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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

     The semctl() system call 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
     argument, 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.
                  Outstanding adjust on exit values for this semaphore in any
                  process are cleared.

     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.  Outstanding
                  adjust on exit values for all semaphores in this set, in any
                  process are cleared.

     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.

ERRORS

     The semctl() system call will fail if:

     [EINVAL]           No semaphore set corresponds to semid.

     [EINVAL]           The semnum argument 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.

     [ERANGE]           SETVAL or SETALL attempted to set a semaphore outside
                        the allowable range [0 .. SEMVMX].

SEE ALSO

     semget(2), semop(2)

BUGS

     SETALL may update some semaphore elements before returning an error.