NAME

shmget — obtain a shared memory identifier

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/shm.h>

int
shmget(key_t key, size_t size, int flag);

DESCRIPTION

Based on the values of key and flag, shmget() returns the identifier of a newly created or previously existing shared memory segment. The key is analogous to a filename: it provides a handle that names an IPC object. There are three ways to specify a key:

IPC_PRIVATE may be specified, in which case a new IPC object will be created.
An integer constant may be specified. If no IPC object corresponding to key is specified and the IPC_CREAT bit is set in flag, a new one will be created.
The ftok(3) may be used to generate a key from a pathname.

The mode of a newly created IPC object is determined by which are set by ORing these constants into the flag argument:

0400: Read access for owner.
0200: Write access for owner.
0040: Read access for group.
0020: Write access for group.
0004: Read access for other.
0002: Write access for other.

When creating a new shared memory segment, size indicates the desired size of the new segment in bytes. The size of the segment may be rounded up to a multiple convenient to the kernel (i.e., the page size).

RETURN VALUES

Upon successful completion, shmget() returns the positive integer identifier of a shared memory segment. Otherwise, -1 is returned and errno set to indicate the error.

ERRORS

The shmget() system call will fail if:

[EINVAL]: Size specified is greater than the size of the previously existing segment. Size specified is less than the system imposed minimum, or greater than the system imposed maximum.
[ENOENT]: No shared memory segment was found matching key, and IPC_CREAT was not specified.
[ENOSPC]: The kernel was unable to allocate enough memory to satisfy the request.
[EEXIST]: IPC_CREAT and IPC_EXCL were specified, and a shared memory segment corresponding to key already exists.