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

NAME
     semctl -- control operations on a semaphore set

SYNOPSIS
     #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:

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

     [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.

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

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

     The include files <sys/types.h> and <sys/ipc.h> are necessary.

SEE ALSO
     semget(2), semop(2), compat(5)

BUGS
     SETALL may update some semaphore elements before returning an error.

BSD                                          September 12, 1995                                          BSD