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SHMCTL(2)                   BSD System Calls Manual                  SHMCTL(2)

     shmctl -- shared memory control operations

     #include <&lt;sys/shm.h>&gt;

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

     The shmctl() system call performs some control operations on the shared
     memory area specified by shmid.

     Each shared memory segment has a data structure associated with it, parts
     of which may be altered by shmctl() and parts of which determine the
     actions of shmctl().

     This structure is defined as follows in <sys/shm.h>:

     struct shmid_ds {
             struct ipc_perm shm_perm;     /* operation permissions */
             int             shm_segsz;    /* size of segment in bytes */
             pid_t           shm_lpid;     /* pid of last shm op */
             pid_t           shm_cpid;     /* pid of creator */
             short           shm_nattch;   /* # of current attaches */
             time_t          shm_atime;    /* last shmat() time*/
             time_t          shm_dtime;    /* last shmdt() time */
             time_t          shm_ctime;    /* last change by shmctl() */
             void            *shm_internal; /* sysv stupidity */

     The ipc_perm structure used inside the shmid_ds structure is defined in
     <sys/ipc.h> and looks like this:

     struct ipc_perm {
             uid_t           cuid;   /* creator user id */
             gid_t           cgid;   /* creator group id */
             uid_t           uid;    /* user id */
             gid_t           gid;    /* group id */
             mode_t          mode;   /* r/w permission (see chmod(2)) */
             u_short         seq;    /* sequence # (to generate unique msg/sem/shm id) */
             key_t           key;    /* user specified msg/sem/shm key */

     The operation to be performed by shmctl() is specified in cmd and is one

     IPC_STAT   Gather information about the shared memory segment and place
                it in the structure pointed to by buf.

     IPC_SET    Set the value of the shm_perm.uid, shm_perm.gid and
                shm_perm.mode fields in the structure associated with shmid.
                The values are taken from the corresponding fields in the
                structure pointed to by buf.  This operation can only be exe-
                cuted by the superuser, or a process that has an effective
                user ID equal to either shm_perm.cuid or shm_perm.uid in the
                data structure associated with the shared memory segment.

     IPC_RMID   Mark the shared memory segment specified by shmid for removal
                when it is no longer in use by any process.  When it is
                removed, all data associated with it will be destroyed too.
                Only the superuser or a process with an effective UID equal to
                the shm_perm.cuid or shm_perm.uid values in the data structure
                associated with the queue can do this.

     The read and write permissions on a shared memory identifier are deter-
     mined by the shm_perm.mode field in the same way as is done with files
     (see chmod(2)), but the effective UID can match either the shm_perm.cuid
     field or the shm_perm.uid field, and the effective GID can match either
     shm_perm.cgid or shm_perm.gid.

     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the

     shmctl() will fail if:

     [EPERM]            cmd is equal to IPC_SET or IPC_RMID and the caller is
                        not the superuser, nor does the effective UID match
                        either the shm_perm.uid or shm_perm.cuid fields of the
                        data structure associated with the shared memory seg-

                        An attempt is made to increase the value of shm_qbytes
                        through IPC_SET but the caller is not the superuser.

     [EACCES]           The command is IPC_STAT and the caller has no read
                        permission for this shared memory segment.

     [EINVAL]           shmid is not a valid shared memory segment identifier.

                        cmd is not a valid command.

     [EFAULT]           buf specifies an invalid address.

     shmat(2), shmget(2)

     Segments which are marked for removal (but not yet removed since they are
     still in use) can be attached to by new callers using shmat(2).  This is
     permitted as an extension beyond the standards.

BSD                            December 10, 2014                           BSD