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Chapter 11
Input and Output

## 11.1 Ports

The global variable `eof` is bound to the end-of-file value. The standard Scheme predicate `eof-object?` returns `#t` only when applied to this value. The predicate `port?` returns `#t` only for values for which either `input-port?` or `output-port?` returns `#t`.

### 11.1.1 Current Ports

The standard Scheme procedures `current-input-port` and `current-output-port` are implemented as parameters in MzScheme. See section 7.7.1.2 for more information.

### 11.1.2 Opening File Ports

The `open-input-file` and `open-output-file` procedures accept an optional flag argument after the filename that specifies a mode for the file:

- `'binary` -- characters are returned from the port exactly as they are read from the file. Binary mode is the default mode.

- `'text` -- return and linefeed characters written to and read from the file are filtered by the port in a platform specific manner:

  - Unix and Mac OS X: no filtering occurs.

  - Windows reading: a return-linefeed combination from a file is returned by the port as a single linefeed; no filtering occurs for return characters that are not followed by a linefeed, or for a linefeed that is not preceded by a return.

  - Windows writing: a linefeed written to the port is translated into a return-linefeed combination in the file; no filtering occurs for returns.

  - Mac OS Classic reading: a return character read from the file is returned as a linefeed by the port; no filtering occurs for linefeeds.

  - Mac OS Classic writing: a return character written to the port is translated into a linefeed in the file; no filtering occurs for linefeeds.

  In Windows, `'text` mode works only with regular files; attempting to use `'text` with other kinds of files triggers an `exn:i/o:filesystem` exception.

The `open-output-file` procedure can also take a flag argument that specifies how to proceed when a file with the specified name already exists:

- `'error` -- raise `exn:i/o:filesystem` (this is the default)

- `'replace` -- remove the old file and write a new one

- `'truncate` -- overwrite the old data

- `'truncate/replace` -- try `'truncate`; if it fails, try `'replace`

- `'append` -- append to the end of the file under Unix and Mac OS; under Windows, `'append` is equivalent to `'update`, except that the file position is immediately set to the end of the file after opening it

- `'update` -- open an existing file without truncating it; if the file does not exist, the `exn:i/o:filesystem` exception is raised

The `open-input-output-file` procedure takes the same arguments as ``open-output-file``, but it produces two values: an input port and an output port. The two ports are connected in that they share the underlying file device. This procedure is intended for use with special devices that can be opened by only one process, such as `COM1` in Windows. For regualr files, sharing the device can be confusing. For example, using one port does not automatically flush the other port's buffer (see section 11.1.5 for more information about buffers), and reading or writing in one port moves the file position (if any) for the other port. For regular files, use separate `open-input-file` and `open-output-file` calls to avoid confusion.

Extra flag arguments are passed to ``open-output-file`` in any order. Appropriate flag arguments can also be passed as the last argument(s) to `call-with-input-file`, `with-input-from-file`, `call-with-output-file`, and `with-output-to-file`. When conflicting flag arguments (e.g., both `'error` and `'replace`) are provided to ``open-output-file``, `with-output-to-file`, or `call-with-output-file`, the `exn:application:mismatch` exception is raised.

Both `with-input-from-file` and `with-output-to-file` close the port they create if control jumps out of the supplied thunk (either through a continuation or an exception), and the port remains closed if control jumps back into the thunk. The current input or output port is installed and restored with `parameterize` (see section 7.7.2).

See section 11.1.5 for more information on file ports. When an input or output file-stream port is created, it is placed into the management of the current custodian (see section 9.2).

### 11.1.3 Pipes

`(make-pipe``` [`limit-k`]`)` returns two port values (see section 2.2): the first port is an input port and the second is an output port. Data written to the output port is read from the input port. The ports do not need to be explicitly closed.

The optional `limit-k` argument can be `#f` or a positive exact integer. If `limit-k` is omitted or `#f`, the new pipe holds an unlimited number of unread characters (i.e., limited only by the available memory). If `limit-k` is a positive number, then the pipe will hold at most `limit-k` unread characters; writing to the pipe's output port thereafter will block until a read from the input port makes more space available.

