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.
SEE ALSO
ipcrm(1),
ipcs(1),
mmap(2),
shmat(2),
shmctl(2),
ftok(3),
sysctl(7)
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.