c - Whats is purpose ftok in Message queues -


i have started reading message queues 1 of ipc mechanism on linux .but @ first step have basic questions.

  1. use of ftok() generate unique id (key) , unique id generated.

  2. can't use simple number our keys rather using ftok()?

  3. what purpose of argument key in msget function?

    #include "sys/msg.h" key = ftok("/home/beej/somefile", 'b'); msqid = msgget(key, 0666 | ipc_creat);

  4. what difference between msqid , key?

the ftok function creates sort of identifier used system v ipc functions (semget, shmget, msgget). think of filedescriptor: when open file, pass path open , number in return used read , write identify file. ftok function serves similar purpose, while filedescriptor's scope limited process called open (and children), ftok token valid across system.

the reason system scope want 2 or more independent processes have access same ipc resources. if have 2 programs, both of execute key = ftok("/home/beej/somefile", 'b');, both same token , can therefor access same resources (semaphores, shared memory, message queues). that's whole point of inter process communication.

you cannot use "simple number" don't know whether token might example index system-internal table or something. in other words, don't know how token used internally need use ftok.

the man page says: "the specified path must specify existing file accessible calling process or call fail. also, note links files return same key, given same id." assume @ least ftok implementations create token looking inode number of file specified path , combine second argument create token. second argument exists can create bunch of ipc resources (like several semaphores protect different resources).

as difference of key_t (the value returned ftok) , value retured msgget: former gives access bunch of ipc resources (semaphore, shared memory , message queue), while later identifies specific message queue.


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