### 11.1.4 String Ports

Scheme input and output can be read from or collected into a string:

- `(open-input-string``` `string``)` creates an input port that reads characters from `string`.

- `(open-output-string``)` creates an output port that accumulates the output into a string.

- `(get-output-string``` `string-output-port``)` returns the characters accumulated in `string-output-port` so far in a freshly-allocated string. The characters also remain in the port for further accumulation or for later calls to `get-output-string`.

String input and output ports do not need to be explicitly closed. The ``file-position`` procedure, described in section 11.1.5, works for string ports in position-setting mode.

Example:

    (define i (open-input-string "hello world"))
    (define o (open-output-string))
    (write (read i) o)
    (get-output-string o) ; => "hello"

### 11.1.5 File-Stream Ports

A port created by ``open-input-file``, ``open-output-file``, ``subprocess``, and related functions is a file-stream port. The initial input, output, and error ports in stand-alone MzScheme are also file-stream ports.

`(file-stream-port?``` `port``)` returns `#t` if the given port is a file-stream port, `#f` otherwise.

Both input and output file-stream ports use a buffer. For an input port, a buffer is filled with immediately-available characters to speed up future reads. Thus, if a file is modified between a pair of reads to the file, the second read can produce stale data. Calling ``file-position`` to set an input port's file position flushes its buffer. For an output port, a buffer is filled to with a sequence of written to be committed as a group, typically when a newline is written. An output port's buffer use can be controlled via ``file-stream-buffer-mode`` (described below). The two ports produced by `open-input-output-file` have independent buffers.

Three procedures work primarily on file-stream ports:

- `(flush-output``` [`output-port`]`)` forces all buffered data in the given output port to be physically written. If `output-port` is omitted, then the current output port is flushed. Only file-stream ports and custom ports (see section 11.1.6) use buffers; when called on a port without a buffer, ``flush-output`` has no effect.

  By default, a file-stream port flushes its buffer automatically after each newline, but this behavior can be modified with `file-stream-buffer-mode`. In addition, the initial current output and error ports are automatically flushed when ``read``18, ``read-line``, ``read-string``, or ``read-string-avail!`` are performed on the initial standard input port.

- `(file-stream-buffer-mode``` `file-stream-output-port``` [`mode-symbol`]`)` gets or sets the buffer mode for `file-stream-output-port`. If `mode-symbol` is provided, it must be one of `'none`, `'line`, or `'block`, and the port's buffering is set accordingly. If `mode-symbol` is not provided, the current mode is returned. If the mode cannot be set or returned, the `exn:i/o:port` exception is raised.

- `(file-position``` `port``)` returns the current read/write position of `port`, and `(file-position``` `port k``)` sets the read/write position to `k`. The latter works only for file-stream and string ports, and raises the `exn:application:mismatch` exception for other port kinds. Calling ``file-position`` without a position on a non-file/non-string input port returns the number of characters that have been read from that port if the position is known (see section 11.2.3), otherwise the `exn:i/o:port` exception is raised.

  When `(file-position `port` `k`)` sets the position `k` beyond the current size of an output file or string, the file/string is enlarged to size `k` and the new region is filled with `#\nul`. If `k` is beyond the end of an input file or string, then reading thereafter returns ``eof`` without changing the port's position.

  Not all file-stream ports support setting the position. If ``file-position`` is called with a position argument on such a file-stream port, the `exn:i/o:port` exception is raised.

  When changing the file position for an output port, the port is first flushed if its buffer is not empty. Similarly, setting the position for an input port clears the port's buffer (even if the new position is the same as the old position). However, although input and output ports produced by `open-input-output-file` share the file position, setting the position via one port does not flush the other port's buffer.

- `(port-file-identity``` `file-stream-port``)` returns an exact positive integer that represents the identity of the device and file read or written by `file-stream-port`. For two ports whose open times overlap, the result of `port-file-identity` is the same for both ports if and only if the ports access the same device and file. For ports whose open times do not overlap, no guarantee is provided for the port identities (even if the ports actually access the same file) -- except as can be inferred through relationships with other ports. If `file-stream-port` is closed, the `exn:i/o:port:closed` exception is raised. Under Windows 95, 98, and Me, if `file-stream-port` is connected to a pipe instead of a file, the `exn:i/o:port` exception is raised.

