ftok - convert a pathname and a project identifier to a System V IPC key
#include <sys/types.h>
#include <sys/ipc.h>
key_t ftok(const char *pathname, int
proj_id);
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.
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.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| ftok () |
Thread safety |
MT-Safe |
POSIX.1-2001, POSIX.1-2008.
On some ancient systems, 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 behavior 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 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/hda1 and files on
/dev/sda1.
See
semget(2).
msgget(2),
semget(2),
shmget(2),
stat(2),
sysvipc(7)