NAME
ftok - generate an IPC key
SYNOPSIS
#include <sys/ipc.h>
key_t ftok(const char *path, int id);
DESCRIPTION
The ftok() function shall return a key based on path and id that is
usable in subsequent calls to msgget(), semget(), and shmget(). The
application shall ensure that the path argument is the pathname of an
existing file that the process is able to stat().
The ftok() function shall return the same key value for all paths that
name the same file, when called with the same id value, and return
different key values when called with different id values or with paths
that name different files existing on the same file system at the same
time. It is unspecified whether ftok() shall return the same key value
when called again after the file named by path is removed and recreated
with the same name.
Only the low-order 8-bits of id are significant. The behavior of ftok()
is unspecified if these bits are 0.
RETURN VALUE
Upon successful completion, ftok() shall return a key. Otherwise,
ftok() shall return (key_t)-1 and set errno to indicate the error.
ERRORS
The ftok() function shall fail if:
EACCES Search permission is denied for a component of the path prefix.
ELOOP A loop exists in symbolic links encountered during resolution of
the path argument.
ENAMETOOLONG
The length of the path argument exceeds {PATH_MAX} or a pathname
component is longer than {NAME_MAX}.
ENOENT A component of path does not name an existing file or path is an
empty string.
ENOTDIR
A component of the path prefix is not a directory.
The ftok() function may fail if:
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path argument.
ENAMETOOLONG
Pathname resolution of a symbolic link produced an intermediate
result whose length exceeds {PATH_MAX}.
The following sections are informative.
EXAMPLES
Getting an IPC Key
The following example gets a unique key that can be used by the IPC
functions semget(), msgget(), and shmget(). The key returned by ftok()
for this example is based on the ID value S and the pathname /tmp.
#include <sys/ipc.h>
...
key_t key;
char *path = "/tmp";
int id = ’S’;
key = ftok(path, id);
Saving an IPC Key
The following example gets a unique key based on the pathname /tmp and
the ID value a. It also assigns the value of the resulting key to the
semkey variable so that it will be available to a later call to
semget(), msgget(), or shmget().
#include <sys/ipc.h>
...
key_t semkey;
if ((semkey = ftok("/tmp", ’a’)) == (key_t) -1) {
perror("IPC error: ftok"); exit(1);
}
APPLICATION USAGE
For maximum portability, id should be a single-byte character.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
msgget() , semget() , shmget() , the Base Definitions volume of
IEEE Std 1003.1-2001, <sys/ipc.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .