guile/doc/ref/api-io.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007, 2009,
@c   2010, 2011, 2013, 2016, 2019, 2021  Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Input and Output
@section Input and Output

@menu
* Ports::                       What's a port?
* Binary I/O::                  Reading and writing bytes.
* Encoding::                    Characters as bytes.
* Textual I/O::                 Reading and writing characters.
* Simple Output::               Simple syntactic sugar solution.
* Buffering::                   Controlling when data is written to ports.
* Random Access::               Moving around a random access port.
* Line/Delimited::              Read and write lines or delimited text.
* Default Ports::               Defaults for input, output and errors.
* Port Types::                  Types of port and how to make them.
* Venerable Port Interfaces::   Procedures from the last millenium.
* Using Ports from C::          Nice interfaces for C.
* I/O Extensions::              Implementing new port types in C.
* Non-Blocking I/O::            How Guile deals with EWOULDBLOCK.
* BOM Handling::                Handling of Unicode byte order marks.
@end menu


@node Ports
@subsection Ports
@cindex Port

Ports are the way that Guile performs input and output.  Guile can read
in characters or bytes from an @dfn{input port}, or write them out to an
@dfn{output port}.  Some ports support both interfaces.

There are a number of different port types implemented in Guile.  File
ports provide input and output over files, as you might imagine.  For
example, we might display a string to a file like this:

@example
(let ((port (open-output-file "foo.txt")))
  (display "Hello, world!\n" port)
  (close-port port))
@end example

There are also string ports, for taking input from a string, or
collecting output to a string; bytevector ports, for doing the same but
using a bytevector as a source or sink of data; and soft ports, for
arranging to call Scheme functions to provide input or handle output.
@xref{Port Types}.

Ports should be @dfn{closed} when they are not needed by calling
@code{close-port} on them, as in the example above.  This will make sure
that any pending output is successfully written out to disk, in the case
of a file port, or otherwise to whatever mutable store is backed by the
port.  Any error that occurs while writing out that buffered data would
also be raised promptly at the @code{close-port}, and not later when the
port is closed by the garbage collector.  @xref{Buffering}, for more on
buffered output.

Closing a port also releases any precious resource the file might have.
Usually in Scheme a programmer doesn't have to clean up after their data
structures (@pxref{Memory Management}), but most systems have strict
limits on how many files can be open, both on a per-process and a
system-wide basis.  A program that uses many files should take care not
to hit those limits.  The same applies to similar system resources such
as pipes and sockets.

Indeed for these reasons the above example is not the most idiomatic way
to use ports.  It is more common to acquire ports via procedures like
@code{call-with-output-file}, which handle the @code{close-port}
automatically:

@example
(call-with-output-file "foo.txt"
  (lambda (port)
    (display "Hello, world!\n" port)))
@end example

Finally, all ports have associated input and output buffers, as
appropriate.  Buffering is a common strategy to limit the overhead of
small reads and writes: without buffering, each character fetched from a
file would involve at least one call into the kernel, and maybe more
depending on the character and the encoding.  Instead, Guile will batch
reads and writes into internal buffers.  However, sometimes you want to
make output on a port show up immediately.  @xref{Buffering}, for more
on interfaces to control port buffering.

@deffn {Scheme Procedure} port? x
@deffnx {C Function} scm_port_p (x)
Return a boolean indicating whether @var{x} is a port.
@end deffn

@rnindex input-port?
@deffn {Scheme Procedure} input-port? x
@deffnx {C Function} scm_input_port_p (x)
Return @code{#t} if @var{x} is an input port, otherwise return
@code{#f}.  Any object satisfying this predicate also satisfies
@code{port?}.
@end deffn

@rnindex output-port?
@deffn {Scheme Procedure} output-port? x
@deffnx {C Function} scm_output_port_p (x)
Return @code{#t} if @var{x} is an output port, otherwise return
@code{#f}.  Any object satisfying this predicate also satisfies
@code{port?}.
@end deffn

@cindex Closing ports
@cindex Port, close
@deffn {Scheme Procedure} close-port port
@deffnx {C Function} scm_close_port (port)
Close the specified port object.  Return @code{#t} if it successfully
closes a port or @code{#f} if it was already closed.  An exception may
be raised if an error occurs, for example when flushing buffered output.
@xref{Buffering}, for more on buffered output.  @xref{Ports and File
Descriptors, close}, for a procedure which can close file descriptors.
@end deffn

@deffn {Scheme Procedure} port-closed? port
@deffnx {C Function} scm_port_closed_p (port)
Return @code{#t} if @var{port} is closed or @code{#f} if it is
open.
@end deffn

@deffn {Scheme Procedure} call-with-port port proc
Call @var{proc}, passing it @var{port} and closing @var{port} upon exit
of @var{proc}.  Return the return values of @var{proc}.
@end deffn


@node Binary I/O
@subsection Binary I/O

Guile's ports are fundamentally binary in nature: at the lowest level,
they work on bytes.  This section describes Guile's core binary I/O
operations.  @xref{Textual I/O}, for input and output of strings and
characters.

To use these routines, first include the binary I/O module:

@example
(use-modules (ice-9 binary-ports))
@end example

Note that although this module's name suggests that binary ports are
some different kind of port, that's not the case: all ports in Guile are
both binary and textual ports.

@cindex binary input
@anchor{x-get-u8}
@deffn {Scheme Procedure} get-u8 port
@deffnx {C Function} scm_get_u8 (port)
Return an octet read from @var{port}, an input port, blocking as
necessary, or the end-of-file object.
@end deffn

@anchor{x-lookahead-u8}
@deffn {Scheme Procedure} lookahead-u8 port
@deffnx {C Function} scm_lookahead_u8 (port)
Like @code{get-u8} but does not update @var{port}'s position to point
past the octet.
@end deffn

The end-of-file object is unlike any other kind of object: it's not a
pair, a symbol, or anything else.  To check if a value is the
end-of-file object, use the @code{eof-object?} predicate.

@rnindex eof-object?
@cindex End of file object
@deffn {Scheme Procedure} eof-object? x
@deffnx {C Function} scm_eof_object_p (x)
Return @code{#t} if @var{x} is an end-of-file object, or @code{#f}
otherwise.
@end deffn

Note that unlike other procedures in this module, @code{eof-object?} is
defined in the default environment.

@deffn {Scheme Procedure} get-bytevector-n port count
@deffnx {C Function} scm_get_bytevector_n (port, count)
Read @var{count} octets from @var{port}, blocking as necessary and
return a bytevector containing the octets read.  If fewer bytes are
available, a bytevector smaller than @var{count} is returned.
@end deffn

@deffn {Scheme Procedure} get-bytevector-n! port bv start count
@deffnx {C Function} scm_get_bytevector_n_x (port, bv, start, count)
Read @var{count} bytes from @var{port} and store them in @var{bv}
starting at index @var{start}.  Return either the number of bytes
actually read or the end-of-file object.
@end deffn

@deffn {Scheme Procedure} get-bytevector-some port
@deffnx {C Function} scm_get_bytevector_some (port)
Read from @var{port}, blocking as necessary, until bytes are available
or an end-of-file is reached.  Return either the end-of-file object or a
new bytevector containing some of the available bytes (at least one),
and update the port position to point just past these bytes.
@end deffn

@deffn {Scheme Procedure} get-bytevector-some! port bv start count
@deffnx {C Function} scm_get_bytevector_some_x (port, bv, start, count)
Read up to @var{count} bytes from @var{port}, blocking as necessary
until at least one byte is available or an end-of-file is reached.
Store them in @var{bv} starting at index @var{start}.  Return the number
of bytes actually read, or an end-of-file object.
@end deffn

@deffn {Scheme Procedure} get-bytevector-all port
@deffnx {C Function} scm_get_bytevector_all (port)
Read from @var{port}, blocking as necessary, until the end-of-file is
reached.  Return either a new bytevector containing the data read or the
end-of-file object (if no data were available).
@end deffn

@deffn {Scheme Procedure} unget-bytevector port bv [start [count]]
@deffnx {C Function} scm_unget_bytevector (port, bv, start, count)
Place the contents of @var{bv} in @var{port}, optionally starting at
index @var{start} and limiting to @var{count} octets, so that its bytes
will be read from left-to-right as the next bytes from @var{port} during
subsequent read operations.  If called multiple times, the unread bytes
will be read again in last-in first-out order.
@end deffn

@cindex binary output
To perform binary output on a port, use @code{put-u8} or
@code{put-bytevector}.

@anchor{x-put-u8}
@deffn {Scheme Procedure} put-u8 port octet
@deffnx {C Function} scm_put_u8 (port, octet)
Write @var{octet}, an integer in the 0--255 range, to @var{port}, a
binary output port.
@end deffn

@deffn {Scheme Procedure} put-bytevector port bv [start [count]]
@deffnx {C Function} scm_put_bytevector (port, bv, start, count)
Write the contents of @var{bv} to @var{port}, optionally starting at
index @var{start} and limiting to @var{count} octets.
@end deffn

@subsubheading Binary I/O in R7RS

@ref{R7RS Standard Libraries,R7RS} defines the following binary I/O
procedures.  Access them with

@example
(use-modules (scheme base))
@end example

@anchor{x-open-output-bytevector}
@deffn {Scheme Procedure} open-output-bytevector
Returns a binary output port that will accumulate bytes
for retrieval by @ref{x-get-output-bytevector,@code{get-output-bytevector}}.
@end deffn

@deffn {Scheme Procedure} write-u8 byte [out]
Writes @var{byte} to the given binary output port @var{out} and returns
an unspecified value.  @var{out} defaults to @code{(current-output-port)}.

See also @ref{x-put-u8,@code{put-u8}}.
@end deffn

@deffn {Scheme Procedure} read-u8 [in]
Returns the next byte available from the binary input port @var{in},
updating the port to point to the following byte.  If no more bytes are
available, an end-of-file object is returned.  @var{in} defaults to
@code{(current-input-port)}.

