ftok(3) - c6

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

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

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

33       POSIX.1-2001.
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NOTES

36       Under libc4 and libc5 (and under SunOS 4.x) the prototype was:
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38              key_t ftok(char *pathname, char proj_id);
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40       Today proj_id is an int, but still only 8 bits are used.  Typical usage
41       has an ASCII character proj_id, that is why the behavior is said to  be
42       undefined when proj_id is zero.
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44       Of course no guarantee can be given that the resulting key_t is unique.
45       Typically, a best effort attempt combines the given proj_id  byte,  the
46       lower  16  bits of the inode number, and the lower 8 bits of the device
47       number into a 32-bit result.  Collisions may easily happen, for example
48       between files on /dev/hda1 and files on /dev/sda1.
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COLOPHON

54       This  page  is  part of release 3.22 of the Linux man-pages project.  A
55       description of the project, and information about reporting  bugs,  can
56       be found at http://www.kernel.org/doc/man-pages/.
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60GNU                               2001-11-28                           FTOK(3)