FTOK(3P)

💖 FTOK(3P) — the POSIX Programmer's Manual, bestie edition 💖

okay so this one lives in the POSIX Programmer's Manual, and I need you to know something before we even start:

⚠️ The Linux implementation of this interface may differ — so go check the corresponding Linux manual page for the deets on Linux behavior. Or honestly? The interface might not even be implemented on Linux at all. Rude, but true.

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✨ 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.

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🌸 NAME 🌸

ftok — generate an IPC key 💅

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🎀 SYNOPSIS 🎀

#include <sys/ipc.h>

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

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🦋 DESCRIPTION 🦋

so the ftok() function shall return a key based on path and id that's usable in subsequent calls to msgget(), semget(), and shmget(). That's the whole job. Making keys so your IPC stuff can find each other. iconic behavior honestly.

Your application shall ensure that the path argument is the pathname of an existing file that the process is able to stat() — with one exception, and it's a cute one: if stat() would fail with [EOVERFLOW] because of file size, ftok() shall still succeed. We love a forgiving queen. 👑

Here's the vibe:

• ftok() shall return the same key value for all paths that name the same file, when it's called with the same id value.
• It 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. So don't come for it. 🫠

And the spicy bit: only the low-order 8-bits of id are significant. The behavior of ftok() is unspecified if these bits are 0. So like… don't do that. fr fr.

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💜 RETURN VALUE 💜

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

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🔥 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 resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.

🌈 heads up bestie: the following sections are informative. They're the fun bonus content, not the binding rules.

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⭐ 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'll be available to a later call to semget(), msgget(), or shmget(). Look at her go. 👑

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

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

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🌈 APPLICATION USAGE 🌈

• For maximum portability, id should be a single-byte character. Simple. Elegant. Do that. ✨
• Applications should not assume that the resulting key value is unique. Like, it might be! But don't bet your whole personality on it. 🙈

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💀 RATIONALE 💀

None.

(i know. i know. literally nothing. moving on. 💅)

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🧚 FUTURE DIRECTIONS 🧚

Future versions of this standard may add new interfaces to provide unique keys. so watch this space i guess 👀

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💖 SEE ALSO 💖

• msgget()
• semget()
• shmget()

The Base Definitions volume of POSIX.1‐2017, <sys_ipc.h>

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👑 COPYRIGHT 👑

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.

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okay that's the whole page, bestie — mwah 💋 this edition was brought to you by IEEE/The Open Group, dated 2017, filed under FTOK(3P). go make some keys. slay. 🫶