FTOK(3C)FTOK(3C)NAMEftok - 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 unintention‐
ally 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(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Standard │
├────────────────────┼─────────────────┤
│MT-Level │ MT-Safe │
└────────────────────┴─────────────────┘
SEE ALSOmsgget(2), semget(2), shmget(2), stat(2), attributes(5), standards(5)
Jul 24, 2002 FTOK(3C)