See also @ref{x-get-u8,@code{get-u8}}.
@end deffn

@deffn {Scheme Procedure} peek-u8 [in]
Returns the next byte available from the binary input port @var{in},
but without updating the port to point to the following
byte.  If no more bytes are available, an end-of-file object
is returned.  @var{in} defaults to @code{(current-input-port)}.

See also @ref{x-lookahead-u8,@code{lookahead-u8}}.
@end deffn

@anchor{x-get-output-bytevector}
@deffn {Scheme Procedure} get-output-bytevector port
Returns a bytevector consisting of the bytes that have been output to
@var{port} so far in the order they were output.  It is an error if
@var{port} was not created with
@ref{x-open-output-bytevector,@code{open-output-bytevector}}.

@example
(define out (open-output-bytevector))
(write-u8 1 out)
(write-u8 2 out)
(write-u8 3 out)
(get-output-bytevector out) @result{} #vu8(1 2 3)
@end example
@end deffn

@deffn {Scheme Procedure} open-input-bytevector bv
Takes a bytevector @var{bv} and returns a binary input port that
delivers bytes from @var{bv}.

@example
(define in (open-input-bytevector #vu8(1 2 3)))
(read-u8 in) @result{} 1
(peek-u8 in) @result{} 2
(read-u8 in) @result{} 2
(read-u8 in) @result{} 3
(read-u8 in) @result{} #<eof>
@end example
@end deffn

@deffn {Scheme Procedure} read-bytevector! bv [port [start [end]]]
Reads the next @var{end} - @var{start} bytes, or as many as are
available before the end of file, from the binary input port into the
bytevector @var{bv} in left-to-right order beginning at the @var{start}
position.  If @var{end} is not supplied, reads until the end of @var{bv}
has been reached.  If @var{start} is not supplied, reads beginning at
position 0.

Returns the number of bytes read.  If no bytes are available, an
end-of-file object is returned.

@example
(define in (open-input-bytevector #vu8(1 2 3)))
(define bv (make-bytevector 5 0))
(read-bytevector! bv in 1 3) @result{} 2
bv @result{} #vu8(0 1 2 0 0 0)
@end example

@end deffn

@deffn {Scheme Procedure} read-bytevector k in
Reads the next @var{k} bytes, or as many as are available before the end
of file if that is less than @var{k}, from the binary input port
@var{in} into a newly allocated bytevector in left-to-right order, and
returns the bytevector.  If no bytes are available before the end of
file, an end-of-file object is returned.

@example
(define bv #vu8(1 2 3))
(read-bytevector 2 (open-input-bytevector bv)) @result{} #vu8(1 2)
(read-bytevector 10 (open-input-bytevector bv)) @result{} #vu8(1 2 3)
@end example

@end deffn

@deffn {Scheme Procedure} write-bytevector bv [port [start [end]]]
Writes the bytes of bytevector @var{bv} from @var{start} to @var{end} in
left-to-right order to the binary output @var{port}.  @var{start}
defaults to 0 and @var{end} defaults to the length of @var{bv}.

@example
(define out (open-output-bytevector))
(write-bytevector #vu8(0 1 2 3 4) out 2 4)
(get-output-bytevector out) @result{} #vu8(2 3)
@end example

@end deffn

@node Encoding
@subsection Encoding

Textual input and output on Guile ports is layered on top of binary
operations.  To this end, each port has an associated character encoding
that controls how bytes read from the port are converted to characters,
and how characters written to the port are converted to bytes.

@deffn {Scheme Procedure} port-encoding port
@deffnx {C Function} scm_port_encoding (port)
Returns, as a string, the character encoding that @var{port} uses to
interpret its input and output.
@end deffn

@deffn {Scheme Procedure} set-port-encoding! port enc
@deffnx {C Function} scm_set_port_encoding_x (port, enc)
Sets the character encoding that will be used to interpret I/O to
@var{port}.  @var{enc} is a string containing the name of an encoding.
Valid encoding names are those
@url{http://www.iana.org/assignments/character-sets, defined by IANA},
for example @code{"UTF-8"} or @code{"ISO-8859-1"}.
@end deffn

When ports are created, they are assigned an encoding.  The usual
process to determine the initial encoding for a port is to take the
value of the @code{%default-port-encoding} fluid.

@defvr {Scheme Variable} %default-port-encoding
A fluid containing name of the encoding to be used by default for newly
created ports (@pxref{Fluids and Dynamic States}).  As a special case,
the value @code{#f} is equivalent to @code{"ISO-8859-1"}.
@end defvr

The @code{%default-port-encoding} itself defaults to the encoding
appropriate for the current locale, if @code{setlocale} has been called.
@xref{Locales}, for more on locales and when you might need to call
@code{setlocale} explicitly.

Some port types have other ways of determining their initial locales.
String ports, for example, default to the UTF-8 encoding, in order to be
able to represent all characters regardless of the current locale.  File
ports can optionally sniff their file for a @code{coding:} declaration;
@xref{File Ports}.  Binary ports might be initialized to the ISO-8859-1
encoding in which each codepoint between 0 and 255 corresponds to a byte
with that value.

Currently, the ports only work with @emph{non-modal} encodings.  Most
encodings are non-modal, meaning that the conversion of bytes to a
string doesn't depend on its context: the same byte sequence will always
return the same string.  A couple of modal encodings are in common use,
like ISO-2022-JP and ISO-2022-KR, and they are not yet supported.

@cindex port conversion strategy
@cindex conversion strategy, port
@cindex decoding error
@cindex encoding error
Each port also has an associated conversion strategy, which determines
what to do when a Guile character can't be converted to the port's
encoded character representation for output.  There are three possible
strategies: to raise an error, to replace the character with a hex
escape, or to replace the character with a substitute character.  Port
conversion strategies are also used when decoding characters from an
input port.

@deffn {Scheme Procedure} port-conversion-strategy port
@deffnx {C Function} scm_port_conversion_strategy (port)
Returns the behavior of the port when outputting a character that is not
representable in the port's current encoding.

If @var{port} is @code{#f}, then the current default behavior will be
returned.  New ports will have this default behavior when they are
created.
@end deffn

@deffn {Scheme Procedure} set-port-conversion-strategy! port sym
@deffnx {C Function} scm_set_port_conversion_strategy_x (port, sym)
Sets the behavior of Guile when outputting a character that is not
representable in the port's current encoding, or when Guile encounters a
decoding error when trying to read a character.  @var{sym} can be either
@code{error}, @code{substitute}, or @code{escape}.

If @var{port} is an open port, the conversion error behavior is set for
that port.  If it is @code{#f}, it is set as the default behavior for
any future ports that get created in this thread.
@end deffn

As with port encodings, there is a fluid which determines the initial
conversion strategy for a port.

@deffn {Scheme Variable} %default-port-conversion-strategy
The fluid that defines the conversion strategy for newly created ports,
and also for other conversion routines such as @code{scm_to_stringn},
@code{scm_from_stringn}, @code{string->pointer}, and
@code{pointer->string}.

Its value must be one of the symbols described above, with the same
semantics: @code{error}, @code{substitute}, or @code{escape}.

When Guile starts, its value is @code{substitute}.

Note that @code{(set-port-conversion-strategy! #f @var{sym})} is
equivalent to @code{(fluid-set! %default-port-conversion-strategy
@var{sym})}.
@end deffn

As mentioned above, for an output port there are three possible port
conversion strategies.  The @code{error} strategy will throw an error
when a nonconvertible character is encountered.  The @code{substitute}
strategy will replace nonconvertible characters with a question mark
(@samp{?}).  Finally the @code{escape} strategy will print
nonconvertible characters as a hex escape, using the escaping that is
recognized by Guile's string syntax.  Note that if the port's encoding
is a Unicode encoding, like @code{UTF-8}, then encoding errors are
impossible.

For an input port, the @code{error} strategy will cause Guile to throw
an error if it encounters an invalid encoding, such as might happen if
you tried to read @code{ISO-8859-1} as @code{UTF-8}.  The error is
thrown before advancing the read position.  The @code{substitute}
strategy will replace the bad bytes with a U+FFFD replacement character,
in accordance with Unicode recommendations.  When reading from an input
port, the @code{escape} strategy is treated as if it were @code{error}.


@node Textual I/O
@subsection Textual I/O
@cindex textual input
@cindex textual output

This section describes Guile's core textual I/O operations on characters
and strings.  @xref{Binary I/O}, for input and output of bytes and
bytevectors.  @xref{Encoding}, for more on how characters relate to
bytes.  To read general S-expressions from ports, @xref{Scheme Read}.
@xref{Scheme Write}, for interfaces that write generic Scheme datums.

To use these routines, first include the textual I/O module:

@example
(use-modules (ice-9 textual-ports))
@end example

Note that although this module's name suggests that textual ports are
some different kind of port, that's not the case: all ports in Guile are
both binary and textual ports.

@deffn {Scheme Procedure} get-char input-port
Reads from @var{input-port}, blocking as necessary, until a
complete character is available from @var{input-port},
or until an end of file is reached.

If a complete character is available before the next end of file,
@code{get-char} returns that character and updates the input port to
point past the character. If an end of file is reached before any
character is read, @code{get-char} returns the end-of-file object.
@end deffn

@deffn {Scheme Procedure} lookahead-char input-port
The @code{lookahead-char} procedure is like @code{get-char}, but it does
not update @var{input-port} to point past the character.
@end deffn

In the same way that it's possible to "unget" a byte or bytes, it's
possible to "unget" the bytes corresponding to an encoded character.

