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SHMGET(2) Linux Programmer’s Manual SHMGET(2)
NAME
shmget - allocates a shared memory segment
SYNOPSIS
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);
DESCRIPTION
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 asserted in shmflg (i.e. shmflg&IPC_CREAT isn’t
zero).
The value shmflg is composed of:
SHM_HUGETLB used for allocating HUGETLB pages for shared memory. IPC_CREAT to cre-
ate 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 per-
mission to access the segment.
IPC_EXCL used with IPC_CREAT to ensure failure if the segment already exists.
mode_flags (lowest 9 bits)
specifying the permissions granted to the owner, group, and world.
Presently, the execute permissions are not used by the system.
If a new segment is created, the access permissions from shmflg are copied into the
shm_perm member of the shmid_ds structure that defines the segment. The shmid_ds
structure has the following form:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation perms */
size_t shm_segsz; /* size of segment (bytes) */
time_t shm_atime; /* last attach time */
time_t shm_dtime; /* last detach time */
time_t shm_ctime; /* last change time */
unsigned short shm_cpid; /* pid of creator */
unsigned short shm_lpid; /* pid of last operator */
short shm_nattch; /* no. of current attaches */
};
struct ipc_perm {
key_t key;
ushort uid; /* owner euid and egid */
ushort gid;
ushort cuid; /* creator euid and egid */
ushort cgid;
ushort mode; /* lower 9 bits of shmflg */
ushort seq; /* sequence number */
};
When creating a new shared memory segment, the system call initializes the shmid_ds
data structure shmid_ds as follows:
shm_perm.cuid and shm_perm.uid are set to the effective user-ID of the call-
ing process.
shm_perm.cgid and shm_perm.gid are set to the effective group-ID of the
calling process.
The lowest order 9 bits of shm_perm.mode are set to the lowest order 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 access permissions are verified,
and a check is made to see if it is marked for destruction.
SYSTEM CALLS
fork() After a fork() the child inherits the attached shared memory segments.
exec() After an exec() all attached shared memory segments are detached (not
destroyed).
exit() Upon exit() all attached shared memory segments are detached (not
destroyed).
RETURN VALUE
A valid segment identifier, shmid, is returned on success, -1 on error.
ERRORS
On failure, errno is set to one of the following:
EINVAL if 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.
EEXIST if IPC_CREAT | IPC_EXCL was specified and the segment exists.
ENOSPC if all possible shared memory id’s have been taken (SHMMNI) or if allo-
cating a segment of the requested size would cause the system to exceed
the system-wide limit on shared memory (SHMALL).
ENOENT if no segment exists for the given key, and IPC_CREAT was not speci-
fied.
EACCES if the user does not have permission to access the shared memory seg-
ment.
ENOMEM if no memory could be allocated for segment overhead.
NOTES
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 lowest order 9 bits of shmflg and
creates a new shared memory segment (on success).
The followings are limits on shared memory segment resources affecting a shmget
call:
SHMALL System wide maximum of shared memory pages: policy dependent.
SHMMAX Maximum size in bytes for a shared memory segment: implementation depen-
dent (currently 4M).
SHMMIN Minimum size in bytes for a shared memory segment: implementation depen-
dent (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).
The implementation has no specific limits for the per process maximum number of
shared memory segments (SHMSEG).
BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly
show its function.
CONFORMING TO
SVr4, SVID. SVr4 documents an additional error condition EEXIST. Until version
2.3.30 Linux would return EIDRM for a shmget on a shared memory segment scheduled
for deletion.
SEE ALSO
ftok(3), ipc(5), shmctl(2), shmat(2), shmdt(2)
Linux 0.99.11 1993-11-28 SHMGET(2)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2009-01-10 11:07 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)