shmget — allocates a shared memory segment
#include <sys/ipc.h> #include <sys/shm.h>
int
shmget( |
key_t | key, |
size_t | size, | |
int | shmflg) ; |
shmget
() returns the
identifier of the shared memory segment associated with the
value of the argument key
. A new shared memory
segment, with size equal to the value of size
rounded up to a multiple
of PAGE_SIZE
, is created if
key
has the value
IPC_PRIVATE
or key
isn't IPC_PRIVATE
, no shared memory segment
corresponding to key
exists, and IPC_CREAT
is
specified in shmflg
.
If shmflg
specifies both IPC_CREAT
and
IPC_EXCL
and a shared memory
segment already exists for key
, then shmget
() fails with errno
set to EEXIST. (This is analogous to the effect
of the combination O_CREAT |
O_EXCL for open(2).)
The value shmflg
is composed of:
IPC_CREAT
to create a new segment. If this flag is not used,
then shmget
() will find
the segment associated with key
and check to see if
the user has permission to access the segment.
IPC_EXCL
used with IPC_CREAT
to
ensure failure if the segment already exists.
mode_flags
(least significant 9 bits) specifying the
permissions granted to the owner, group, and world.
These bits have the same format, and the same meaning,
as the mode
argument of open(2). Presently,
the execute permissions are not used by the system.
SHM_HUGETLB
(since Linux
2.6)Allocate the segment using "huge pages." See the
kernel source file Documentation/vm/hugetlbpage.txt
for further information.
SHM_NORESERVE
(since Linux
2.6.15)This flag serves the same purpose as the mmap(2) MAP_NORESERVE
flag. Do not reserve
swap space for this segment. When swap space is
reserved, one has the guarantee that it is possible to
modify the segment. When swap space is not reserved one
might get SIGSEGV
upon a
write if no physical memory is available. See also the
discussion of the file /proc/sys/vm/overcommit_memory
in
proc(5).
When a new shared memory segment is created, its contents
are initialized to zero values, and its associated data
structure, shmid_ds
(see shmctl(2)), is initialized
as follows:
shm_perm.cuid
and
shm_perm.uid
are set to the effective user ID of the calling
process.
shm_perm.cgid
and
shm_perm.gid
are set to the effective group ID of the calling
process.
The least significant 9 bits of shm_perm.mode
are set
to the least significant 9 bit of shmflg
.
shm_segsz
is set to the value of size
.
shm_lpid
,
shm_nattch
,
shm_atime
and
shm_dtime
are
set to 0.
shm_ctime
is set to the current time.
If the shared memory segment already exists, the permissions are verified, and a check is made to see if it is marked for destruction.
On failure, errno
is set to
one of the following:
The user does not have permission to access the
shared memory segment, and does not have the
CAP_IPC_OWNER
capability.
IPC_CREAT | IPC_EXCL was specified and the segment exists.
A new segment was to be created and size
< SHMMIN
or size
> SHMMAX
, or no new segment was to be
created, a segment with given key existed, but
size
is greater
than the size of that segment.
The system limit on the total number of open files has been reached.
No segment exists for the given key
, and IPC_CREAT
was not specified.
No memory could be allocated for segment overhead.
All possible shared memory IDs have been taken
(SHMMNI
), or allocating a
segment of the requested size
would cause the
system to exceed the system-wide limit on shared memory
(SHMALL
).
The SHM_HUGETLB
flag
was specified, but the caller was not privileged (did
not have the CAP_IPC_LOCK
capability).
IPC_PRIVATE
isn't a flag
field but a key_t
type. If this special value is used for key
, the system call ignores
everything but the least significant 9 bits of shmflg
and creates a new shared
memory segment (on success).
The following limits on shared memory segment resources
affect the shmget
() call:
SHMALL
System wide maximum of shared memory pages (on
Linux, this limit can be read and modified via
/proc/sys/kernel/shmall
).
SHMMAX
Maximum size in bytes for a shared memory segment:
policy dependent (on Linux, this limit can be read and
modified via /proc/sys/kernel/shmmax
).
SHMMIN
Minimum size in bytes for a shared memory segment:
implementation dependent (currently 1 byte, though
PAGE_SIZE
is the
effective minimum size).
SHMMNI
System wide maximum number of shared memory
segments: implementation dependent (currently 4096, was
128 before Linux 2.3.99; on Linux, this limit can be
read and modified via /proc/sys/kernel/shmmni
).
The implementation has no specific limits for the per
process maximum number of shared memory segments
(SHMSEG
).
The name choice IPC_PRIVATE
was perhaps unfortunate, IPC_NEW would more clearly show its
function.
shmat(2), shmctl(2), shmdt(2), ftok(3), capabilities(7), svipc(7)
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