stdipc(S)

stdipc: ftok -- standard interprocess communication package

Synopsis

   #include <sys/types.h>
   #include <sys/ipc.h>
   
   key_t ftok(const char path, int 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. It is still possible to interface intentionally. 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 that 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.

Errors

ftok returns (key_t) -1 if path does not exist or if it is not accessible to the process.

References

intro(S), msgget(S), semget(S), shmget(S)

Notices

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.