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SHMCTL(2)                           Linux Programmer's Manual                           SHMCTL(2)

       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

           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 segment.

       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  effec‐
                 tive  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 only actually  be  destroyed
                 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 ignored.

                 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

                     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

       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 set.

       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 mem‐
       ory to be locked falls within the RLIMIT_MEMLOCK resource limit (see setrlimit(2)).

       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  iden‐
       tifier  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.

       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

       SVr4, POSIX.1-2001.

       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

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

       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-02-07                                  SHMCTL(2)

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