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A Generalized SysV Resource ID Creation - ftok()


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A Generalized SysV Resource ID Creation - ftok()


One of the problems with SysV IPC methods is the need to choose a unique identifier for our processes. How can we make sure that the identifier of a semaphore in our project won't collide with the identifier of a semaphore in some other program installed on the system?

A D V E R T I S E M E N T

To help with that, the ftok() system call was introduced. This system call accepts two parameters, a path to a file and a character, and generates a more-or-less unique identifier. It does that by finding the "i-node" number of the file (more or less the number of the disk sector containing this file's information), combines it with the second parameter, and thus generates an identifier, that can be later fed to semget, shmget() or msgget(). Here is how to use ftok():

/* identifier returned by ftok() */
key_t set_key;

/* generate a "unique" key for our set, using the */
/* directory "/usr/local/lib/ourprojectdir".      */
set_key = ftok("/usr/local/lib/ourprojectdir", 'a');
if (set_key == -1) {
    perror("ftok: ");
    exit(1);
}

/* now we can use 'set_key' to generate a set id, for example. */
sem_set_id = semget(set_key, 1, IPC_CREAT | 0600);
.
.

One note should be taken: if we remove the file and then re-create it, the system is very likely to allocate a new disk sector for this file, and thus activating the same ftok call with this file will generate a different key. Thus, the file used should be a steady file, and not one that is likely to be moved to a different disk or erased and re-created.



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Keywords: A Generalized SysV Resource ID Creation - ftok(), Inter-Process Communications, Inter-Process Communications, Inter-Process Communications tutorial, Inter-Process Communications tutorial pdf, history of Inter-Process Communications, Custamizing Style Sheet, learn Inter-Process Communications

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