NAME
shmget - allocates a shared memory segment
SYNOPSIS
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget(key_t key, int size, int shmflg
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 seg-
ment 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 asso-
ciated 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 per-
missions 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:
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 */
};
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 fol-
lows:
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 per-
missions 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 speci-
fied and the segment exists.
EIDRM is returned if the segment is marked as des-
troyed, or was removed.
ENOSPC is returned if all possible shared memory id's
have been taken or if allocating a segment of
the requested size would cause the system to
exceed the system-wide limit on shared memory
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: pol-
icy dependent.
SHMMAX Maximum size in bytes for a shared memory seg-
ment: implementation dependent (currently 4M).
SHMMIN Minimum size in bytes for a shared memory seg-
ment: implementation dependent (currently 1 byte,
though PAGE_SIZE is the effective minimum size).
SHMMNI System wide maximum number of shared memory seg-
ments: implementation dependent (currently 4096).
The implementation has no specific limits for the per pro-
cess 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 seg-
ment. 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 seg-
ment.
CONFORMING TO
SVr4, SVID. SVr4 documents an additional error condition
EEXIST. Neither SVr4 nor SVID documents an EIDRM condition.
SEE ALSO
ftok(3), ipc(5), shmctl(2), shmat(2), shmdt(2).