NAME
shmget —
obtain a shared memory
identifier
LIBRARY
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.