@deffn {Scheme Procedure} unget-char port char
Place character @var{char} in @var{port} so that it will be read by the
next read operation.  If called multiple times, the unread characters
will be read again in last-in first-out order.
@end deffn

@deffn {Scheme Procedure} unget-string port str
Place the string @var{str} in @var{port} so that its characters will
be read from left-to-right as the next characters from @var{port}
during subsequent read operations.  If called multiple times, the
unread characters will be read again in last-in first-out order.
@end deffn

Reading in a character at a time can be inefficient.  If it's possible
to perform I/O over multiple characters at a time, via strings, that
might be faster.

@deffn {Scheme Procedure} get-string-n input-port count
The @code{get-string-n} procedure reads from @var{input-port}, blocking
as necessary, until @var{count} characters are available, or until an
end of file is reached.  @var{count} must be an exact, non-negative
integer, representing the number of characters to be read.

If @var{count} characters are available before end of file,
@code{get-string-n} returns a string consisting of those @var{count}
characters. If fewer characters are available before an end of file, but
one or more characters can be read, @code{get-string-n} returns a string
containing those characters. In either case, the input port is updated
to point just past the characters read. If no characters can be read
before an end of file, the end-of-file object is returned.
@end deffn

@deffn {Scheme Procedure} get-string-n! input-port string start count
The @code{get-string-n!} procedure reads from @var{input-port} in the
same manner as @code{get-string-n}.  @var{start} and @var{count} must be
exact, non-negative integer objects, with @var{count} representing the
number of characters to be read.  @var{string} must be a string with at
least $@var{start} + @var{count}$ characters.

If @var{count} characters are available before an end of file, they are
written into @var{string} starting at index @var{start}, and @var{count}
is returned. If fewer characters are available before an end of file,
but one or more can be read, those characters are written into
@var{string} starting at index @var{start} and the number of characters
actually read is returned as an exact integer object. If no characters
can be read before an end of file, the end-of-file object is returned.
@end deffn

@deffn {Scheme Procedure} get-string-all input-port
Reads from @var{input-port} until an end of file, decoding characters in
the same manner as @code{get-string-n} and @code{get-string-n!}.

If characters are available before the end of file, a string containing
all the characters decoded from that data are returned. If no character
precedes the end of file, the end-of-file object is returned.
@end deffn

@deffn {Scheme Procedure} get-line input-port
Reads from @var{input-port} up to and including the linefeed
character or end of file, decoding characters in the same manner as
@code{get-string-n} and @code{get-string-n!}.

If a linefeed character is read, a string containing all of the text up
to (but not including) the linefeed character is returned, and the port
is updated to point just past the linefeed character. If an end of file
is encountered before any linefeed character is read, but some
characters have been read and decoded as characters, a string containing
those characters is returned. If an end of file is encountered before
any characters are read, the end-of-file object is returned.
@end deffn

Finally, there are just two core procedures to write characters to a
port.

@deffn {Scheme Procedure} put-char port char
Writes @var{char} to the port. The @code{put-char} procedure returns
an unspecified value.
@end deffn

@deffn {Scheme Procedure} put-string port string
@deffnx {Scheme Procedure} put-string port string start
@deffnx {Scheme Procedure} put-string port string start count
Write the @var{count} characters of @var{string} starting at index
@var{start} to the port.

@var{start} and @var{count} must be non-negative exact integer objects.
@var{string} must have a length of at least @math{@var{start} +
@var{count}}.  @var{start} defaults to 0.  @var{count} defaults to
@math{@code{(string-length @var{string})} - @var{start}}$.

Calling @code{put-string} is equivalent in all respects to calling
@code{put-char} on the relevant sequence of characters, except that it
will attempt to write multiple characters to the port at a time, even if
the port is unbuffered.

The @code{put-string} procedure returns an unspecified value.
@end deffn

Textual ports have a textual position associated with them: a line and a
column.  Reading in characters or writing them out advances the line and
the column appropriately.

@deffn {Scheme Procedure} port-column port
@deffnx {Scheme Procedure} port-line port
@deffnx {C Function} scm_port_column (port)
@deffnx {C Function} scm_port_line (port)
Return the current column number or line number of @var{port}.
@end deffn

Port lines and positions are represented as 0-origin integers, which is
to say that the the first character of the first line is line 0, column
0.  However, when you display a line number, for example in an error
message, we recommend you add 1 to get 1-origin integers.  This is
because lines numbers traditionally start with 1, and that is what
non-programmers will find most natural.

@deffn {Scheme Procedure} set-port-column! port column
@deffnx {Scheme Procedure} set-port-line! port line
@deffnx {C Function} scm_set_port_column_x (port, column)
@deffnx {C Function} scm_set_port_line_x (port, line)
Set the current column or line number of @var{port}.
@end deffn

@node Simple Output
@subsection Simple Textual Output

Guile exports a simple formatted output function, @code{simple-format}.
For a more capable formatted output facility, @xref{Formatted Output}.

@deffn {Scheme Procedure} simple-format destination message . args
@deffnx {C Function} scm_simple_format (destination, message, args)
Write @var{message} to @var{destination}, defaulting to the current
output port.  @var{message} can contain @code{~A} and @code{~S} escapes.
When printed, the escapes are replaced with corresponding members of
@var{args}: @code{~A} formats using @code{display} and @code{~S} formats
using @code{write}.  If @var{destination} is @code{#t}, then use the
current output port, if @var{destination} is @code{#f}, then return a
string containing the formatted text.  Does not add a trailing newline.
@end deffn

Somewhat confusingly, Guile binds the @code{format} identifier to
@code{simple-format} at startup.  Once @code{(ice-9 format)} loads, it
actually replaces the core @code{format} binding, so depending on
whether you or a module you use has loaded @code{(ice-9 format)}, you
may be using the simple or the more capable version.

@node Buffering
@subsection Buffering
@cindex Port, buffering

Every port has associated input and output buffers.  You can think of
ports as being backed by some mutable store, and that store might be far
away.  For example, ports backed by file descriptors have to go all the
way to the kernel to read and write their data.  To avoid this
round-trip cost, Guile usually reads in data from the mutable store in
chunks, and then services small requests like @code{get-char} out of
that intermediate buffer.  Similarly, small writes like
@code{write-char} first go to a buffer, and are sent to the store when
the buffer is full (or when port is flushed).  Buffered ports speed up
your program by reducing the number of round-trips to the mutable store,
and they do so in a way that is mostly transparent to the user.

There are two major ways, however, in which buffering affects program
semantics.  Building correct, performant programs requires understanding
these situations.

The first case is in random-access read/write ports (@pxref{Random
Access}).  These ports, usually backed by a file, logically operate over
the same mutable store when both reading and writing.  So, if you read a
character, causing the buffer to fill, then write a character, the bytes
you filled in your read buffer are now invalid.  Every time you switch
between reading and writing, Guile has to flush any pending buffer.  If
this happens frequently, the cost can be high.  In that case you should
reduce the amount that you buffer, in both directions.  Similarly, Guile
has to flush buffers before seeking.  None of these considerations apply
to sockets, which don't logically read from and write to the same
mutable store, and are not seekable.  Note also that sockets are
unbuffered by default.  @xref{Network Sockets and Communication}.

The second case is the more pernicious one.  If you write data to a
buffered port, it probably doesn't go out to the mutable store directly.
(This ``probably'' introduces some indeterminism in your program: what
goes to the store, and when, depends on how full the buffer is.  It is
something that the user needs to explicitly be aware of.)  The data is
written to the store later -- when the buffer fills up due to another
write, or when @code{force-output} is called, or when @code{close-port}
is called, or when the program exits, or even when the garbage collector
runs.  The salient point is, @emph{the errors are signalled then too}.
Buffered writes defer error detection (and defer the side effects to the
mutable store), perhaps indefinitely if the port type does not need to
be closed at GC.

One common heuristic that works well for textual ports is to flush
output when a newline (@code{\n}) is written.  This @dfn{line buffering}
mode is on by default for TTY ports.  Most other ports are @dfn{block
buffered}, meaning that once the output buffer reaches the block size,
which depends on the port and its configuration, the output is flushed
as a block, without regard to what is in the block.  Likewise reads are
read in at the block size, though if there are fewer bytes available to
read, the buffer may not be entirely filled.

Note that binary reads or writes that are larger than the buffer size go
directly to the mutable store without passing through the buffers.  If
your access pattern involves many big reads or writes, buffering might
not matter so much to you.

To control the buffering behavior of a port, use @code{setvbuf}.

@deffn {Scheme Procedure} setvbuf port mode [size]
@deffnx {C Function} scm_setvbuf (port, mode, size)
@cindex port buffering
Set the buffering mode for @var{port}.  @var{mode} can be one of the
following symbols:

@table @code
@item none
non-buffered
@item line
line buffered
@item block
block buffered, using a newly allocated buffer of @var{size} bytes.
If @var{size} is omitted, a default size will be used.
@end table
@end deffn

Another way to set the buffering, for file ports, is to open the file
with @code{0} or @code{l} as part of the mode string, for unbuffered or
line-buffered ports, respectively.  @xref{File Ports}, for more.

Any buffered output data will be written out when the port is closed.
To make sure to flush it at specific points in your program, use
@code{force-otput}.

@findex fflush
@deffn {Scheme Procedure} force-output [port]
@deffnx {C Function} scm_force_output (port)
Flush the specified output port, or the current output port if
@var{port} is omitted.  The current output buffer contents, if any, are
passed to the underlying port implementation.

The return value is unspecified.
@end deffn

@deffn {Scheme Procedure} flush-all-ports
@deffnx {C Function} scm_flush_all_ports ()
Equivalent to calling @code{force-output} on all open output ports.  The
return value is unspecified.
@end deffn

Similarly, sometimes you might want to switch from using Guile's ports
to working directly on file descriptors.  In that case, for input ports
use @code{drain-input} to get any buffered input from that port.

