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 path name of an existing file that is accessible to the process. ID is a character which uniquely identifies a project. Note: 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.