LIBRARY

Lb libc

SYNOPSIS

In sys/types.h In sys/ipc.h In sys/sem.h Ft int Fn semctl int semid int semnum int cmd ...

DESCRIPTION

The Fn semctl system call performs the operation indicated by Fa cmd on the semaphore set indicated by Fa semid . A fourth argument, a Fa union semun arg , is required for certain values of Fa cmd . For the commands that use the Fa arg argument, Fa 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 Fa struct semid_ds , storing it in the memory pointed to by Fa arg.buf .

IPC_SET

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

GETVAL

Return the value of semaphore number Fa semnum .

SETVAL

Set the value of semaphore number Fa semnum to Fa 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 Fa semnum .

GETNCNT

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

GETZCNT

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

GETALL

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

SETALL

Set the values of all of the semaphores in the set to the values in the array pointed to by Fa arg.array . Outstanding adjust on exit values for all semaphores in this set, in any process are cleared.

The Vt 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 */
        time_t  sem_ctime;      /* last change time */
                                /* Times measured in secs since */
                                /* 00:00:00 GMT, Jan. 1, 1970 */
};

RETURN VALUES

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