FTOK(3P) POSIX Programmer's Manual FTOK(3P)PROLOG
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.
NAMEftok — generate an IPC key
SYNOPSIS
#include <sys/ipc.h>
key_t ftok(const char *path, int id);
DESCRIPTION
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.
RETURN VALUE
Upon successful completion, ftok() shall return a key. Otherwise,
ftok() shall return (key_t)−1 and set errno to indicate the error.
ERRORS
The ftok() function shall fail if:
EACCES Search permission is denied for a component of the path prefix.
EIO An error occurred while reading from the file system.
ELOOP A loop exists in symbolic links encountered during resolution of
the path argument.
ENAMETOOLONG
The length of a component of a pathname is longer than
{NAME_MAX}.
ENOENT A component of path does not name an existing file or path is an
empty string.
ENOTDIR
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:
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path argument.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
tion of a symbolic link produced an intermediate result with a
length that exceeds {PATH_MAX}.
The following sections are informative.
EXAMPLES
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);
}
APPLICATION USAGE
For maximum portability, id should be a single-byte character.
Applications should not assume that the resulting key value is unique.
RATIONALE
None.
FUTURE DIRECTIONS
Future versions of this standard may add new interfaces to provide
unique keys.
SEE ALSOmsgget(), semget(), shmget()
The Base Definitions volume of POSIX.1‐2008, <sys_ipc.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 FTOK(3P)
