FTOK(3C) Standard C Library Functions FTOK(3C)

NAME

ftok - generate an IPC key

SYNOPSIS

#include <sys/ipc.h>

key_t ftok(const char *path, int id);

DESCRIPTION

The ftok() function returns a key based on path and id that is usable in
subsequent calls to msgget(2), semget(2) and shmget(2). The path argument
must be the pathname of an existing file that the process is able to
stat(2).


The ftok() function will return the same key value for all paths that
name the same file, when called with the same id value, and will return
different key values when called with different id values.


If the file named by path is removed while still referred to by a key, a
call to ftok() with the same path and id returns an error. If the same
file is recreated, then a call to ftok() with the same path and id is
likely to return a different key.


Only the low order 8-bits of id are significant. The behavior of ftok()
is unspecified if these bits are 0.

RETURN VALUES

Upon successful completion, ftok() returns a key. Otherwise, ftok()
returns (key_t)-1 and sets errno to indicate the error.

ERRORS

The ftok() function will fail if:

EACCES
Search permission is denied for a component of the path
prefix.


ELOOP
Too many symbolic links were encountered in resolving
path.


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:

ENAMETOOLONG
Pathname resolution of a symbolic link produced an
intermediate result whose length exceeds {PATH_MAX}.

USAGE

For maximum portability, id should be a single-byte character.


Another way to compose keys is to include the project ID in the most
significant byte and to use the remaining portion as a sequence number.
There are many other ways to form keys, but it is necessary for each
system to define standards for forming them. If some standard is not
adhered to, it will be possible for unrelated processes to
unintentionally interfere with each other's operation. It is still
possible to interfere intentionally. Therefore, it is strongly suggested
that the most significant byte of a key in some sense refer to a project
so that keys do not conflict across a given system.

NOTES

Since the ftok() function returns a value based on the id given and the
file serial number of the file named by path in a type that is no longer
large enough to hold all file serial numbers, it may return the same key
for paths naming different files on large filesystems.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO

msgget(2), semget(2), shmget(2), stat(2), attributes(7), standards(7)

February 17, 2023 FTOK(3C)