@deffn {Scheme Procedure} drain-input port
@deffnx {C Function} scm_drain_input (port)
This procedure clears a port's input buffers, similar
to the way that force-output clears the output buffer.  The
contents of the buffers are returned as a single string, e.g.,

@lisp
(define p (open-input-file ...))
(drain-input p) => empty string, nothing buffered yet.
(unread-char (read-char p) p)
(drain-input p) => initial chars from p, up to the buffer size.
@end lisp
@end deffn

All of these considerations are very similar to those of streams in the
C library, although Guile's ports are not built on top of C streams.
Still, it is useful to read what other systems do.
@xref{Streams,,,libc,The GNU C Library Reference Manual}, for more
discussion on C streams.


@node Random Access
@subsection Random Access
@cindex Random access, ports
@cindex Port, random access

@deffn {Scheme Procedure} seek fd_port offset whence
@deffnx {C Function} scm_seek (fd_port, offset, whence)
Sets the current position of @var{fd_port} to the integer
@var{offset}.  For a file port, @var{offset} is expressed
as a number of bytes; for other types of ports, such as string
ports, @var{offset} is an abstract representation of the
position within the port's data, not necessarily expressed
as a number of bytes.  @var{offset} is interpreted according to
the value of @var{whence}.

One of the following variables should be supplied for
@var{whence}:
@defvar SEEK_SET
Seek from the beginning of the file.
@end defvar
@defvar SEEK_CUR
Seek from the current position.
@end defvar
@defvar SEEK_END
Seek from the end of the file.
@end defvar
If @var{fd_port} is a file descriptor, the underlying system
call is @code{lseek}.  @var{port} may be a string port.

The value returned is the new position in @var{fd_port}.  This means
that the current position of a port can be obtained using:
@lisp
(seek port 0 SEEK_CUR)
@end lisp
@end deffn

@deffn {Scheme Procedure} ftell fd_port
@deffnx {C Function} scm_ftell (fd_port)
Return an integer representing the current position of
@var{fd_port}, measured from the beginning.  Equivalent to:

@lisp
(seek port 0 SEEK_CUR)
@end lisp
@end deffn

@findex truncate
@findex ftruncate
@deffn {Scheme Procedure} truncate-file file [length]
@deffnx {C Function} scm_truncate_file (file, length)
Truncate @var{file} to @var{length} bytes.  @var{file} can be a
filename string, a port object, or an integer file descriptor.  The
return value is unspecified.

For a port or file descriptor @var{length} can be omitted, in which
case the file is truncated at the current position (per @code{ftell}
above).

On most systems a file can be extended by giving a length greater than
the current size, but this is not mandatory in the POSIX standard.
@end deffn

@node Line/Delimited
@subsection Line Oriented and Delimited Text
@cindex Line input/output
@cindex Port, line input/output

The delimited-I/O module can be accessed with:

@lisp
(use-modules (ice-9 rdelim))
@end lisp

It can be used to read or write lines of text, or read text delimited by
a specified set of characters.

@deffn {Scheme Procedure} read-line [port] [handle-delim]
Return a line of text from @var{port} if specified, otherwise from the
value returned by @code{(current-input-port)}.  Under Unix, a line of text
is terminated by the first end-of-line character or by end-of-file.

If @var{handle-delim} is specified, it should be one of the following
symbols:
@table @code
@item trim
Discard the terminating delimiter.  This is the default, but it will
be impossible to tell whether the read terminated with a delimiter or
end-of-file.
@item concat
Append the terminating delimiter (if any) to the returned string.
@item peek
Push the terminating delimiter (if any) back on to the port.
@item split
Return a pair containing the string read from the port and the
terminating delimiter or end-of-file object.
@end table
@end deffn

@deffn {Scheme Procedure} read-line! buf [port]
Read a line of text into the supplied string @var{buf} and return the
number of characters added to @var{buf}.  If @var{buf} is filled, then
@code{#f} is returned.  Read from @var{port} if specified, otherwise
from the value returned by @code{(current-input-port)}.
@end deffn

@deffn {Scheme Procedure} read-delimited delims [port] [handle-delim]
Read text until one of the characters in the string @var{delims} is
found or end-of-file is reached.  Read from @var{port} if supplied,
otherwise from the value returned by @code{(current-input-port)}.
@var{handle-delim} takes the same values as described for
@code{read-line}.
@end deffn

@c begin (scm-doc-string "rdelim.scm" "read-delimited!")
@deffn {Scheme Procedure} read-delimited! delims buf [port] [handle-delim] [start] [end]
Read text into the supplied string @var{buf}.

If a delimiter was found, return the number of characters written,
except if @var{handle-delim} is @code{split}, in which case the return
value is a pair, as noted above.

As a special case, if @var{port} was already at end-of-stream, the EOF
object is returned. Also, if no characters were written because the
buffer was full, @code{#f} is returned.

It's something of a wacky interface, to be honest.
@end deffn

@deffn {Scheme Procedure} %read-delimited! delims str gobble [port [start [end]]]
@deffnx {C Function} scm_read_delimited_x (delims, str, gobble, port, start, end)
Read characters from @var{port} into @var{str} until one of the
characters in the @var{delims} string is encountered.  If
@var{gobble} is true, discard the delimiter character;
otherwise, leave it in the input stream for the next read.  If
@var{port} is not specified, use the value of
@code{(current-input-port)}.  If @var{start} or @var{end} are
specified, store data only into the substring of @var{str}
bounded by @var{start} and @var{end} (which default to the
beginning and end of the string, respectively).

 Return a pair consisting of the delimiter that terminated the
string and the number of characters read.  If reading stopped
at the end of file, the delimiter returned is the
@var{eof-object}; if the string was filled without encountering
a delimiter, this value is @code{#f}.
@end deffn

@deffn {Scheme Procedure} %read-line [port]
@deffnx {C Function} scm_read_line (port)
Read a newline-terminated line from @var{port}, allocating storage as
necessary.  The newline terminator (if any) is removed from the string,
and a pair consisting of the line and its delimiter is returned.  The
delimiter may be either a newline or the @var{eof-object}; if
@code{%read-line} is called at the end of file, it returns the pair
@code{(#<eof> . #<eof>)}.
@end deffn

@deffn {Scheme Procedure} write-line obj [port]
@deffnx {C Function} scm_write_line (obj, port)
Display @var{obj} and a newline character to @var{port}.  If
@var{port} is not specified, @code{(current-output-port)} is
used.  This procedure is equivalent to:
@lisp
(display obj [port])
(newline [port])
@end lisp
@end deffn

@node Default Ports
@subsection Default Ports for Input, Output and Errors
@cindex Default ports
@cindex Port, default

@rnindex current-input-port
@deffn {Scheme Procedure} current-input-port
@deffnx {C Function} scm_current_input_port ()
@cindex standard input
Return the current input port.  This is the default port used
by many input procedures.

Initially this is the @dfn{standard input} in Unix and C terminology.
When the standard input is a tty the port is unbuffered, otherwise
it's fully buffered.

Unbuffered input is good if an application runs an interactive
subprocess, since any type-ahead input won't go into Guile's buffer
and be unavailable to the subprocess.

Note that Guile buffering is completely separate from the tty ``line
discipline''.  In the usual cooked mode on a tty Guile only sees a
line of input once the user presses @key{Return}.
@end deffn

@rnindex current-output-port
@deffn {Scheme Procedure} current-output-port
@deffnx {C Function} scm_current_output_port ()
@cindex standard output
Return the current output port.  This is the default port used
by many output procedures.

Initially this is the @dfn{standard output} in Unix and C terminology.
When the standard output is a tty this port is unbuffered, otherwise
it's fully buffered.

Unbuffered output to a tty is good for ensuring progress output or a
prompt is seen.  But an application which always prints whole lines
could change to line buffered, or an application with a lot of output
could go fully buffered and perhaps make explicit @code{force-output}
calls (@pxref{Buffering}) at selected points.
@end deffn

@deffn {Scheme Procedure} current-error-port
@deffnx {C Function} scm_current_error_port ()
@cindex standard error output
Return the port to which errors and warnings should be sent.

Initially this is the @dfn{standard error} in Unix and C terminology.
When the standard error is a tty this port is unbuffered, otherwise
it's fully buffered.
@end deffn

@deffn {Scheme Procedure} set-current-input-port port
@deffnx {Scheme Procedure} set-current-output-port port
@deffnx {Scheme Procedure} set-current-error-port port
@deffnx {C Function} scm_set_current_input_port (port)
@deffnx {C Function} scm_set_current_output_port (port)
@deffnx {C Function} scm_set_current_error_port (port)
Change the ports returned by @code{current-input-port},
@code{current-output-port} and @code{current-error-port}, respectively,
so that they use the supplied @var{port} for input or output.
@end deffn

@deffn {Scheme Procedure} with-input-from-port port thunk
@deffnx {Scheme Procedure} with-output-to-port port thunk
@deffnx {Scheme Procedure} with-error-to-port port thunk
Call @var{thunk} in a dynamic environment in which
@code{current-input-port}, @code{current-output-port} or
@code{current-error-port} is rebound to the given @var{port}.
@end deffn

@deftypefn {C Function} void scm_dynwind_current_input_port (SCM port)
@deftypefnx {C Function} void scm_dynwind_current_output_port (SCM port)
@deftypefnx {C Function} void scm_dynwind_current_error_port (SCM port)
These functions must be used inside a pair of calls to
@code{scm_dynwind_begin} and @code{scm_dynwind_end} (@pxref{Dynamic
Wind}).  During the dynwind context, the indicated port is set to
@var{port}.

More precisely, the current port is swapped with a `backup' value
whenever the dynwind context is entered or left.  The backup value is
initialized with the @var{port} argument.
@end deftypefn

@node Port Types
@subsection Types of Port
@cindex Types of ports
@cindex Port, types

@menu
* File Ports:: Ports on an operating system file.
* Bytevector Ports:: Ports on a bytevector.
* String Ports:: Ports on a Scheme string.
* Custom Ports:: Ports whose implementation you control.
* Soft Ports:: An older version of custom ports.
* Void Ports:: Ports on nothing at all.
@end menu


@node File Ports
@subsubsection File Ports
@cindex File port
@cindex Port, file

The following procedures are used to open file ports.
See also @ref{Ports and File Descriptors, open}, for an interface
to the Unix @code{open} system call.

