The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


ftok - generate an IPC key


#include <sys/ipc.h>

key_t ftok(const char *path, int id);


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.


Upon successful completion, ftok() returns a key. Otherwise, ftok() returns (key_t)-1 and sets errno to indicate the error.


The ftok() function will fail if:
Search permission is denied for a component of the path prefix.
Too many symbolic links were encountered in resolving path.
The length of the path argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.
A component of path does not name an existing file or path is an empty string.
A component of the path prefix is not a directory.

The ftok() function may fail if:

Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}.




For maximum portability, id should be a single-byte character.




msgget(), semget(), shmget(), <sys/ipc.h>.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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