mirror of https://github.com/mkerrisk/man-pages
105 lines
3.0 KiB
Groff
105 lines
3.0 KiB
Groff
.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" Modified 2001-11-28, by Michael Kerrisk, <mtk.manpages@gmail.com>
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.\" Changed data type of proj_id; minor fixes
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.\" aeb: further fixes; added notes.
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.\"
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.TH FTOK 3 2001-11-28 "GNU" "Linux Programmer's Manual"
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.SH NAME
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ftok \- convert a pathname and a project identifier to a System V IPC key
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.SH SYNOPSIS
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.nf
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.B #include <sys/types.h>
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.B #include <sys/ipc.h>
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.fi
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.sp
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.BI "key_t ftok(const char *" pathname ", int " proj_id );
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.SH DESCRIPTION
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The
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.BR ftok ()
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function uses the identity of the file named by the given
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.I pathname
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(which must refer to an existing, accessible file)
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and the least significant 8 bits of
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.I proj_id
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(which must be non-zero) to generate a
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.I key_t
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type System V IPC key, suitable for use with
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.BR msgget (2),
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.BR semget (2),
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or
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.BR shmget (2).
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.LP
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The resulting value is the same for all pathnames that
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name the same file, when the same value of
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.I proj_id
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is used.
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The value returned should be different when the
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(simultaneously existing) files or the project IDs differ.
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.SH "RETURN VALUE"
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On success the generated
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.I key_t
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value is returned.
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On failure \-1 is returned, with
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.I errno
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indicating the error as for the
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.BR stat (2)
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system call.
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.SH "CONFORMING TO"
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POSIX.1-2001.
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.SH NOTES
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Under libc4 and libc5 (and under SunOS 4.x) the prototype was:
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.sp
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.RS
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.BI "key_t ftok(char *" pathname ", char " proj_id );
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.RE
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.PP
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Today
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.I proj_id
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is an
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.IR int ,
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but still only 8 bits are used.
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Typical usage has an ASCII character
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.IR proj_id ,
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that is why the behavior is said to be undefined when
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.I proj_id
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is zero.
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.LP
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Of course no guarantee can be given that the resulting
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.I key_t
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is unique.
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Typically, a best effort attempt combines the given
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.I proj_id
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byte, the lower 16 bits of the inode number, and the
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lower 8 bits of the device number into a 32-bit result.
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Collisions may easily happen, for example between files on
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.I /dev/hda1
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and files on
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.IR /dev/sda1 .
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.SH "SEE ALSO"
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.BR msgget (2),
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.BR semget (2),
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.BR shmget (2),
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.BR stat (2),
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.BR svipc (7)
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