NAME
shmget —
get shared memory area
identifier
SYNOPSIS
#include
<sys/shm.h>
int
shmget(key_t
key, size_t size,
int shmflg);
DESCRIPTION
shmget()
returns the shared memory identifier associated with the key
key.
A shared memory segment is created if either
key is equal to IPC_PRIVATE,
or key does not have a shared memory segment
identifier associated with it, and the IPC_CREAT bit
is set in shmflg.
If a new shared memory segment is created, the data structure associated with it (the shmid_ds structure, see shmctl(2)) is initialized as follows:
- shm_perm.cuid and shm_perm.uid are set to the effective uid of the calling process.
- shm_perm.gid and shm_perm.cgid are set to the effective gid of the calling process.
- shm_perm.mode is set to the lower 9 bits of shmflg.
- shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
- shm_ctime is set to the current time.
- shm_segsz is set to the value of size.
RETURN VALUES
Upon successful completion a positive shared memory segment identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.
ERRORS
- [
EACCES] - A shared memory segment is already associated with key and the caller has no permission to access it.
- [
EEXIST] - Both
IPC_CREATandIPC_EXCLare set in shmflg, and a shared memory segment is already associated with key. - [
EINVAL] - A shared memory segment is already associated with key and its size is less than the requested size.
- [
ENOSPC] - A new shared memory identifier could not be created because the system limit for the number of shared memory identifiers has been reached.
- [
ENOENT] IPC_CREATwas not set in shmflg and no shared memory segment associated with key was found.- [
ENOMEM] - There is not enough memory left to create a shared memory segment of the requested size.