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FTOK(3) | Linux Programmer's Manual | FTOK(3) |
NAME¶
ftok - convert a pathname and a project identifier to a System V IPC keySYNOPSIS¶
#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¶
Multithreading (see pthreads(7))¶
The ftok() function is thread-safe.CONFORMING TO¶
POSIX.1-2001.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 3.74 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 http://www.kernel.org/doc/man-pages/.2014-08-19 | GNU |