noble (3) ftok.3.gz

NAME

       ftok - convert a pathname and a project identifier to a System V IPC key

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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 │
       └──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       POSIX.1-2008.

HISTORY

       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.

EXAMPLES

       See semget(2).

SEE ALSO

       msgget(2), semget(2), shmget(2), stat(2), sysvipc(7)