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ftok(3c) [opensolaris man page]

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 num- ber. 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(5) 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(5), standards(5) SunOS 5.11 24 Jul 2002 ftok(3C)

Check Out this Related Man Page

FTOK(3)                                                      Linux Programmer's Manual                                                     FTOK(3)

NAME
ftok - convert a pathname and a project identifier to a System V IPC key SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> key_t ftok(const char *pathname, int proj_id); DESCRIPTION
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. RETURN VALUE
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. ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7). +----------+---------------+---------+ |Interface | Attribute | Value | +----------+---------------+---------+ |ftok() | Thread safety | MT-Safe | +----------+---------------+---------+ CONFORMING TO
POSIX.1-2001, POSIX.1-2008. NOTES
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 ALSO
msgget(2), semget(2), shmget(2), stat(2), svipc(7) COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. GNU 2015-08-08 FTOK(3)

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