ftok (3p) - Linux Manuals

ftok: generate an IPC key


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


ftok - generate an IPC key


#include <sys/ipc.h>

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


The ftok() function shall return a key based on path and id that is usable in subsequent calls to msgget(), semget(), and shmget(). The application shall ensure that the path argument is the pathname of an existing file that the process is able to stat().

The ftok() function shall return the same key value for all paths that name the same file, when called with the same id value, and 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() shall return 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 behavior of ftok() is unspecified if these bits are 0.


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


The ftok() function shall fail if:

Search permission is denied for a component of the path prefix.
A loop exists in symbolic links encountered during resolution of the path argument.
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:

More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}.

The following sections are informative.


Getting an IPC Key

The following example gets a unique key that can be used by the IPC functions semget(), msgget(), and shmget(). The key returned by ftok() for this example is based on the ID value S and the pathname /tmp.

#include <sys/ipc.h>
key_t key;
char *path = "/tmp";
int id = 'S';

key = ftok(path, id);

Saving an IPC Key

The following example gets a unique key based on the pathname /tmp and the ID value a. It also assigns the value of the resulting key to the semkey variable so that it will be available to a later call to semget(), msgget(), or shmget().

#include <sys/ipc.h>
key_t semkey;

if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
    perror("IPC error: ftok"); exit(1);


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






Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .


msgget(), semget(), shmget(), the Base Definitions volume of IEEE Std 1003.1-2001, <sys/ipc.h>