All file access uses the ``LFS'' large file support functions when
available, so files bigger than 2 Gbytes (@math{2^31} bytes) can be
read and written on a 32-bit system.

Most systems have limits on how many files can be open, so it's
strongly recommended that file ports be closed explicitly when no
longer required (@pxref{Ports}).

@deffn {Scheme Procedure} open-file filename mode @
                          [#:guess-encoding=#f] [#:encoding=#f]
@deffnx {C Function} scm_open_file_with_encoding @
                     (filename, mode, guess_encoding, encoding)
@deffnx {C Function} scm_open_file (filename, mode)
Open the file whose name is @var{filename}, and return a port
representing that file.  The attributes of the port are
determined by the @var{mode} string.  The way in which this is
interpreted is similar to C stdio.  The first character must be
one of the following:

@table @samp
@item r
Open an existing file for input.
@item w
Open a file for output, creating it if it doesn't already exist
or removing its contents if it does.
@item a
Open a file for output, creating it if it doesn't already
exist.  All writes to the port will go to the end of the file.
The "append mode" can be turned off while the port is in use
@pxref{Ports and File Descriptors, fcntl}
@end table

The following additional characters can be appended:

@table @samp
@item b
Open the underlying file in binary mode, if supported by the system.
Also, open the file using the binary-compatible character encoding
"ISO-8859-1", ignoring the default port encoding.
@item +
Open the port for both input and output.  E.g., @code{r+}: open
an existing file for both input and output.
@item e
Mark the underlying file descriptor as close-on-exec, as per the
@code{O_CLOEXEC} flag.
@item 0
Create an "unbuffered" port.  In this case input and output
operations are passed directly to the underlying port
implementation without additional buffering.  This is likely to
slow down I/O operations.  The buffering mode can be changed
while a port is in use (@pxref{Buffering}).
@item l
Add line-buffering to the port.  The port output buffer will be
automatically flushed whenever a newline character is written.
@item b
Use binary mode, ensuring that each byte in the file will be read as one
Scheme character.

To provide this property, the file will be opened with the 8-bit
character encoding "ISO-8859-1", ignoring the default port encoding.
@xref{Ports}, for more information on port encodings.

Note that while it is possible to read and write binary data as
characters or strings, it is usually better to treat bytes as octets,
and byte sequences as bytevectors.  @xref{Binary I/O}, for more.

This option had another historical meaning, for DOS compatibility: in
the default (textual) mode, DOS reads a CR-LF sequence as one LF byte.
The @code{b} flag prevents this from happening, adding @code{O_BINARY}
to the underlying @code{open} call.  Still, the flag is generally useful
because of its port encoding ramifications.
@end table

Unless binary mode is requested, the character encoding of the new port
is determined as follows: First, if @var{guess-encoding} is true, the
@code{file-encoding} procedure is used to guess the encoding of the file
(@pxref{Character Encoding of Source Files}).  If @var{guess-encoding}
is false or if @code{file-encoding} fails, @var{encoding} is used unless
it is also false.  As a last resort, the default port encoding is used.
@xref{Ports}, for more information on port encodings.  It is an error to
pass a non-false @var{guess-encoding} or @var{encoding} if binary mode
is requested.

If a file cannot be opened with the access requested, @code{open-file}
throws an exception.
@end deffn

@rnindex open-input-file
@deffn {Scheme Procedure} open-input-file filename @
       [#:guess-encoding=#f] [#:encoding=#f] [#:binary=#f]

Open @var{filename} for input.  If @var{binary} is true, open the port
in binary mode, otherwise use text mode.  @var{encoding} and
@var{guess-encoding} determine the character encoding as described above
for @code{open-file}.  Equivalent to
@lisp
(open-file @var{filename}
           (if @var{binary} "rb" "r")
           #:guess-encoding @var{guess-encoding}
           #:encoding @var{encoding})
@end lisp
@end deffn

@rnindex open-output-file
@deffn {Scheme Procedure} open-output-file filename @
       [#:encoding=#f] [#:binary=#f]

Open @var{filename} for output.  If @var{binary} is true, open the port
in binary mode, otherwise use text mode.  @var{encoding} specifies the
character encoding as described above for @code{open-file}.  Equivalent
to
@lisp
(open-file @var{filename}
           (if @var{binary} "wb" "w")
           #:encoding @var{encoding})
@end lisp
@end deffn

@deffn {Scheme Procedure} call-with-input-file filename proc @
        [#:guess-encoding=#f] [#:encoding=#f] [#:binary=#f]
@deffnx {Scheme Procedure} call-with-output-file filename proc @
        [#:encoding=#f] [#:binary=#f]
@rnindex call-with-input-file
@rnindex call-with-output-file
Open @var{filename} for input or output, and call @code{(@var{proc}
port)} with the resulting port.  Return the value returned by
@var{proc}.  @var{filename} is opened as per @code{open-input-file} or
@code{open-output-file} respectively, and an error is signaled if it
cannot be opened.

When @var{proc} returns, the port is closed.  If @var{proc} does not
return (e.g.@: if it throws an error), then the port might not be
closed automatically, though it will be garbage collected in the usual
way if not otherwise referenced.
@end deffn

@deffn {Scheme Procedure} with-input-from-file filename thunk @
        [#:guess-encoding=#f] [#:encoding=#f] [#:binary=#f]
@deffnx {Scheme Procedure} with-output-to-file filename thunk @
        [#:encoding=#f] [#:binary=#f]
@deffnx {Scheme Procedure} with-error-to-file filename thunk @
        [#:encoding=#f] [#:binary=#f]
@rnindex with-input-from-file
@rnindex with-output-to-file
Open @var{filename} and call @code{(@var{thunk})} with the new port
setup as respectively the @code{current-input-port},
@code{current-output-port}, or @code{current-error-port}.  Return the
value returned by @var{thunk}.  @var{filename} is opened as per
@code{open-input-file} or @code{open-output-file} respectively, and an
error is signaled if it cannot be opened.

When @var{thunk} returns, the port is closed and the previous setting
of the respective current port is restored.

The current port setting is managed with @code{dynamic-wind}, so the
previous value is restored no matter how @var{thunk} exits (eg.@: an
exception), and if @var{thunk} is re-entered (via a captured
continuation) then it's set again to the @var{filename} port.

The port is closed when @var{thunk} returns normally, but not when
exited via an exception or new continuation.  This ensures it's still
ready for use if @var{thunk} is re-entered by a captured continuation.
Of course the port is always garbage collected and closed in the usual
way when no longer referenced anywhere.
@end deffn

@deffn {Scheme Procedure} port-mode port
@deffnx {C Function} scm_port_mode (port)
Return the port modes associated with the open port @var{port}.
These will not necessarily be identical to the modes used when
the port was opened, since modes such as "append" which are
used only during port creation are not retained.
@end deffn

@deffn {Scheme Procedure} port-filename port
@deffnx {C Function} scm_port_filename (port)
Return the filename associated with @var{port}, or @code{#f} if no
filename is associated with the port.

@var{port} must be open; @code{port-filename} cannot be used once the
port is closed.
@end deffn

@deffn {Scheme Procedure} set-port-filename! port filename
@deffnx {C Function} scm_set_port_filename_x (port, filename)
Change the filename associated with @var{port}, using the current input
port if none is specified.  Note that this does not change the port's
source of data, but only the value that is returned by
@code{port-filename} and reported in diagnostic output.
@end deffn

@deffn {Scheme Procedure} file-port? obj
@deffnx {C Function} scm_file_port_p (obj)
Determine whether @var{obj} is a port that is related to a file.
@end deffn


@node Bytevector Ports
@subsubsection Bytevector Ports

@deffn {Scheme Procedure} open-bytevector-input-port bv [transcoder]
@deffnx {C Function} scm_open_bytevector_input_port (bv, transcoder)
Return an input port whose contents are drawn from bytevector @var{bv}
(@pxref{Bytevectors}).

@c FIXME: Update description when implemented.
The @var{transcoder} argument is currently not supported.
@end deffn

@deffn {Scheme Procedure} open-bytevector-output-port [transcoder]
@deffnx {C Function} scm_open_bytevector_output_port (transcoder)
Return two values: a binary output port and a procedure.  The latter
should be called with zero arguments to obtain a bytevector containing
the data accumulated by the port, as illustrated below.

