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

       shmctl - System V shared memory control

       #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

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

       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 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:

       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 privileged.

              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

              See also the description of /proc/sys/kernel/shm_rmid_forced in

       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

                  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
              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 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

       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

              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 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)

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       latest version of this page, can be found at

Linux                              2021-03-22                          SHMCTL(2)