SHMGET (2)
allocates a shared memory segment
SYNOPSIS
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#include <sys/ipc.h>
#include <sys/shm.h>
int shmget(key_t key , int size , int shmflg );
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DESCRIPTION
shmget()
returns the identifier of the shared memory segment
associated to the value of the argument
key .
A new shared memory segment, with size equal to the round up of
size
to a multiple of
PAGE_SIZE ,
is created if
key
has value
IPC_PRIVATE
or
key
isn't
IPC_PRIVATE ,
no shared memory segment is associated to
key ,
and
IPC_CREAT
is asserted in
shmflg
(i.e.
shmflg &IPC_CREAT
isn't zero).
The presence in
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, check to see if
the user has permission to receive the shmid associated with the
segment, and ensure the segment is not marked for destruction.
IPC_EXCL
used with IPC_CREAT to ensure failure if the segment 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:
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struct shmid_ds {
struct ipc_perm shm_perm; /* operation perms */
int 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 */
};
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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 */
};
Furthermore, while creating,
the system call initializes the system shared memory segment data structure
shmid_ds
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 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
is returned if SHMMIN > size or
size > SHMMAX, or size is greater than size of segment.
EEXIST
is returned if
IPC_CREAT | IPC_EXCL
was specified and the segment exists.
EIDRM
is returned if the segment is marked as destroyed, or was removed.
ENOSPC
is returned if all possible shared memory id's have been taken
( SHMMNI )
or if allocating a segment of the requested
size
would cause the system to exceed the system-wide limit on shared memory
( SHMALL ).
ENOENT
is returned if no segment exists for the given key, and
IPC_CREAT
was not specified.
EACCES
is returned if the user does not have permission to access the
shared memory segment.
ENOMEM
is returned 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
dependent (currently 4M).
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).
The implementation has no specific limits for the per process maximum
number of shared memory segments
( SHMSEG ).
BUGS
Use of
IPC_PRIVATE
doesn't inhibit to other processes the access to the allocated
shared memory segment.
As for the files, there is currently no intrinsic way for a process to ensure
exclusive access to a shared memory segment.
Asserting both
IPC_CREAT
and
IPC_EXCL
in
shmflg
only ensures (on success) that a new shared memory segment will be created,
it doesn't imply exclusive access to the segment.
CONFORMING TO
SVr4, SVID. SVr4 documents an additional error condition EEXIST.
Neither SVr4 nor SVID documents an EIDRM condition.
SEE ALSO
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