@lisp
(call-with-values
  (lambda ()
    (open-bytevector-output-port))
  (lambda (port get-bytevector)
    (display "hello" port)
    (get-bytevector)))

@result{} #vu8(104 101 108 108 111)
@end lisp

@c FIXME: Update description when implemented.
The @var{transcoder} argument is currently not supported.
@end deffn

@deffn {Scheme Procedure} call-with-output-bytevector proc
Call the one-argument procedure @var{proc} with a newly created
bytevector output port.  When the function returns, the bytevector
composed of the characters written into the port is returned.
@var{proc} should not close the port.
@end deffn

@deffn {Scheme Procedure} call-with-input-bytevector bytevector proc
Call the one-argument procedure @var{proc} with a newly created input
port from which @var{bytevector}'s contents may be read.  The values
yielded by the @var{proc} is returned.
@end deffn


@node String Ports
@subsubsection String Ports
@cindex String port
@cindex Port, string

@deffn {Scheme Procedure} call-with-output-string proc
@deffnx {C Function} scm_call_with_output_string (proc)
Calls the one-argument procedure @var{proc} with a newly created output
port.  When the function returns, the string composed of the characters
written into the port is returned.  @var{proc} should not close the port.
@end deffn

@deffn {Scheme Procedure} call-with-input-string string proc
@deffnx {C Function} scm_call_with_input_string (string, proc)
Calls the one-argument procedure @var{proc} with a newly
created input port from which @var{string}'s contents may be
read.  The value yielded by the @var{proc} is returned.
@end deffn

@deffn {Scheme Procedure} with-output-to-string thunk
Calls the zero-argument procedure @var{thunk} with the current output
port set temporarily to a new string port.  It returns a string
composed of the characters written to the current output.
@end deffn

@deffn {Scheme Procedure} with-input-from-string string thunk
Calls the zero-argument procedure @var{thunk} with the current input
port set temporarily to a string port opened on the specified
@var{string}.  The value yielded by @var{thunk} is returned.
@end deffn

@deffn {Scheme Procedure} open-input-string str
@deffnx {C Function} scm_open_input_string (str)
Take a string and return an input port that delivers characters
from the string. The port can be closed by
@code{close-input-port}, though its storage will be reclaimed
by the garbage collector if it becomes inaccessible.
@end deffn

@deffn {Scheme Procedure} open-output-string
@deffnx {C Function} scm_open_output_string ()
Return an output port that will accumulate characters for
retrieval by @code{get-output-string}. The port can be closed
by the procedure @code{close-output-port}, though its storage
will be reclaimed by the garbage collector if it becomes
inaccessible.
@end deffn

@deffn {Scheme Procedure} get-output-string port
@deffnx {C Function} scm_get_output_string (port)
Given an output port created by @code{open-output-string},
return a string consisting of the characters that have been
output to the port so far.

@code{get-output-string} must be used before closing @var{port}, once
closed the string cannot be obtained.
@end deffn

With string ports, the port-encoding is treated differently than other
types of ports.  When string ports are created, they do not inherit a
character encoding from the current locale.  They are given a
default locale that allows them to handle all valid string characters.
Typically one should not modify a string port's character encoding
away from its default.  @xref{Encoding}.


@node Custom Ports
@subsubsection Custom Ports

Custom ports allow the user to provide input and handle output via
user-supplied procedures.  Guile currently only provides custom binary
ports, not textual ports; for custom textual ports, @xref{Soft Ports}.
We should add the R6RS custom textual port interfaces though.
Contributions are appreciated.

@cindex custom binary input ports
@deffn {Scheme Procedure} make-custom-binary-input-port id read! get-position set-position! close
Return a new custom binary input port@footnote{This is similar in spirit
to Guile's @dfn{soft ports} (@pxref{Soft Ports}).} named @var{id} (a
string) whose input is drained by invoking @var{read!} and passing it a
bytevector, an index where bytes should be written, and the number of
bytes to read.  The @code{read!}  procedure must return an integer
indicating the number of bytes read, or @code{0} to indicate the
end-of-file.

Optionally, if @var{get-position} is not @code{#f}, it must be a thunk
that will be called when @code{port-position} is invoked on the custom
binary port and should return an integer indicating the position within
the underlying data stream; if @var{get-position} was not supplied, the
returned port does not support @code{port-position}.

Likewise, if @var{set-position!} is not @code{#f}, it should be a
one-argument procedure.  When @code{set-port-position!} is invoked on the
custom binary input port, @var{set-position!} is passed an integer
indicating the position of the next byte is to read.

Finally, if @var{close} is not @code{#f}, it must be a thunk.  It is
invoked when the custom binary input port is closed.

The returned port is fully buffered by default, but its buffering mode
can be changed using @code{setvbuf} (@pxref{Buffering}).

Using a custom binary input port, the @code{open-bytevector-input-port}
procedure (@pxref{Bytevector Ports}) could be implemented as follows:

@lisp
(define (open-bytevector-input-port source)
  (define position 0)
  (define length (bytevector-length source))

  (define (read! bv start count)
    (let ((count (min count (- length position))))
      (bytevector-copy! source position
                        bv start count)
      (set! position (+ position count))
      count))

  (define (get-position) position)

  (define (set-position! new-position)
    (set! position new-position))

  (make-custom-binary-input-port "the port" read!
                                  get-position set-position!
                                  #f))

(read (open-bytevector-input-port (string->utf8 "hello")))
@result{} hello
@end lisp
@end deffn

@cindex custom binary output ports
@deffn {Scheme Procedure} make-custom-binary-output-port id write! get-position set-position! close
Return a new custom binary output port named @var{id} (a string) whose
output is sunk by invoking @var{write!} and passing it a bytevector, an
index where bytes should be read from this bytevector, and the number of
bytes to be ``written''.  The @code{write!}  procedure must return an
integer indicating the number of bytes actually written; when it is
passed @code{0} as the number of bytes to write, it should behave as
though an end-of-file was sent to the byte sink.

The other arguments are as for @code{make-custom-binary-input-port}.
@end deffn

@cindex custom binary input/output ports
@deffn {Scheme Procedure} make-custom-binary-input/output-port id read! write! get-position set-position! close
Return a new custom binary input/output port named @var{id} (a string).
The various arguments are the same as for The other arguments are as for
@code{make-custom-binary-input-port} and
@code{make-custom-binary-output-port}.  If buffering is enabled on the
port, as is the case by default, input will be buffered in both
directions; @xref{Buffering}.  If the @var{set-position!} function is
provided and not @code{#f}, then the port will also be marked as
random-access, causing the buffer to be flushed between reads and
writes.
@end deffn

@node Soft Ports
@subsubsection Soft Ports
@cindex Soft port
@cindex Port, soft

A @dfn{soft port} is a port based on a vector of procedures capable of
accepting or delivering characters.  It allows emulation of I/O ports.

@deffn {Scheme Procedure} make-soft-port pv modes
Return a port capable of receiving or delivering characters as
specified by the @var{modes} string (@pxref{File Ports,
open-file}).  @var{pv} must be a vector of length 5 or 6.  Its
components are as follows:

@enumerate 0
@item
procedure accepting one character for output
@item
procedure accepting a string for output
@item
thunk for flushing output
@item
thunk for getting one character
@item
thunk for closing port (not by garbage collection)
@item
(if present and not @code{#f}) thunk for computing the number of
characters that can be read from the port without blocking.
@end enumerate

For an output-only port only elements 0, 1, 2, and 4 need be
procedures.  For an input-only port only elements 3 and 4 need
be procedures.  Thunks 2 and 4 can instead be @code{#f} if
there is no useful operation for them to perform.

If thunk 3 returns @code{#f} or an @code{eof-object}
(@pxref{Input, eof-object?, ,r5rs, The Revised^5 Report on
Scheme}) it indicates that the port has reached end-of-file.
For example:

@lisp
(define stdout (current-output-port))
(define p (make-soft-port
           (vector
            (lambda (c) (write c stdout))
            (lambda (s) (display s stdout))
            (lambda () (display "." stdout))
            (lambda () (char-upcase (read-char)))
            (lambda () (display "@@" stdout)))
           "rw"))

(write p p) @result{} #<input-output: soft 8081e20>
@end lisp
@end deffn


@node Void Ports
@subsubsection Void Ports
@cindex Void port
@cindex Port, void

This kind of port causes any data to be discarded when written to, and
always returns the end-of-file object when read from.

@deffn {Scheme Procedure} %make-void-port mode
@deffnx {C Function} scm_sys_make_void_port (mode)
Create and return a new void port.  A void port acts like
@file{/dev/null}.  The @var{mode} argument
specifies the input/output modes for this port: see the
documentation for @code{open-file} in @ref{File Ports}.
@end deffn


@node Venerable Port Interfaces
@subsection Venerable Port Interfaces

Over the 25 years or so that Guile has been around, its port system has
evolved, adding many useful features.  At the same time there have been
four major Scheme standards released in those 25 years, which also
evolve the common Scheme understanding of what a port interface should
be.  Alas, it would be too much to ask for all of these evolutionary
branches to be consistent.  Some of Guile's original interfaces don't
mesh with the later Scheme standards, and yet Guile can't just drop old
interfaces.  Sadly as well, the R6RS and R7RS standards both part from a
base of R5RS, but end up in different and somewhat incompatible designs.

Guile's approach is to pick a set of port primitives that make sense
together.  We document that set of primitives, design our internal
interfaces around them, and recommend them to users.  As the R6RS I/O
system is the most capable standard that Scheme has yet produced in this
domain, we mostly recommend that; @code{(ice-9 binary-ports)} and
@code{(ice-9 textual-ports)} are wholly modelled on @code{(rnrs io
ports)}.  Guile does not wholly copy R6RS, however; @xref{R6RS
Incompatibilities}.

At the same time, we have many venerable port interfaces, lore handed
down to us from our hacker ancestors.  Most of these interfaces even
predate the expectation that Scheme should have modules, so they are
present in the default environment.  In Guile we support them as well
and we have no plans to remove them, but again we don't recommend them
for new users.

@rnindex char-ready?
@deffn {Scheme Procedure} char-ready? [port]
Return @code{#t} if a character is ready on input @var{port}
and return @code{#f} otherwise.  If @code{char-ready?} returns
@code{#t} then the next @code{read-char} operation on
@var{port} is guaranteed not to hang.  If @var{port} is a file
port at end of file then @code{char-ready?} returns @code{#t}.

@code{char-ready?} exists to make it possible for a
program to accept characters from interactive ports without
getting stuck waiting for input.  Any input editors associated
with such ports must make sure that characters whose existence
has been asserted by @code{char-ready?} cannot be rubbed out.
If @code{char-ready?} were to return @code{#f} at end of file,
a port at end of file would be indistinguishable from an
interactive port that has no ready characters.

Note that @code{char-ready?} only works reliably for terminals and
sockets with one-byte encodings.  Under the hood it will return
@code{#t} if the port has any input buffered, or if the file descriptor
that backs the port polls as readable, indicating that Guile can fetch
more bytes from the kernel.  However being able to fetch one byte
doesn't mean that a full character is available; @xref{Encoding}.  Also,
on many systems it's possible for a file descriptor to poll as readable,
but then block when it comes time to read bytes.  Note also that on
Linux kernels, all file ports backed by files always poll as readable.
For non-file ports, this procedure always returns @code{#t}, except for
soft ports, which have a @code{char-ready?} handler.  @xref{Soft Ports}.

