SHMCTL(2)                 (2020-12-21)                  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;   /* Creation time/time of last
                                                  modification via shmctl() */
                  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 fields of the shmid_ds structure are as follows:

          shm_perm    This is an ipc_perm structure (see  below)  that
                      specifies  the  access permissions on the shared
                      memory segment.

          shm_segsz   Size in bytes of the shared memory segment.

          shm_atime   Time of  the  last  shmat(2)  system  call  that
                      attached this segment.

          shm_dtime   Time of  the  last  shmdt(2)  system  call  that
                      detached tgis segment.

          shm_ctime   Time of creation of segment or time of the  last
                      shmctl() IPC_SET operation.

          shm_cpid    ID of the process that created the shared memory

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          shm_lpid    ID of the last process that executed a  shmat(2)
                      or shmdt(2) system call on this segment.

          shm_nattch  Number  of  processes  that  have  this  segment

          The ipc_perm structure is  defined  as  follows  (the  high-
          lighted 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 */

          The least significant 9  bits  of  the  mode  field  of  the
          ipc_perm  structure  define  the  access permissions for the
          shared memory segment.  The permission bits are as  follows:
          l  l.   0400 Read  by  user  0200 Write by user 0040 Read by
          group 0020 Write by group 0004 Read by others 0002 Write  by

          Bits 0100, 0010, and 0001 (the execute bits) are  unused  by
          the system.  (It is not necessary to have execute permission
          on a segment in order to perform a shmat(2)  call  with  the
          SHM_EXEC flag.)

          Valid values for cmd are:

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

               Write the values of some members of the shmid_ds struc-
               ture  pointed  to  by  buf to the kernel data structure
               associated with this shared  memory  segment,  updating
               also its shm_ctime member.

               The  following  fields   are   updated:   shm_perm.uid,
               shm_perm.gid,  and  (the  least  significant 9 bits of)

               The effective UID of the calling process must match the
               owner  (shm_perm.uid) or creator (shm_perm.cuid) of the

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               shared memory segment, or the  caller  must  be  privi-

               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 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 lim-
               its  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

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

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                   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.

          SHM_STAT_ANY (Linux-specific, since Linux 4.17)
               Return a shmid_ds structure as for SHM_STAT.   However,
               shm_perm.mode is not checked for read access for shmid,
               meaning that any user can employ this  operation  (just
               as  any  user  may read /proc/sysvipc/shm to obtain the
               same information).

          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  struc-
               ture 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 seg-
          ment, and (for SHM_LOCK) the amount of memory to  be  locked
          falls   within   the   RLIMIT_MEMLOCK  resource  limit  (see


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          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  seg-
          ments.  (This information can be used with repeated SHM_STAT
          or SHM_STAT_ANY operations to obtain information  about  all
          shared   memory  segments  on  the  system.)   A  successful
          SHM_STAT operation returns the identifier of the shared mem-
          ory segment whose index was given in shmid. Other operations
          return 0 on success.

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

               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.

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

               shmid points to a removed identifier.

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

               (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.

               IPC_SET or IPC_RMID is  attempted,  and  the  effective
               user  ID of the calling process is not that of the cre-
               ator (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

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               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.  Appli-
          cations 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), sysvipc(7)

          This page is part of release 5.10  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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