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ftok(3) [freebsd man page]

FTOK(3) 						   BSD Library Functions Manual 						   FTOK(3)

NAME
ftok -- create IPC identifier from path name LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> key_t ftok(const char *path, int id); DESCRIPTION
The ftok() function attempts to create a unique key suitable for use with the msgget(2), semget(2) and shmget(2) functions given the path of an existing file and a user-selectable id. The specified path must specify an existing file that is accessible to the calling process or the call will fail. Also, note that links to files will return the same key, given the same id. RETURN VALUES
The ftok() function will return -1 if path does not exist or if it cannot be accessed by the calling process. SEE ALSO
msgget(2), semget(2), shmget(2) HISTORY
The ftok() function originates with System V and is typically used by programs that use the System V IPC routines. AUTHORS
Thorsten Lockert <tholo@sigmasoft.com> BUGS
The returned key is computed based on the device minor number and inode of the specified path in combination with the lower 8 bits of the given id. Thus it is quite possible for the routine to return duplicate keys. BSD
July 9, 2009 BSD

Check Out this Related Man Page

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

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_t type 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_id is used. The value returned should be different when the (simultaneously existing) files or the project IDs differ. RETURN VALUE
On success the generated key_t value is returned. On failure -1 is returned, with errno indicating the error as for the stat(2) system call. CONFORMING TO
XPG4 NOTES
Under libc4 and libc5 (and under SunOS 4.x) the prototype was key_t ftok(char *pathname, char proj_id); Today proj_id is an int, but still only 8 bits are used. Typical usage has an ASCII character proj_id, that is why the behaviour is said to be undefined when proj_id is zero. Of course no guarantee can be given that the resulting key_t is unique. Typically, a best effort attempt combines the given proj_id byte, the lower 16 bits of the i-node 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/hda1 and files on /dev/sda1. SEE ALSO
ipc(5), msgget(2), semget(2), shmget(2), stat(2) Linux 2.4 2001-11-28 FTOK(3)

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