DragonFly On-Line Manual Pages

SEMCTL(2)                DragonFly System Calls Manual               SEMCTL(2)

NAME
     semctl -- control operations on a semaphore set

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/sem.h>

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

DESCRIPTION
     Semctl() performs the operation indicated by cmd on the semaphore set
     indicated by semid.  A fourth argument, a union semun arg, is required
     for certain values of cmd.  For the commands that use the arg parameter,
     union semun is defined as follows:

     union semun {
             int     val;            /* value for SETVAL */
             struct  semid_ds *buf;  /* buffer for IPC_STAT & IPC_SET */
             u_short *array;         /* array for GETALL & SETALL */
     };

     Commands are performed as follows:

     IPC_STAT  Fetch the semaphore set's struct semid_ds, storing it in the
               memory pointed to by arg.buf.

     IPC_SET   Changes the sem_perm.uid, sem_perm.gid, and sem_perm.mode
               members of the semaphore set's struct semid_ds to match those
               of the struct pointed to by arg.buf.  The calling process's
               effective uid must match either sem_perm.uid or sem_perm.cuid,
               or it must have superuser privileges.

     IPC_RMID  Immediately removes the semaphore set from the system.  The
               calling process's effective uid must equal the semaphore set's
               sem_perm.uid or sem_perm.cuid, or the process must have
               superuser privileges.

     GETVAL    Return the value of semaphore number semnum.

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

     GETPID    Return the pid of the last process to perform an operation on
               semaphore number semnum.

     GETNCNT   Return the number of processes waiting for semaphore number
               semnum's value to become greater than its current value.

     GETZCNT   Return the number of processes waiting for semaphore number
               semnum's value to become 0.

     GETALL    Fetch the value of all of the semaphores in the set into the
               array pointed to by arg.array.

     SETALL    Set the values of all of the semaphores in the set to the
               values in the array pointed to by arg.array.

     The struct semid_ds is defined as follows:

     struct semid_ds {
             struct  ipc_perm sem_perm;      /* operation permission struct */
             struct  sem *sem_base;  /* pointer to first semaphore in set */
             u_short sem_nsems;      /* number of sems in set */
             time_t  sem_otime;      /* last operation time */
             long    sem_pad1;       /* SVABI/386 says I need this here */
             time_t  sem_ctime;      /* last change time */
                                     /* Times measured in secs since */
                                     /* 00:00:00 GMT, Jan. 1, 1970 */
             long    sem_pad2;       /* SVABI/386 says I need this here */
             long    sem_pad3[4];    /* SVABI/386 says I need this here */
     };

RETURN VALUES
     On success, when cmd is one of GETVAL, GETPID, GETNCNT or GETZCNT,
     semctl() returns the corresponding value; otherwise, 0 is returned.  On
     failure, -1 is returned, and errno is set to indicate the error.

ENVIRONMENT
     The XSI Interprocess Communication family of functions is also available
     as an implementation in userspace.  To use it, the sysvipcd(8) daemon has
     to be running.

     If the USR_SYSVIPC variable is set in a process' environment, the process
     and its children will use the userspace implementation.

ERRORS
     Semctl() will fail if:

     [EINVAL]           No semaphore set corresponds to semid.

     [EINVAL]           semnum is not in the range of valid semaphores for
                        given semaphore set.

     [EPERM]            The calling process's effective uid does not match the
                        uid of the semaphore set's owner or creator.

     [EACCES]           Permission denied due to mismatch between operation
                        and mode of semaphore set.

SEE ALSO
     semget(2), semop(2)

AUTHORS
     The DragonFly specific userspace implementation (see ENVIRONMENT) was
     written by Larisa Grigore.

DragonFly 4.3                   January 4, 2014                  DragonFly 4.3