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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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ftok(3) Library Functions Manual ftok(3)
ftok - convert a pathname and a project identifier to a System V
IPC key
Standard C library (libc, -lc)
#include <sys/ipc.h>
key_t ftok(const char *path, int proj_id);
The ftok() function uses the identity of the file named by the
given path (which must refer to an existing, accessible file) and
the least significant 8 bits of proj_id (which must be nonzero) to
generate a key_t type System V IPC key, suitable for use with
msgget(2), semget(2), or shmget(2).
The resulting value is the same for all pathnames that name the
same file, when the same value of proj_id is used. The value
returned should be different when the (simultaneously existing)
files or the project IDs differ.
On success, the generated key_t value is returned. On failure -1
is returned, with errno indicating the error as for the stat(2)
system call.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ ftok() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
POSIX.1-2001.
On some ancient systems, the prototype was:
key_t ftok(char *path, char proj_id);
Today, proj_id is an int, but still only 8 bits are used. Typical
usage has an ASCII character proj_id, that is why the behavior is
said to be undefined when proj_id is zero.
Of course, no guarantee can be given that the resulting key_t is
unique. Typically, a best-effort attempt combines the given
proj_id byte, the lower 16 bits of the inode number, and the lower
8 bits of the device number into a 32-bit result. Collisions may
easily happen, for example between files on /dev/hda1 and files on
/dev/sda1.
See semget(2).
msgget(2), semget(2), shmget(2), stat(2), sysvipc(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 ftok(3)
Pages that refer to this page: ipcrm(1), msgget(2), semget(2), shmget(2), sysvipc(7), migratepages(8), numactl(8)
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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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