SHMGET(2) BSD Programmer's Manual SHMGET(2)NAME
shmget - obtain a shared memory identifier
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int
shmget(key_t key, int size, int flag);
DESCRIPTION
Based on the values of key and flag, shmget() returns the identifier of a
newly created or previously existing shared memory segment. The key is
analogous to a filename: it provides a handle that names an IPC object.
There are three ways to specify a key:
o IPC_PRIVATE may be specified, in which case a new IPC object will be
created.
o An integer constant may be specified. If no IPC object corresponding
to key is specified and the IPC_CREAT bit is set in flag, a new one
will be created.
o ftok() may be used to generate a key from a pathname. See ftok(3).
The mode of a newly created IPC object is determined by OR'ing the fol-
lowing constants into the flag parameter:
SHM_R Read access for user.
SHM_W Write access for user.
(SHM_R>>3) Read access for group.
(SHM_W>>3) Write access for group.
(SHM_R>>6) Read access for other.
(SHM_W>>6) Write access for other.
When creating a new shared memory segment, size indicates the desired
size of the new segment in bytes. The size of the segment may be rounded
up to a multiple convenient to the kernel (i.e., the page size).
RETURN VALUES
Upon successful completion, shmget() returns the positive integer identi-
fier of a shared memory segment. Otherwise, -1 is returned and errno set
to indicate the error.
ERRORSShmget() will fail if:
[EINVAL] Size specified is greater than the size of the previously
existing segment. Size specified is less than the system
imposed minimum, or greater than the system imposed maxi-
mum.
[ENOENT] No shared memory segment was found matching key, and
IPC_CREAT was not specified.
[ENOSPC] The kernel was unable to allocate enough memory to satisfy
the request.
[EEXIST] IPC_CREAT and IPC_EXCL were specified, and a shared memory
segment corresponding to key already exists.
SEE ALSOshmctl(2), shmat(2), shmdt(2), ftok(3)4th Berkeley Distribution July 3, 1996 2