semctl(2) control operations on a semaphore set

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.

ERRORS

The Fn semctl system call will fail if:

Bq Er EINVAL
No semaphore set corresponds to Fa semid .
Bq Er EINVAL
The Fa semnum argument is not in the range of valid semaphores for given semaphore set.
Bq Er EPERM
The calling process's effective uid does not match the uid of the semaphore set's owner or creator.
Bq Er EACCES
Permission denied due to mismatch between operation and mode of semaphore set.
Bq Er ERANGE
SETVAL or SETALL attempted to set a semaphore outside the allowable range Bq 0 .. Dv SEMVMX .

BUGS

SETALL may update some semaphore elements before returning an error.