Manpage of FTOK
FTOKSection: Linux Programmer's Manual (3)
NAMEftok - convert a pathname and a project identifier to a System V IPC key
#include <sys/types.h>#include <sys/ipc.h>
DESCRIPTIONThe 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 VALUEOn success, the generated key_tvalue is returned. On failure -1 is returned, with errnoindicating the error as for the stat(2) system call.
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7).
CONFORMING TOPOSIX.1-2001, POSIX.1-2008.
NOTESOn 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 ALSOmsgget(2), semget(2), shmget(2), stat(2), svipc(7)
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