
             (nonstandard) SHM_DEST flag of the shm_perm.mode field in the
             associated data structure retrieved by IPC_STAT will be set.

             The   caller   must  ensure  that  a  segment  is  eventually
             destroyed; otherwise its pages  that  were  faulted  in  will
             remain in memory or swap.

             See  also the description of /proc/sys/kernel/shm_rmid_forced
             in proc(5).

   IPC_INFO (Linux-specific)
             Return information about system-wide shared memory limits and
             parameters   in  the  structure  pointed  to  by  buf.   This
             structure is of type shminfo  (thus,  a  cast  is  required),
             defined  in <sys/shm.h> if the _GNU_SOURCE feature test macro
             is defined:

                 struct shminfo {
                     unsigned long shmmax; /* Maximum segment size */
                     unsigned long shmmin; /* Minimum segment size;
                                              always 1 */
                     unsigned long shmmni; /* Maximum number of segments */
                     unsigned long shmseg; /* Maximum number of segments
                                              that a process can attach;
                                              unused within kernel */
                     unsigned long shmall; /* Maximum number of pages of
                                              shared memory, system-wide */

             The shmmni, shmmax, and shmall settings can  be  changed  via
             /proc files of the same name; see proc(5) for details.

   SHM_INFO (Linux-specific)
             Return  a shm_info structure whose fields contain information
             about system  resources  consumed  by  shared  memory.   This
             structure  is  defined  in  <sys/shm.h>  if  the  _GNU_SOURCE
             feature test macro is defined:

                 struct shm_info {
                     int           used_ids; /* # of currently existing
                                                segments */
                     unsigned long shm_tot;  /* Total number of shared
                                                memory pages */
                     unsigned long shm_rss;  /* # of resident shared
                                                memory pages */
                     unsigned long shm_swp;  /* # of swapped shared
                                                memory pages */
                     unsigned long swap_attempts;
                                             /* Unused since Linux 2.4 */
                     unsigned long swap_successes;
                                             /* Unused since Linux 2.4 */

   SHM_STAT (Linux-specific)
             Return a shmid_ds structure as for  IPC_STAT.   However,  the
             shmid  argument  is  not a segment identifier, but instead an
             index  into  the  kernel's  internal  array  that   maintains
             information about all shared memory segments on the system.

   The  caller  can  prevent  or allow swapping of a shared memory segment
   with the following cmd values:

   SHM_LOCK (Linux-specific)
             Prevent swapping of the shared memory  segment.   The  caller
             must fault in any pages that are required to be present after
             locking is enabled.  If a segment has been locked,  then  the
             (nonstandard)  SHM_LOCKED  flag of the shm_perm.mode field in
             the associated data structure retrieved by IPC_STAT  will  be

   SHM_UNLOCK (Linux-specific)
             Unlock the segment, allowing it to be swapped out.

   In  kernels  before  2.6.10,  only  a  privileged  process could employ
   SHM_LOCK and SHM_UNLOCK.  Since kernel 2.6.10, an unprivileged  process
   can  employ  these operations if its effective UID matches the owner or
   creator UID of the segment, and (for SHM_LOCK) the amount of memory  to
   be   locked   falls  within  the  RLIMIT_MEMLOCK  resource  limit  (see


   A successful IPC_INFO or SHM_INFO operation returns the  index  of  the
   highest used entry in the kernel's internal array recording information
   about all shared memory segments.  (This information can be  used  with
   repeated  SHM_STAT  operations  to  obtain information about all shared
   memory segments  on  the  system.)   A  successful  SHM_STAT  operation
   returns  the  identifier  of  the shared memory segment whose index was
   given in shmid.  Other operations return 0 on success.

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


   EACCES IPC_STAT or SHM_STAT is requested  and  shm_perm.mode  does  not
          allow  read  access  for shmid, and the calling process does not
          have the CAP_IPC_OWNER capability in  the  user  namespace  that
          governs its IPC namespace.

   EFAULT The  argument  cmd has value IPC_SET or IPC_STAT but the address
          pointed to by buf isn't accessible.

   EIDRM  shmid points to a removed identifier.

   EINVAL shmid is not a valid identifier, or cmd is not a valid  command.
          Or: for a SHM_STAT operation, the index value specified in shmid
          referred to an array slot that is currently unused.

   ENOMEM (In kernels since 2.6.9), SHM_LOCK was specified and the size of
          the  to-be-locked  segment  would  mean  that the total bytes in
          locked shared memory segments would exceed  the  limit  for  the
          real  user  ID of the calling process.  This limit is defined by
          the RLIMIT_MEMLOCK soft resource limit (see setrlimit(2)).

          IPC_STAT is attempted, and the GID or UID value is too large  to
          be stored in the structure pointed to by buf.

   EPERM  IPC_SET  or  IPC_RMID is attempted, and the effective user ID of
          the calling process  is  not  that  of  the  creator  (found  in
          shm_perm.cuid),  or  the  owner (found in shm_perm.uid), and the
          process  was  not  privileged   (Linux:   did   not   have   the
          CAP_SYS_ADMIN capability).

          Or  (in  kernels  before  2.6.9),  SHM_LOCK  or  SHM_UNLOCK  was
          specified, but the process was not privileged  (Linux:  did  not
          have  the  CAP_IPC_LOCK  capability).   (Since Linux 2.6.9, this
          error can also occur if the RLIMIT_MEMLOCK is 0 and  the  caller
          is not privileged.)


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


   The  inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
   or by any version of POSIX.  However, some old implementations required
   the inclusion of these header files, and the SVID also documented their
   inclusion.  Applications intended to be portable to  such  old  systems
   may need to include these header files.

   The  IPC_INFO, SHM_STAT and SHM_INFO operations are used by the ipcs(1)
   program to provide information on allocated resources.  In the  future,
   these may modified or moved to a /proc filesystem interface.

   Linux  permits  a  process to attach (shmat(2)) a shared memory segment
   that has already been marked for deletion using shmctl(IPC_RMID).  This
   feature  is  not  available  on  other  UNIX  implementations; portable
   applications should avoid relying on it.

   Various fields in a struct shmid_ds were typed as short under Linux 2.2
   and  have  become  long  under Linux 2.4.  To take advantage of this, a
   recompilation under glibc-2.1.91 or later should suffice.  (The  kernel
   distinguishes old and new calls by an IPC_64 flag in cmd.)


   mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7), svipc(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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