- Reference manual
- Built-in Predicates
- Notation of Predicate Descriptions
- Character representation
- Loading Prolog source files
- Editor Interface
- Verify Type of a Term
- Comparison and Unification of Terms
- Control Predicates
- Meta-Call Predicates
- Delimited continuations
- Exception handling
- Printing messages
- Handling signals
- DCG Grammar rules
- Declaring predicate properties
- Examining the program
- Input and output
- Status of streams
- Primitive character I/O
- Term reading and writing
- Analysing and Constructing Terms
- Analysing and Constructing Atoms
- Localization (locale) support
- Character properties
- Character Conversion
- Misc arithmetic support predicates
- Built-in list operations
- Finding all Solutions to a Goal
- Formatted Write
- Global variables
- Terminal Control
- Operating System Interaction
- File System Interaction
- User Top-level Manipulation
- Creating a Protocol of the User Interaction
- Debugging and Tracing Programs
- Debugging and declaring determinism
- Obtaining Runtime Statistics
- Execution profiling
- Memory Management
- Windows DDE interface
- Built-in Predicates
- Reference manual
- [det]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.
Each element of ListOfStreams is either a stream or an
integer. Integers are consider waitable OS handles. This can be used to
(also) wait for handles that are not associated with Prolog streams such
as UDP sockets. See tcp_setopt/2.
This predicate waits for at most TimeOut seconds. TimeOut may be specified as a floating point number to specify fractions of a second. If TimeOut equals
infinite, wait_for_input/3 waits indefinitely. If Timeout is 0 or 0.0 this predicate returns without waiting.105Prior to 7.3.23, the integer value‘0' was the same as
This predicate can be used to implement timeout while reading and to handle input from multiple sources and is typically used to wait for multiple (network) sockets. On Unix systems it may be used on any stream that is associated with a system file descriptor. On Windows it can only be used on sockets. If ListOfStreams contains a stream that is not associated with a supported device, a
domain_error(waitable_stream, Stream)is raised.
The example below waits for input from the user and an explicitly opened secondary terminal stream. On return, Inputs may hold
user_inputor P4 or both.
?- open('/dev/ttyp4', read, P4), wait_for_input([user_input, P4], Inputs, 0).
When available, the implementation is based on the poll() system call. The poll() puts no additional restriction on the number of open files the process may have. It does limit the time to 2^31-1 milliseconds (a bit less than 25 days). Specifying a too large timeout raises a
representation_error(timeout)exception. If poll() is not supported by the OS, select() is used. The select() call can only handle file descriptors up to
FD_SETSIZE. If the set contains a descriptor that exceeds this limit a
Note that wait_for_input/3 returns streams that have data waiting. This does not mean you can, for example, call read/2 on the stream without blocking as the stream might hold an incomplete term. The predicate set_stream/2 using the option
timeout(Seconds)can be used to make the stream generate an exception if no new data arrives within the timeout period. Suppose two processes communicate by exchanging Prolog terms. The following code makes the server immune for clients that write an incomplete term:
..., tcp_accept(Server, Socket, _Peer), tcp_open(Socket, In, Out), set_stream(In, timeout(10)), catch(read(In, Term), _, (close(Out), close(In), fail)), ...,
- byte_count(+Stream, -Count)
- Byte position in Stream. For binary streams this is the same as character_count/2. For text files the number may be different due to multi-byte encodings or additional record separators (such as Control-M in Windows).
- 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.