Sistemi operativi - Syscall shmget

shmget - get shared memory segment

The Single UNIX ® Specification, Version 2

Copyright © 1997 The Open Group

NAME

shmget - get shared memory segment

SYNOPSIS

#include <sys/shm.h>
int shmget(key_t size_t intkey, size, shmflg);

DESCRIPTION

The shmget() function returns the shared memory identifier associated with key.

A shared memory identifier, associated data structure and shared memory segment of at least size bytes, see <sys/shm.h>, are created for key if one of the following is true:

• The argument key is equal to IPC_PRIVATE.
• The argument key does not already have a shared memory identifier associated with it and (shmflg&IPC_CREAT) is non-zero.

Upon creation, the data structure associated with the new shared memory identifier is initialised as follows:

• The values of shm_perm.cuid, shm_perm.uid, shm_perm.cgid and shm_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process.

The low-order nine bits of shm_perm.mode are set equal to the low-order nine bits of shmflg.

bits of shm_perm.mode are set equal to the low-order nine bits of shmflg. The value of shm_segsz is set equal to the value of size. The values of shm_lpid, shm_nattch, shm_atime and shm_dtime are set equal to 0. The value of shm_ctime is set equal to the current time. When the shared memory segment is created, it will be initialised with all zero values.

RETURN VALUE

Upon successful completion, shmget() returns a non-negative integer, namely a shared memory identifier; otherwise, it returns -1 and errno will be set to indicate the error.

ERRORS

The shmget() function will fail if:

• [EACCES] A shared memory identifier exists for key but operation permission as specified by the