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       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
                 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

       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 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.

       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

       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
       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 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
              or SHM_STAT_ANY 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


       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


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


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