table of contents
- jessie 3.74-1
- jessie-backports 4.10-2~bpo8+1
- stretch 4.10-2
- testing 4.16-1
- stretch-backports 4.16-1~bpo9+1
- unstable 4.16-1
|FTOK(3)||Linux Programmer's Manual||FTOK(3)|
NAME¶ftok - convert a pathname and a project identifier to a System V IPC key
#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.
Multithreading (see pthreads(7))¶The ftok() function is thread-safe.
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/.