4.17 Status of streams

wait_for_input(+ListOfStreams, -ReadyList, +TimeOut)

Wait for input on one of the streams in ListOfStreams and return a list of streams on which input is available in ReadyList. wait_for_input/3 waits for at most TimeOut seconds. Timeout may be specified as a floating point number to specify fractions of a second. If Timeout equals 0, wait_for_input/3 waits indefinitely. This predicate can be used to implement timeout while reading and to handle input from multiple sources. The following example will wait for input from the user and an explicitly opened second terminal. On return, Inputs may hold user or P4 or both.

?- open('/dev/ttyp4', read, P4), wait_for_input([user, P4], Inputs, 0).

This predicate relies on the select() call on most operating systems. On Unix this call is implemented for any stream referring to a file-handle, which implies all OS-based streams: sockets, terminals, pipes, etc. On non-Unix systems select() is generally only implemented for socket-based streams. See also library(socket) from the clib package.

character_count(+Stream, -Count)

Unify Count with the current character index. For input streams this is the number of characters read since the open, for output streams this is the number of characters written. Counting starts at 0.

line_count(+Stream, -Count)

Unify Count with the number of lines read or written. Counting starts at 1.

line_position(+Stream, -Count)

Unify Count with the position on the current line. Note that this assumes the position is 0 after the open. Tabs are assumed to be defined on each 8-th character and backspaces are assumed to reduce the count by one, provided it is positive.

fileerrors(-Old, +New)

Define error behaviour on errors when opening a file for reading or writing. Valid values are the atoms on (default) and off. First Old is unified with the current value. Then the new value is set to New. (27)

With the introduction of exception-handling, it is adviced to use catch/3 to catch possibly file-errors and act accordingly. Note that if fileerrors is off, no exception is generated.