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FTOK(3P) POSIX Programmer's Manual FTOK(3P)
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(), 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:
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
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.
None.
Future versions of this standard may add new interfaces to provide
unique keys.
msgget(3p), semget(3p), shmget(3p)
The Base Definitions volume of POSIX.1‐2008, sys_ipc.h(0p)
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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 FTOK(3P)
Pages that refer to this page: sys_ipc.h(0p), msgget(3p), semget(3p), shmget(3p)