shmctl(2) — Linux manual page


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

NAME         top

       shmctl - System V shared memory control

SYNOPSIS         top

       #include <sys/ipc.h>
       #include <sys/shm.h>

       int shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION         top

       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_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.

       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.

       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 struc‐
       ture 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 sys‐
       tem.  (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 mem‐
              ber.  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 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 (non‐
              standard) 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 struc‐
              ture 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 informa‐
              tion 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, mean‐
              ing 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

       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)).

RETURN VALUE         top

       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.

ERRORS         top

       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 privileged.)

CONFORMING TO         top

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

NOTES         top

       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

       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.)

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-04-11                        SHMCTL(2)

Pages that refer to this page: ipcrm(1)ipcs(1)getrlimit(2)ipc(2)mlock2(2)mlock(2)mlockall(2)munlock(2)munlockall(2)prlimit(2)prlimit64(2)setrlimit(2)shmat(2)shmdt(2)shmget(2)shmop(2)syscalls(2)ugetrlimit(2)vlimit(3)proc(5)procfs(5)capabilities(7)svipc(7)sysvipc(7)