STDIPC(S) UNIX System V STDIPC(S)
Name
stdipc: ftok - standard interprocess communication package
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
key_t ftok(path, id)
char *path;
char id;
Description
All interprocess communication facilities require the user
to supply a key to be used by the msgget(S), semget(S), and
shmget(S) system calls to obtain interprocess communication
identifiers. One suggested method for forming a key is to
use the ftok subroutine described below. Another way to
compose keys is to include the project ID in the most
significant byte and to use the remaining portion as a
sequence number. There are many other ways to form keys,
but it is necessary for each system to define standards for
forming them. If some standard is not adhered to, it will
be possible for unrelated processes to unintentionally
interfere with each other's operation. Therefore, it is
strongly suggested that the most significant byte of a key
in some sense refer to a project so that keys do not
conflict across a given system.
ftok returns a key based on path and id that is usable in
subsequent msgget, semget, and shmget system calls. path
must be the path name of an existing file that is accessible
to the process. id is a character which uniquely identifies
a project. Note that ftok will return the same key for
linked files when called with the same id, and that it will
return different keys when called with the same file name
but different ids.
See Also
intro(S), msgget(S), semget(S), shmget(S)
Diagnostics
ftok returns (key_t) -1 if path does not exist or if it is
not accessible to the process.
Warning
If the file whose path is passed to ftok is removed when
keys still refer to the file, future calls to ftok with the
same path and id will return an error. If the same file is
recreated, then ftok is likely to return a different key
than it did the original time it was called.
(printed 6/20/89)