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SEMCTL(2)                   BSD System Calls Manual                  SEMCTL(2)

NAME
     semctl -- semaphore control operations

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

     int
     semctl(int semid, int semnum, int cmd, union semun arg);

DESCRIPTION
     The semctl() system call provides a number of control operations on the
     semaphore specified by semnum and semid.  The operation to be performed
     is specified in cmd (see below).  arg is a union of the following fields:

             int     val;                    /* value for SETVAL */
             struct  semid_ds *buf;          /* buffer for IPC_{STAT,SET} */
             u_short *array;                 /* array for GETALL & SETALL */

     The semid_ds structure used in the IPC_SET and IPC_STAT commands is
     defined as follows in <sys/sem.h>:

     struct semid_ds {
             struct ipc_perm sem_perm;       /* operation permissions */
             struct  sem *sem_base;          /* semaphore set */
             u_short sem_nsems;              /* number of sems in set */
             time_t  sem_otime;              /* last operation time */
             time_t  sem_ctime;              /* last change time */
     };

     The ipc_perm structure used inside the semid_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 */
     };

     semctl() provides the following operations:

     GETVAL     Return the value of the semaphore.

     SETVAL     Set the value of the semaphore to arg.val.

     GETPID     Return the pid of the last process that did an operation on
                this semaphore.

     GETNCNT    Return the number of processes waiting to acquire the sema-
                phore.

     GETZCNT    Return the number of processes waiting for the value of the
                semaphore to reach 0.

     GETALL     Return the values for all the semaphores associated with
                semid.

     SETALL     Set the values for all the semaphores that are associated with
                the semaphore identifier semid to the corresponding values in
                arg.array.

     IPC_STAT   Gather statistics about a semaphore and place the information
                in the semid_ds structure pointed to by arg.buf (see above).

     IPC_SET    Set the value of the sem_perm.uid, sem_perm.gid and
                sem_perm.mode fields in the structure associated with the sem-
                aphore.  The values are taken from the corresponding fields in
                the structure pointed to by arg.buf.  This operation can only
                be executed by the superuser, or a process that has an effec-
                tive user ID equal to either sem_perm.cuid or sem_perm.uid in
                the data structure associated with the message queue.

     IPC_RMID   Remove the semaphores associated with semid from the system
                and destroy the data structures associated with it.  Only the
                superuser or a process with an effective UID equal to the
                sem_perm.cuid or sem_perm.uid values in the data structure
                associated with the semaphore can do this.

     The permission to read or change a message queue (see semop(2)) is deter-
     mined by the sem_perm.mode field in the same way as is done with files
     (see chmod(2)), but the effective UID can match either the sem_perm.cuid
     field or the sem_perm.uid field, and the effective GID can match either
     sem_perm.cgid or sem_perm.gid.

RETURN VALUES
     For the GETVAL, GETPID, GETNCNT, and GETZCNT operations, semctl() returns
     one of the values described above if successful.  All other operations
     will make semctl() return 0 if no errors occur.  Otherwise -1 is returned
     and errno set to reflect the error.

ERRORS
     semctl() 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 sem_perm.uid or sem_perm.cuid fields of the
                        data structure associated with the message queue.

     [EACCES]           The caller has no operation permission for this sema-
                        phore.

     [EINVAL]           semid is not a valid message semaphore identifier.

                        cmd is not a valid command.

     [EFAULT]           arg.buf or arg.array specify an invalid address.

     [ERANGE]           cmd is equal to SETVAL or SETALL and arg.val or the
                        values in arg.array are greater than the system-
                        imposed limit.

SEE ALSO
     semget(2), semop(2)

BSD                            November 15, 2014                           BSD