Manpage of FTOK

FTOK

Section: Linux Programmer's Manual (3)
Updated: 2015-08-08
Index
 

NAME

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

SYNOPSIS

#include <sys/types.h>#include <sys/ipc.h>

key_t ftok(const char *pathname, int proj_id); 

DESCRIPTION

The ftok() function uses the identity of the file named by the given pathname(which must refer to an existing, accessible file) and the least significant 8 bits of proj_id(which must be nonzero) to generate a key_ttype System V IPC key, suitable for use with msgget(2), semget(2), or shmget(2).

The resulting value is the same for all pathnames that name the same file, when the same value of proj_idis used. The value returned should be different when the (simultaneously existing) files or the project IDs differ.  

RETURN VALUE

On success, the generated key_tvalue is returned. On failure -1 is returned, with errnoindicating the error as for the stat(2) system call.  

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).
InterfaceAttributeValue
ftok() Thread safetyMT-Safe
 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008.  

NOTES

On some ancient systems, the prototype was:

key_t ftok(char *pathname, char proj_id);

Today, proj_idis an int, but still only 8 bits are used. Typical usage has an ASCII character proj_id, that is why the behavior is said to be undefined when proj_idis zero.

Of course, no guarantee can be given that the resulting key_tis unique. Typically, a best-effort attempt combines the given proj_idbyte, the lower 16 bits of the inode number, and the lower 8 bits of the device number into a 32-bit result. Collisions may easily happen, for example between files on /dev/hda1and files on /dev/sda1.  

SEE ALSO

msgget(2), semget(2), shmget(2), stat(2), svipc(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ATTRIBUTES
CONFORMING TO
NOTES
SEE ALSO

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