ftok(3) - f37

1FTOK(3)                    Linux Programmer's Manual                   FTOK(3)
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NAME

6       ftok  -  convert  a pathname and a project identifier to a System V IPC
7       key
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SYNOPSIS

10       #include <sys/ipc.h>
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12       key_t ftok(const char *pathname, int proj_id);
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DESCRIPTION

15       The ftok() function uses the identity of the file named  by  the  given
16       pathname  (which  must  refer  to an existing, accessible file) and the
17       least significant 8 bits of proj_id (which must be nonzero) to generate
18       a  key_t  type  System  V  IPC  key,  suitable  for use with msgget(2),
19       semget(2), or shmget(2).
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21       The resulting value is the same for all pathnames that  name  the  same
22       file,  when  the  same  value  of  proj_id is used.  The value returned
23       should be different when the (simultaneously  existing)  files  or  the
24       project IDs differ.
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RETURN VALUE

27       On  success,  the  generated key_t value is returned.  On failure -1 is
28       returned, with errno indicating the error as  for  the  stat(2)  system
29       call.
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ATTRIBUTES

32       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
33       tributes(7).
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35       ┌────────────────────────────────────────────┬───────────────┬─────────┐
36       │Interface                                   │ Attribute     │ Value   │
37       ├────────────────────────────────────────────┼───────────────┼─────────┤
38       │ftok()                                      │ Thread safety │ MT-Safe │
39       └────────────────────────────────────────────┴───────────────┴─────────┘
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CONFORMING TO

42       POSIX.1-2001, POSIX.1-2008.
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NOTES

45       On some ancient systems, the prototype was:
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47           key_t ftok(char *pathname, char proj_id);
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49       Today, proj_id is an int, but still only 8 bits are used.  Typical  us‐
50       age has an ASCII character proj_id, that is why the behavior is said to
51       be undefined when proj_id is zero.
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53       Of course, no guarantee can  be  given  that  the  resulting  key_t  is
54       unique.   Typically,  a  best-effort attempt combines the given proj_id
55       byte, the lower 16 bits of the inode number, and the lower  8  bits  of
56       the  device number into a 32-bit result.  Collisions may easily happen,
57       for example between files on /dev/hda1 and files on /dev/sda1.
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EXAMPLES

60       See semget(2).
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COLOPHON

66       This page is part of release 5.13 of the Linux  man-pages  project.   A
67       description  of  the project, information about reporting bugs, and the
68       latest    version    of    this    page,    can     be     found     at
69       https://www.kernel.org/doc/man-pages/.
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73GNU                               2021-03-22                           FTOK(3)