In short, this is a legacy procedure whose semantics are hard to
provide.  However it is a useful check to see if any input is buffered.
@xref{Non-Blocking I/O}.
@end deffn

@rnindex read-char
@deffn {Scheme Procedure} read-char [port]
The same as @code{get-char}, except that @var{port} defaults to the
current input port.  @xref{Textual I/O}.
@end deffn

@rnindex peek-char
@deffn {Scheme Procedure} peek-char [port]
The same as @code{lookahead-char}, except that @var{port} defaults to
the current input port.  @xref{Textual I/O}.
@end deffn

@deffn {Scheme Procedure} unread-char cobj [port]
The same as @code{unget-char}, except that @var{port} defaults to the
current input port, and the arguments are swapped.  @xref{Textual I/O}.
@end deffn

@deffn {Scheme Procedure} unread-string str port
@deffnx {C Function} scm_unread_string (str, port)
The same as @code{unget-string}, except that @var{port} defaults to the
current input port, and the arguments are swapped.  @xref{Textual I/O}.
@end deffn

@rnindex newline
@deffn {Scheme Procedure} newline [port]
Send a newline to @var{port}.  If @var{port} is omitted, send to the
current output port.  Equivalent to @code{(put-char port #\newline)}.
@end deffn

@rnindex write-char
@deffn {Scheme Procedure} write-char chr [port]
The same as @code{put-char}, except that @var{port} defaults to the
current input port, and the arguments are swapped.  @xref{Textual I/O}.
@end deffn

@node Using Ports from C
@subsection Using Ports from C

Guile's C interfaces provides some niceties for sending and receiving
bytes and characters in a way that works better with C.

@deftypefn {C Function} size_t scm_c_read (SCM port, void *buffer, size_t size)
Read up to @var{size} bytes from @var{port} and store them in
@var{buffer}.  The return value is the number of bytes actually read,
which can be less than @var{size} if end-of-file has been reached.

Note that as this is a binary input procedure, this function does not
update @code{port-line} and @code{port-column} (@pxref{Textual I/O}).
@end deftypefn

@deftypefn {C Function} void scm_c_write (SCM port, const void *buffer, size_t size)
Write @var{size} bytes at @var{buffer} to @var{port}.

Note that as this is a binary output procedure, this function does not
update @code{port-line} and @code{port-column} (@pxref{Textual I/O}).
@end deftypefn

@deftypefn {C Function} size_t scm_c_read_bytes (SCM port, SCM bv, size_t start, size_t count)
@deftypefnx {C Function} void scm_c_write_bytes (SCM port, SCM bv, size_t start, size_t count)
Like @code{scm_c_read} and @code{scm_c_write}, but reading into or
writing from the bytevector @var{bv}.  @var{count} indicates the byte
index at which to start in the bytevector, and the read or write will
continue for @var{count} bytes.
@end deftypefn

@deftypefn {C Function} void scm_unget_bytes (const unsigned char *buf, size_t len, SCM port)
@deftypefnx {C Function} void scm_unget_byte (int c, SCM port)
@deftypefnx {C Function} void scm_ungetc (scm_t_wchar c, SCM port)
Like @code{unget-bytevector}, @code{unget-byte}, and @code{unget-char},
respectively.  @xref{Textual I/O}.
@end deftypefn

@deftypefn {C Function} void scm_c_put_latin1_chars (SCM port, const scm_t_uint8 *buf, size_t len)
@deftypefnx {C Function} void scm_c_put_utf32_chars (SCM port, const scm_t_uint32 *buf, size_t len);
Write a string to @var{port}.  In the first case, the
@code{scm_t_uint8*} buffer is a string in the latin-1 encoding.  In the
second, the @code{scm_t_uint32*} buffer is a string in the UTF-32
encoding.  These routines will update the port's line and column.
@end deftypefn

@node I/O Extensions
@subsection Implementing New Port Types in C

This section describes how to implement a new port type in C.  Although
ports support many operations, as a data structure they present an
opaque interface to the user.  To the port implementor, you have two
pieces of information to work with: the port type, and the port's
``stream''.  The port type is an opaque pointer allocated when defining
your port type.  It is your key into the port API, and it helps you
identify which ports are actually yours.  The ``stream'' is a pointer
you control, and which you set when you create a port.  Get a stream
from a port using the @code{SCM_STREAM} macro.  Note that your port
methods are only ever called with ports of your type.

A port type is created by calling @code{scm_make_port_type}.  Once you
have your port type, you can create ports with @code{scm_c_make_port},
or @code{scm_c_make_port_with_encoding}.

@deftypefun scm_t_port_type* scm_make_port_type (char *name, size_t (*read) (SCM port, SCM dst, size_t start, size_t count), size_t (*write) (SCM port, SCM src, size_t start, size_t count))
Define a new port type.  The @var{name}, @var{read} and @var{write}
parameters are initial values for those port type fields, as described
below.  The other fields are initialized with default values and can be
changed later.
@end deftypefun

@deftypefun SCM scm_c_make_port_with_encoding (scm_t_port_type *type, unsigned long mode_bits, SCM encoding, SCM conversion_strategy, scm_t_bits stream)
@deftypefunx SCM scm_c_make_port (scm_t_port_type *type, unsigned long mode_bits, scm_t_bits stream)
Make a port with the given @var{type}.  The @var{stream} indicates the
private data associated with the port, which your port implementation
may later retrieve with @code{SCM_STREAM}.  The mode bits should include
one or more of the flags @code{SCM_RDNG} or @code{SCM_WRTNG}, indicating
that the port is an input and/or an output port, respectively.  The mode
bits may also include @code{SCM_BUF0} or @code{SCM_BUFLINE}, indicating
that the port should be unbuffered or line-buffered, respectively.  The
default is that the port will be block-buffered.  @xref{Buffering}.

As you would imagine, @var{encoding} and @var{conversion_strategy}
specify the port's initial textual encoding and conversion strategy.
Both are symbols.  @code{scm_c_make_port} is the same as
@code{scm_c_make_port_with_encoding}, except it uses the default port
encoding and conversion strategy.
@end deftypefun

The port type has a number of associate procedures and properties which
collectively implement the port's behavior.  Creating a new port type
mostly involves writing these procedures.

@table @code
@item name
A pointer to a NUL terminated string: the name of the port type.  This
property is initialized via the first argument to
@code{scm_make_port_type}.

@item read
A port's @code{read} implementation fills read buffers.  It should copy
bytes to the supplied bytevector @code{dst}, starting at offset
@code{start} and continuing for @code{count} bytes, returning the number
of bytes read.

@item write
A port's @code{write} implementation flushes write buffers to the
mutable store.
It should write out bytes from the supplied bytevector @code{src},
starting at offset @code{start} and continuing for @code{count} bytes,
and return the number of bytes that were written.

@item read_wait_fd
@itemx write_wait_fd
If a port's @code{read} or @code{write} function returns @code{(size_t)
-1}, that indicates that reading or writing would block.  In that case
to preserve the illusion of a blocking read or write operation, Guile's
C port run-time will @code{poll} on the file descriptor returned by
either the port's @code{read_wait_fd} or @code{write_wait_fd} function.
Set using

@deftypefun void scm_set_port_read_wait_fd (scm_t_port_type *type, int (*wait_fd) (SCM port))
@deftypefunx void scm_set_port_write_wait_fd (scm_t_port_type *type, int (*wait_fd) (SCM port))
@end deftypefun

Only a port type which implements the @code{read_wait_fd} or
@code{write_wait_fd} port methods can usefully return @code{(size_t) -1}
from a read or write function.  @xref{Non-Blocking I/O}, for more on
non-blocking I/O in Guile.

@item print
Called when @code{write} is called on the port, to print a port
description.  For example, for a file port it may produce something
like: @code{#<input: /etc/passwd 3>}.  Set using

@deftypefun void scm_set_port_print (scm_t_port_type *type, int (*print) (SCM port, SCM dest_port, scm_print_state *pstate))
The first argument @var{port} is the port being printed, the second
argument @var{dest_port} is where its description should go.
@end deftypefun

@item close
Called when the port is closed.  It should free any resources used by
the port.  Set using

@deftypefun void scm_set_port_close (scm_t_port_type *type, void (*close) (SCM port))
@end deftypefun

By default, ports that are garbage collected just go away without
closing.  If your port type needs to release some external resource like
a file descriptor, or needs to make sure that its internal buffers are
flushed even if the port is collected while it was open, then mark the
port type as needing a close on GC.

@deftypefun void scm_set_port_needs_close_on_gc (scm_t_port_type *type, int needs_close_p)
@end deftypefun

@item seek
Set the current position of the port.  Guile will flush read and/or
write buffers before seeking, as appropriate.

@deftypefun void scm_set_port_seek (scm_t_port_type *type, scm_t_off (*seek) (SCM port, scm_t_off offset, int whence))
@end deftypefun

@item truncate
Truncate the port data to be specified length.  Guile will flush buffers
before hand, as appropriate.  Set using

@deftypefun void scm_set_port_truncate (scm_t_port_type *type, void (*truncate) (SCM port, scm_t_off length))
@end deftypefun

@item random_access_p
Determine whether this port is a random-access port.

@cindex random access
Seeking on a random-access port with buffered input, or switching to
writing after reading, will cause the buffered input to be discarded and
Guile will seek the port back the buffered number of bytes.  Likewise
seeking on a random-access port with buffered output, or switching to
reading after writing, will flush pending bytes with a call to the
@code{write} procedure.  @xref{Buffering}.

