Provided by: freebsd-manpages_8.2-1_all
shmget — obtain a shared memory identifier
Standard C Library (libc, -lc)
shmget(key_t key, size_t size, int flag);
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:
· IPC_PRIVATE may be specified, in which case a new IPC object will be
· 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.
· The ftok(3) may be used to generate a key from a pathname.
The mode of a newly created IPC object is determined by OR'ing the
following constants into the flag argument:
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).
Upon successful completion, shmget() returns the positive integer
identifier of a shared memory segment. Otherwise, -1 is returned and
errno set to indicate the error.
The shmget() system call 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 maximum.
[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.
shmat(2), shmctl(2), shmdt(2), ftok(3)