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.