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       shmctl - System V shared memory control


       Standard C library (libc, -lc)


       #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 segment.

       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 attached.

       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 */

       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:

       0400   Read by user
       0200   Write by user
       0040   Read by group
       0020   Write by group
       0004   Read by others
       0002   Write by others

       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  associated  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 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  are  updated:  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

              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 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.

       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

       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.

       Before Linux 2.6.10, only a privileged  process  could  employ  SHM_LOCK  and  SHM_UNLOCK.
       Since  Linux  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 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  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 memory segment whose index was given in
       shmid.  Other operations return 0 on success.

       On error, -1 is returned, and errno is set to indicate the error.


       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
              or SHM_STAT_ANY operation, the index value specified in shmid referred to an  array
              slot that is currently unused.

       ENOMEM (Since  Linux  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 (before Linux 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


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


       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), sysvipc(7)