semctl (2)
Leading comments
Copyright (c) 1995 David Hovemeyer <daveho@infocom.com> All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the d...
NAME
semctl - control operations on a semaphore setLIBRARY
Lb libcSYNOPSIS
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 .