Indicate to Guile that your port needs this behavior by returning a
nonzero value from your @code{random_access_p} function.  The default
implementation of this function returns nonzero if the port type
supplies a seek implementation.

@deftypefun void scm_set_port_random_access_p (scm_t_port_type *type, int (*random_access_p) (SCM port));
@end deftypefun

@item get_natural_buffer_sizes
Guile will internally attach buffers to ports.  An input port always has
a read buffer and an output port always has a write buffer.
@xref{Buffering}.  A port buffer consists of a bytevector, along with
some cursors into that bytevector denoting where to get and put data.

Port implementations generally don't have to be concerned with
buffering: a port type's @code{read} or @code{write} function will
receive the buffer's bytevector as an argument, along with an offset and
a length into that bytevector, and should then either fill or empty that
bytevector.  However in some cases, port implementations may be able to
provide an appropriate default buffer size to Guile.

@deftypefun void scm_set_port_get_natural_buffer_sizes @
  (scm_t_port_type *type, void (*get_natural_buffer_sizes) (SCM, size_t *read_buf_size, size_t *write_buf_size))
Fill in @var{read_buf_size} and @var{write_buf_size} with an appropriate buffer size for this port, if one is known.
@end deftypefun

File ports implement a @code{get_natural_buffer_sizes} to let the
operating system inform Guile about the appropriate buffer sizes for the
particular file opened by the port.
@end table

Note that calls to all of these methods can proceed in parallel and
concurrently and from any thread up until the point that the port is
closed.  The call to @code{close} will happen when no other method is
running, and no method will be called after the @code{close} method is
called.  If your port implementation needs mutual exclusion to prevent
concurrency, it is responsible for locking appropriately.

@node Non-Blocking I/O
@subsection Non-Blocking I/O

Most ports in Guile are @dfn{blocking}: when you try to read a character
from a port, Guile will block on the read until a character is ready, or
end-of-stream is detected.  Likewise whenever Guile goes to write
(possibly buffered) data to an output port, Guile will block until all
the data is written.

Interacting with ports in blocking mode is very convenient: you can
write straightforward, sequential algorithms whose code flow reflects
the flow of data.  However, blocking I/O has two main limitations.

The first is that it's easy to get into a situation where code is
waiting on data.  Time spent waiting on data when code could be doing
something else is wasteful and prevents your program from reaching its
peak throughput.  If you implement a web server that sequentially
handles requests from clients, it's very easy for the server to end up
waiting on a client to finish its HTTP request, or waiting on it to
consume the response.  The end result is that you are able to serve
fewer requests per second than you'd like to serve.

The second limitation is related: a blocking parser over user-controlled
input is a denial-of-service vulnerability.  Indeed the so-called ``slow
loris'' attack of the early 2010s was just that: an attack on common web
servers that drip-fed HTTP requests, one character at a time.  All it
took was a handful of slow loris connections to occupy an entire web
server.

In Guile we would like to preserve the ability to write straightforward
blocking networking processes of all kinds, but under the hood to allow
those processes to suspend their requests if they would block.

To do this, the first piece is to allow Guile ports to declare
themselves as being nonblocking.  This is currently supported only for
file ports, which also includes sockets, terminals, or any other port
that is backed by a file descriptor.  To do that, we use an arcane UNIX
incantation:

@example
(let ((flags (fcntl socket F_GETFL)))
  (fcntl socket F_SETFL (logior O_NONBLOCK flags)))
@end example

Now the file descriptor is open in non-blocking mode.  If Guile tries to
read or write from this file and the read or write returns a result
indicating that more data can only be had by doing a blocking read or
write, Guile will block by polling on the socket's @code{read-wait-fd}
or @code{write-wait-fd}, to preserve the illusion of a blocking read or
write.  @xref{I/O Extensions} for more on those internal interfaces.

So far we have just reproduced the status quo: the file descriptor is
non-blocking, but the operations on the port do block.  To go farther,
it would be nice if we could suspend the ``thread'' using delimited
continuations, and only resume the thread once the file descriptor is
readable or writable.  (@xref{Prompts}).

But here we run into a difficulty.  The ports code is implemented in C,
which means that although we can suspend the computation to some outer
prompt, we can't resume it because Guile can't resume delimited
continuations that capture the C stack.

To overcome this difficulty we have created a compatible but entirely
parallel implementation of port operations.  To use this implementation,
do the following:

@example
(use-modules (ice-9 suspendable-ports))
(install-suspendable-ports!)
@end example

This will replace the core I/O primitives like @code{get-char} and
@code{put-bytevector} with new versions that are exactly the same as the
ones in the standard library, but with two differences.  One is that
when a read or a write would block, the suspendable port operations call
out the value of the @code{current-read-waiter} or
@code{current-write-waiter} parameter, as appropriate.
@xref{Parameters}.  The default read and write waiters do the same thing
that the C read and write waiters do, which is to poll.  User code can
parameterize the waiters, though, enabling the computation to suspend
and allow the program to process other I/O operations.  Because the new
suspendable ports implementation is written in Scheme, that suspended
computation can resume again later when it is able to make progress.
Success!

The other main difference is that because the new ports implementation
is written in Scheme, it is slower than C, currently by a factor of 3 or
4, though it depends on many factors.  For this reason we have to keep
the C implementations as the default ones.  One day when Guile's
compiler is better, we can close this gap and have only one port
operation implementation again.

Note that Guile does not currently include an implementation of the
facility to suspend the current thread and schedule other threads in the
meantime.  Before adding such a thing, we want to make sure that we're
providing the right primitives that can be used to build schedulers and
other user-space concurrency patterns, and that the patterns that we
settle on are the right patterns.  In the meantime, have a look at 8sync
(@url{https://gnu.org/software/8sync}) for a prototype of an
asynchronous I/O and concurrency facility.

@deffn {Scheme Procedure} install-suspendable-ports!
Replace the core ports implementation with suspendable ports, as
described above.  This will mutate the values of the bindings like
@code{get-char}, @code{put-u8}, and so on in place.
@end deffn

@deffn {Scheme Procedure} uninstall-suspendable-ports!
Restore the original core ports implementation, un-doing the effect of
@code{install-suspendable-ports!}.
@end deffn

@deffn {Scheme Parameter} current-read-waiter
@deffnx {Scheme Parameter} current-write-waiter
Parameters whose values are procedures of one argument, called when a
suspendable port operation would block on a port while reading or
writing, respectively.  The default values of these parameters do a
blocking @code{poll} on the port's file descriptor.  The procedures are
passed the port in question as their one argument.
@end deffn


@node BOM Handling
@subsection Handling of Unicode Byte Order Marks
@cindex BOM
@cindex byte order mark

This section documents the finer points of Guile's handling of Unicode
byte order marks (BOMs).  A byte order mark (U+FEFF) is typically found
at the start of a UTF-16 or UTF-32 stream, to allow readers to reliably
determine the byte order.  Occasionally, a BOM is found at the start of
a UTF-8 stream, but this is much less common and not generally
recommended.

Guile attempts to handle BOMs automatically, and in accordance with the
recommendations of the Unicode Standard, when the port encoding is set
to @code{UTF-8}, @code{UTF-16}, or @code{UTF-32}.  In brief, Guile
automatically writes a BOM at the start of a UTF-16 or UTF-32 stream,
and automatically consumes one from the start of a UTF-8, UTF-16, or
UTF-32 stream.

As specified in the Unicode Standard, a BOM is only handled specially at
the start of a stream, and only if the port encoding is set to
@code{UTF-8}, @code{UTF-16} or @code{UTF-32}.  If the port encoding is
set to @code{UTF-16BE}, @code{UTF-16LE}, @code{UTF-32BE}, or
@code{UTF-32LE}, then BOMs are @emph{not} handled specially, and none of
the special handling described in this section applies.

@itemize @bullet
@item
To ensure that Guile will properly detect the byte order of a UTF-16 or
UTF-32 stream, you must perform a textual read before any writes, seeks,
or binary I/O.  Guile will not attempt to read a BOM unless a read is
explicitly requested at the start of the stream.

@item
If a textual write is performed before the first read, then an arbitrary
byte order will be chosen.  Currently, big endian is the default on all
platforms, but that may change in the future.  If you wish to explicitly
control the byte order of an output stream, set the port encoding to
@code{UTF-16BE}, @code{UTF-16LE}, @code{UTF-32BE}, or @code{UTF-32LE},
and explicitly write a BOM (@code{#\xFEFF}) if desired.

@item
If @code{set-port-encoding!} is called in the middle of a stream, Guile
treats this as a new logical ``start of stream'' for purposes of BOM
handling, and will forget about any BOMs that had previously been seen.
Therefore, it may choose a different byte order than had been used
previously.  This is intended to support multiple logical text streams
embedded within a larger binary stream.

@item
Binary I/O operations are not guaranteed to update Guile's notion of
whether the port is at the ``start of the stream'', nor are they
guaranteed to produce or consume BOMs.

@item
For ports that support seeking (e.g. normal files), the input and output
streams are considered linked: if the user reads first, then a BOM will
be consumed (if appropriate), but later writes will @emph{not} produce a
BOM.  Similarly, if the user writes first, then later reads will
@emph{not} consume a BOM.

@item
For ports that are not random access (e.g. pipes, sockets, and
terminals), the input and output streams are considered
@emph{independent} for purposes of BOM handling: the first read will
consume a BOM (if appropriate), and the first write will @emph{also}
produce a BOM (if appropriate).  However, the input and output streams
will always use the same byte order.

@item
Seeks to the beginning of a file will set the ``start of stream'' flags.
Therefore, a subsequent textual read or write will consume or produce a
BOM.  However, unlike @code{set-port-encoding!}, if a byte order had
already been chosen for the port, it will remain in effect after a seek,
and cannot be changed by the presence of a BOM.  Seeks anywhere other
than the beginning of a file clear the ``start of stream'' flags.
@end itemize

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