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#include <sys/ipc.h> key_t ftok(const char *path, int id);
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.
Upon successful completion, ftok() returns a key. Otherwise, ftok() returns (key_t)-1 and sets errno to indicate the error.
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} .
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.
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.
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Standard |
| MT-Level | MT-Safe |
msgget(2), semget(2), shmget(2), stat(2), attributes(5), standards(5)