man3/ftok.3

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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  23 .\" Modified 2001-11-28, by Michael Kerrisk, <mtk.manpages@gmail.com>
  24 .\"     Changed data type of proj_id; minor fixes
  25 .\"     aeb: further fixes; added notes.
  26 .\"
  27 .TH FTOK 3 2001-11-28 "GNU" "Linux Programmer's Manual"
  28 .SH NAME
  29 ftok \- convert a pathname and a project identifier to a System V IPC key
  30 .SH SYNOPSIS
  31 .nf
  32 .B
  33 #include <sys/types.h>
  34 .B
  35 #include <sys/ipc.h>
  36 .fi
  37 .sp
  38 .BI "key_t ftok(const char *" pathname ", int " proj_id );
  39 .SH DESCRIPTION
  40 The
  41 .BR ftok ()
  42 function uses the identity of the file named by the given
  43 .I pathname
  44 (which must refer to an existing, accessible file)
  45 and the least significant 8 bits of
  46 .I proj_id
  47 (which must be nonzero) to generate a
  48 .I key_t
  49 type System V IPC key, suitable for use with
  50 .BR msgget (2),
  51 .BR semget (2),
  52 or
  53 .BR shmget (2).
  54 .LP
  55 The resulting value is the same for all pathnames that
  56 name the same file, when the same value of
  57 .I proj_id
  58 is used.
  59 The value returned should be different when the
  60 (simultaneously existing) files or the project IDs differ.
  61 .SH "RETURN VALUE"
  62 On success the generated
  63 .I key_t
  64 value is returned.
  65 On failure \-1 is returned, with
  66 .I errno
  67 indicating the error as for the
  68 .BR stat (2)
  69 system call.
  70 .SH "CONFORMING TO"
  71 POSIX.1-2001.
  72 .SH NOTES
  73 Under libc4 and libc5 (and under SunOS 4.x) the prototype was:
  74 .sp
  75 .RS
  76 .BI "key_t ftok(char *" pathname ", char " proj_id );
  77 .RE
  78 .PP
  79 Today
  80 .I proj_id
  81 is an
  82 .IR int ,
  83 but still only 8 bits are used.
  84 Typical usage has an ASCII character
  85 .IR proj_id ,
  86 that is why the behavior is said to be undefined when
  87 .I proj_id
  88 is zero.
  89 .LP
  90 Of course no guarantee can be given that the resulting
  91 .I key_t
  92 is unique.
  93 Typically, a best effort attempt combines the given
  94 .I proj_id
  95 byte, the lower 16 bits of the i-node number, and the
  96 lower 8 bits of the device number into a 32-bit result.
  97 Collisions may easily happen, for example between files on
  98 .I /dev/hda1
  99 and files on
 100 .IR /dev/sda1 .
 101 .SH "SEE ALSO"
 102 .BR msgget (2),
 103 .BR semget (2),
 104 .BR shmget (2),
 105 .BR stat (2),
 106 .BR svipc (7)