STDIPC(3C)                           SysV                           STDIPC(3C)



NAME
     stdipc: ftok - standard interprocess communication package

SYNOPSIS
     #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(2), semget(2), and shmget(2) 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 pathname 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 filename but different ids.

SEE ALSO
     intro(2), msgget(2), semget(2), shmget(2).

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.