Manpages - ftok.3

ftok - convert a pathname and a project identifier to a System V IPC key

  #include <sys/ipc.h>

key_t ftok(const char **/pathname/, int proj_id);*

The *ftok*() function uses the identity of the file named by the given pathname (which must refer to an existing, accessible file) and the least significant 8 bits of proj_id (which must be nonzero) to generate a key_t type System V IPC key, suitable for use with *msgget*(2), *semget*(2), or *shmget*(2).

The resulting value is the same for all pathnames that name the same file, when the same value of proj_id is used. The value returned should be different when the (simultaneously existing) files or the project IDs differ.

On success, the generated key_t value is returned. On failure -1 is returned, with errno indicating the error as for the *stat*(2) system call.

For an explanation of the terms used in this section, see *attributes*(7).

POSIX.1-2001, POSIX.1-2008.

On some ancient systems, the prototype was:

  key_t ftok(char *pathname, char proj_id);

Today, proj_id is an int, but still only 8 bits are used. Typical usage has an ASCII character proj_id, that is why the behavior is said to be undefined when proj_id is zero.

Of course, no guarantee can be given that the resulting key_t is unique. Typically, a best-effort attempt combines the given proj_id byte, the lower 16 bits of the inode number, and the lower 8 bits of the device number into a 32-bit result. Collisions may easily happen, for example between files on /dev/hda1 and files on /dev/sda1.

See *semget*(2).

*msgget*(2), *semget*(2), *shmget*(2), *stat*(2), *sysvipc*(7)

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