The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


ftok - generate an IPC key


[XSI] [Option Start] #include <sys/ipc.h>

key_t ftok(const char *
path, int id); [Option End]


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(), with the exception that if stat() would fail with [EOVERFLOW] due to file size, ftok() shall still succeed.

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 should 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.
An error occurred while reading from the file system.
A loop exists in symbolic links encountered during resolution of the path argument.
The length of a component of a pathname 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 names an existing file that is neither a directory nor a symbolic link to a directory, or the path argument contains at least one non- <slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory.

The ftok() function may fail if:

More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.

The following sections are informative.


Getting an IPC Key

The following example gets a 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.

Applications should not assume that the resulting key value is unique.




Future versions of this standard may add new interfaces to provide unique keys.


msgget, semget, shmget

XBD <sys/ipc.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Issue 6

The normative text is updated to avoid use of the term "must" for application requirements.

The wording of the mandatory [ELOOP] error condition is updated, and a second optional [ELOOP] error condition is added.

Issue 7

Austin Group Interpretation 1003.1-2001 #143 is applied.

The [ENOTDIR] error condition is clarified to cover the condition where the last component of a pathname exists but is not a directory or a symbolic link to a directory.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0209 [343], XSH/TC1-2008/0210 [366], XSH/TC1-2008/0211 [343], XSH/TC1-2008/0212 [324], XSH/TC1-2008/0213 [366], XSH/TC1-2008/0214 [366], XSH/TC1-2008/0215 [366], and XSH/TC1-2008/0216 [366] are applied.

End of informative text.


return to top of page

UNIX ® is a registered Trademark of The Open Group.
POSIX ™ is a Trademark of The IEEE.
Copyright © 2001-2018 IEEE and The Open Group, All Rights Reserved
[ Main Index | XBD | XSH | XCU | XRAT ]