shmget(2)shmget(2)NAMEshmget - get shared memory segment identifier
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);
DESCRIPTIONshmget returns the shared memory identifier associated with key.
A shared memory identifier and associated data structure and shared
memory segment of at least size bytes [see intro(2)] are created for key
if one of the following are true:
key is equal to IPC_PRIVATE.
key does not already have a shared memory identifier associated with
it, and (shmflg&IPC_CREAT) is true.
Upon creation, the contents of the shared memory segment are initialized
to zero, and the data structure associated with the new shared memory
identifier is initialized as follows:
shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and shm_perm.gid are set
equal to the effective user ID and effective group ID, respectively,
of the calling process.
The access permission bits of shm_perm.mode are set equal to the
access permission bits of shmflg. shm_segsz is set equal to the
value of size.
shm_lpid, shm_nattch shm_atime, and shm_dtime are set equal to 0.
shm_ctime is set equal to the current time.
If the shmflg IPC_AUTORESRV is true, logical swap space is
automatically reserved as each page is first referenced with a load
or store operation instead of when the shared memory segment is
created. When using this option, no logical swap space is reserved
when the shared memory segment is created. Therefore, the system
cannot guarantee that space will be available when needed. If all
the logical swap space has been taken by other processes when a page
in an IPC_AUTORESRV segment is first stored to, then the process
will be sent a SIGBUS. (See swap(1m) for a discussion of logical
swap space.)
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shmget(2)shmget(2)shmget fails if one or more of the following are true:
EINVAL size is less than the system-imposed minimum or greater
than the system-imposed maximum.
EACCES A shared memory identifier exists for key but operation
permission [see intro(2)] as specified by the low-order 9
bits of shmflg would not be granted.
EINVAL A shared memory identifier exists for key but the size of
the segment associated with it is less than size and size
is not equal to zero.
ENOENT A shared memory identifier does not exist for key and
(shmflg&IPC_CREAT) is false.
ENOSPC A shared memory identifier is to be created but the
system-imposed limit on the maximum number of allowed
shared memory identifiers system wide would be exceeded.
ENOMEM A shared memory identifier and associated shared memory
segment are to be created but the amount of available
memory is not sufficient to fill the request.
EEXIST A shared memory identifier exists for key but both
(shmflg&IPC_CREAT) and (shmflg&IPC_EXCL) are true.
SEE ALSOintro(2), shmctl(2), shmop(2), stdipc(3C), ipcs(1), ipcrm(1), swap(1m).
DIAGNOSTICS
Upon successful completion, a non-negative integer, namely a shared
memory identifier is returned. Otherwise, a value of -1 is returned and
errno is set to indicate the error.
NOTES
The user must explicitly remove shared memory segments after the last
reference to them has been removed.
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