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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017, sys_ipc.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 FTOK(3P)
Pages that refer to this page: sys_ipc.h(0p), msgget(3p), semget(3p), shmget(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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