The ftok() function returns a key based on path and id that is usable in subsequent calls to msgget(), semget() and shmget(). The path argument must be the pathname of an existing file that the process is able to stat().
The ftok() function will return the same key value for all paths that name the same file, when called with the same id value, and will return different key values when called with different id values or with paths that name different files existing on the same file system at the same time. It is unspecified whether ftok() returns the same key value when called again after the file named by path is removed and recreated with the same name.
Only the low order 8-bits of id are significant. The behaviour of ftok() is unspecified if these bits are 0.