SEMOP

Section: System Calls (2)
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BSD mandoc
NetBSD

NAME

semop - semaphore operations

SYNOPSIS

Fd #include <sys/types.h> Fd #include <sys/ipc.h> Fd #include <sys/sem.h> Ft int Fn semop int semid struct sembuf *sops int nsops

DESCRIPTION

Fn semop provides a number of atomic operations on a set of semaphores. The semaphore set is specified by Fa semid . Fa sops is an array of semaphore operations, Fa nsops is the number of operations in this array. The sembuf structures in the array contain the following members:

        u_short sem_num;        /* semaphore # */
        short   sem_op;         /* semaphore operation */
        short   sem_flg;        /* operation flags */

Each operation (specified in sem_op is applied to semaphore number sem_num in the set of semaphores specified by Fa semid . The value of sem_op determines the action taken in the following way:

sem_op is less than 0. The current process is blocked until the value of the semaphore is greater than or equal to the absolute value of sem_op The absolute value of sem_op is then subtracted from the value of the semaphore, and the calling process continues. Negative values of sem_op are thus used to enter critical regions.
sem_op is greater than 0. Its value is added to the value of the specified semaphore. This is used to leave critical regions.
sem_op is equal to 0. The calling process is blocked until the value of the specified semaphore reaches 0.

The behaviour of each operation is influenced by the flags set in sem_flg in the following way:

IPC_NOWAIT: In the case where the calling process would normally block, waiting for a semaphore to reach a certain value, IPC_NOWAIT makes the call return immediately, returning a value of -1 and setting errno to EAGAIN.
SEM_UNDO: Keep track of the changes that this call makes to the value of a semaphore, so that they can be undone when the calling process terminates. This is useful to prevent other processes waiting on a semaphore to block forever, should the process that has the semaphore locked terminate in a critical section.

RETURN VALUES

Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

ERRORS

Fn semop will fail if:

Bq Er EINVAL

There is no semaphore associated with Fa semid .

The semaphore set was removed while the process was waiting for one of its semaphores to reach a certain value.

Bq Er EACCESS

The calling process has no permission to access the specified semaphore set.

Bq Er E2BIG

The value of Fa nsops is too big. The maximum is specified in MAX_SOPS in <sys/sem.h>

Bq Er EFBIG

sem_num in one of the sem_buf structures is less than 0, or greater than the actual number of semaphores in the set specified by Fa semid .

Bq Er ENOSPC

SEM_UNDO was requested, and there is not enough space left in the kernel to store the unfo information.

Bq Er EAGAIN

The requested operation can not immediately be performed, and IPC_NOWAIT was set in sem_flg

Bq Er EFAULT

Fa sops points to an illegal address.

BUGS

In case of a removed semaphore identifier, errno should be set to EIDRM, but NetBSD does not define this error.

This document was created by man2html, using the manual pages.
Time: 16:28:59 GMT, April 18, 2022