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NAME    [Toc]    [Back]

     semctl - semaphore control operations

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <sys/sem.h>

     semctl(int semid, int semnum, int cmd, ...);

DESCRIPTION    [Toc]    [Back]

     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).  The fourth argument is optional and
     depends upon the opration requested.  If required, it 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 in <sys/sem.h> and contains the following members:

         struct ipc_perm sem_perm; /* operation permissions */
         unsigned 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 contains the following members:

         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; /* permission (lower 9 bits) */

     semctl() provides the following operations:

     GETVAL     Return the value of the semaphore.

     SETVAL     Set the value of the semaphore to arg.val, where arg is the
                fourth arument to semctl().

     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

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

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

     SETALL     Set the values of all the semaphores that are associated with
                the semaphore identifier semid to the corresponding values in
                arg.array, where arg is the fourth argument to semctl().

     IPC_STAT   Gather information about a semaphore and place the information
                in the structure pointed to by arg.buf, where arg is the
                fourth argument to semctl().

     IPC_SET    Set the value of the sem_perm.uid, sem_perm.gid and
                sem_perm.mode fields in the structure associated with the
                semaphore.  The values are taken from the corresponding fields
                in the structure pointed to by arg.buf, there arg is the
                fourth argument to semctl().  This operation can only be executed
 by the super-user, or a process that has an effective
                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
                super-user 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 determined
 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    [Toc]    [Back]

     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    [Toc]    [Back]

     semctl() will fail if:

     [EPERM]            cmd is equal to IPC_SET or IPC_RMID and the caller is
                        not the super-user, 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

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

                        cmd is not a valid command.

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

SEE ALSO    [Toc]    [Back]

     semget(2), semop(2)

STANDARDS    [Toc]    [Back]

     The semctl system call conforms to X/Open System Interfaces and Headers
     Issue 5 (``XSH5'').

HISTORY    [Toc]    [Back]

     Semaphores appeared in the first release of AT&T System V UNIX.

BSD                             August 25, 1999                            BSD
[ Back ]
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