SHMGET(2) |
System Calls Manual |
SHMGET(2) |
NAME
shmget — get shared memory segment
LIBRARY
Standard C Library (libc, -lc)
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 both the IPC_CREAT bit and the IPC_EXCL bit are set in shmflg, and key has a shared memory segment identifier associated with it already, the operation will fail.
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_CREAT and IPC_EXCL are set in shmflg, and a shared memory segment is already associated with key.
-
[EINVAL]
-
No shared memory segment is to be created, and a shared memory segment exists for key, but the size of the segment associated with it is less than size, which is non-zero.
A shared memory segment is to be created, and size is less than the system imposed minimum, or greater than the system imposed maximum (refer to the kern.ipc
values in sysctl(7)).
-
[ENOENT]
-
IPC_CREAT is 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.
-
[ENOSPC]
-
A new shared memory identifier could not be created because the system limit for the number of shared memory identifiers has been reached.
STANDARDS
The shmget system call conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”).
HISTORY
Shared memory segments appeared in the first release of AT&T System V UNIX.