ftok(3) - f39

1ftok(3)                    Library Functions 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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LIBRARY

10       Standard C library (libc, -lc)
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SYNOPSIS

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

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

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

35       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
36       tributes(7).
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38       ┌────────────────────────────────────────────┬───────────────┬─────────┐
39       │Interface                                   │ Attribute     │ Value   │
40       ├────────────────────────────────────────────┼───────────────┼─────────┤
41       │ftok()                                      │ Thread safety │ MT-Safe │
42       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

45       POSIX.1-2008.
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HISTORY

48       POSIX.1-2001.
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NOTES

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

66       See semget(2).
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