SHMGET(2)

NAME
shmget - obtain a shared memory identifier

LIBRARY
libc

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.

SEE ALSO
shmat(2), shmctl(2), shmdt(2), ftok(3)