NAME
shmget —
obtain a shared memory
identifier
LIBRARY
library “libc”
SYNOPSIS
#include
<machine/param.h>
#include <sys/types.h>
#include <sys/ipc.h>
#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_PRIVATEmay 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_CREATbit is set in flag, a new one will be created. ftok() may be used to generate a key from a pathname. See ftok(3).
The mode of a newly created IPC object is determined by OR'ing the following constants into the flag parameter:
SHM_R- Read access for user.
SHM_W- Write access for user.
- (
SHM_R>>3) - Read access for group.
- (
SHM_W>>3) - Write access for group.
- (
SHM_R>>6) - Read access for other.
- (
SHM_W>>6) - 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.
ENVIRONMENT
The XSI Interprocess Communication family of functions is also available as an implementation in userspace. To use it, the sysvipcd(8) daemon has to be running.
If the USR_SYSVIPC variable is set in a
process' environment, the process and its children will use the userspace
implementation.
ERRORS
Shmget() 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_CREATwas not specified. - [
ENOSPC] - The kernel was unable to allocate enough memory to satisfy the request.
- [
EEXIST] IPC_CREATandIPC_EXCLwere specified, and a shared memory segment corresponding to key already exists.
SEE ALSO
AUTHORS
The DragonFly specific userspace implementation (see ENVIRONMENT) was written by Larisa Grigore.