ftok(3)
Name
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(,), semget(,), and shmget() system calls to obtain interprocess communication identifiers. One suggested method for forming a key is to use the ftok, file to key, 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.
The ftok subroutine returns a key based on path and id that is usable in subsequent msgget, semget, and shmget system calls. The path must be the path name of an existing file that is accessible to the process. The 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.
Return Values
The ftok subroutine 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.