shmctl(2) — Linux manual page


shmctl(2)                  System Calls Manual                 shmctl(2)

NAME         top

       shmctl - System V shared memory control

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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

       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:

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

              Size in bytes of the shared memory segment.

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

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

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

              ID of the process that created the shared memory segment.

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

              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

       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

              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)

              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 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

       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 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.

       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)).

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 to indicate the error.

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 (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 privileged.)

VERSIONS         top

       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

STANDARDS         top


HISTORY         top

       POSIX.1-2001, SVr4.

       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.)

NOTES         top

       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

SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                        shmctl(2)

Pages that refer to this page: ipcrm(1)ipcs(1)getrlimit(2)ipc(2)mlock(2)shmget(2)shmop(2)syscalls(2)proc(5)capabilities(7)sysvipc(7)