stdipc(3C)             UNIX System V(C Development Set)              stdipc(3C)


NAME
      stdipc: ftok - standard interprocess communication package

SYNOPSIS
      #include <sys/types.h>
      #include <sys/ipc.h>

      keyt ftok(const char *path, int id);

DESCRIPTION
      All interprocess communication facilities require the user to supply a
      key to be used by the msgget(2), semget(2), and shmget(2) system calls to
      obtain interprocess communication identifiers.  One suggested method for
      forming a key is to use the ftok subroutine described below.  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 interface 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.

      ftok returns a key based on path and id that is usable in subsequent
      msgget, semget, and shmget system calls.  path must be the path name of
      an existing file that is accessible to the process.  id is a character
      that uniquely identifies a project.  Note that ftok will return the same
      key for linked files when called with the same id and that it will return
      different keys when called with the same file name but different ids.

SEE ALSO
      intro(2), msgget(2), semget(2), shmget(2).

DIAGNOSTICS
      ftok returns (keyt) -1 if path does not exist or if it is not accessible
      to the process.

NOTES
      If the file whose path is passed to ftok is removed when keys still refer
      to the file, future calls to ftok with the same path and id will return
      an error.  If the same file is recreated, then ftok is likely to return a
      different key than it did the original time it was called.











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