### 11.1.6 Custom Ports

The ``make-custom-input-port`` and ``make-custom-output-port`` procedures create ports with arbitrary control procedures.

#### 11.1.6.1 Custom Input

`(make-custom-input-port``` `read-string-proc peek-string-proc-or-false close-proc``)` creates an input port. The port is immediately open for reading. If `close-proc` procedure has no side effects, then the port need not be explicitly closed.

- `read-string-proc` -- a procedure that takes one argument, which is a non-empty string to fill with read characters, and returns either the number of characters read, a procedure for special inputs, a waitable for blocking (see section 7.6), or ``eof``. The reading procedure should never itself block; if no input is immediately available, it should return `0` or a waitable (preferably the latter). If a value other than a non-negative exact integer, ``eof``, waitable, or procedure-of-arity-four value is returned, the `exn:application:type` exception is raised. If the returned integer is larger than the supplied string, the `exn:application:mismatch` exception is raised.

  The reading procedure can report an error by raising an exception, but only if no characters are read. Similarly, no characters should be read if ``eof``, a waitable, or a procedure is returned. In other words, no characters should be lost due to spurious exceptions or non-character data.

  A port's reading procedure may be called in multiple threads simultaneously (if the port is accessible in multiple threads). The port is responsible for its own internal synchronization. Note that improper implementation of such synchronization mechanisms might cause the reading procedure to block.

  If the reading procedure returns a waitable object (see section 7.6), the waitable is treated like a `0` result, except that the system may use the waitable to block until input is available. If the waitable becomes unblocked, a later or concurrent call to the port's reading or peeking procedure must not return `0` or a waitable (otherwise, `char-ready?` might return `#t` for the port when do data is in fact available). Although the waitable allows the system to block on the port without spinning, it may also choose to treat a waitable result exactly as `0`; in other words, the system may call the reading procedure again (even in the same thread) without completing a wait. The waitable will not be made externally available by the system.

  See below for information about returning a procedure for special inputs.

- `peek-string-proc-or-false` -- usually `#f`, which means that string peeking should be implemented by the system in terms of the reading procedure.

  Otherwise, `peek-string-proc-or-false` must be a procedure that takes two arguments: a string to fill with peeked characters and a non-negative exact integer indicating a number of characters to skip in the input stream (but not in the string to fill) before writing peeked characters into the string.

  The results and conventions for the procedure are the same as for `read-string-proc`. When the peeking procedure is called with a positive number for characters to skip, a waitable result applies specifically to the skip value, not to peeking in general. Peeking past a non-character result should be disallowed by by raising `exn:application:mismatch`.

  The system does not check that multiple peeks return consistent results, or that peeking and reading produce consistent results. If peeking produces a procedure, then a future call to the reading procedure is expected to produce the same procedure, and the one returned by peeking is never invoked.

- `close-proc` -- a procedure of zero arguments that is called to close the port. The port is not considered closed until the closing procedure returns. The port's waitable, reading procedure, peeking procedure, and closing procedure will never be used again via the port if it is closed. However, the closing procedure can be called simultaneously in multiple threads (if the port is accessible in multiple threads).

When `read-string-proc` returns a procedure, the procedure is called by ``read``,19 ``read-syntax``, or ``read-char-or-special`` to ``read'' non-character input from the port. The procedure is called exactly once before additional characters are read from the port, and the procedure must return two values: an arbitrary value and an exact, non-negative integer. The first return value is used as the read result, and the second is used as the width in characters of the result (for port position tracking). If `read-string-proc` or `peek-string-proc` returns a procedure when called by any reading procedure other than ``read``, ``read-syntax``, ``read-char-or-special``, or ``peek-char-or-special``, then the `exn:application:mismatch` exception is raised.

The four arguments to the procedure represent the source location of the non-character value, as much as it is known (see section 11.2.3). The first argument is an arbitrary value representing the source for read values -- the one passed to `read-syntax` -- or `#f` if `read` or `read-char-or-special` was called. The second argument is a line number (exact, positive integer) if known, or `#f` otherwise. The third is a column number (exact, non-negative integer) or `#f`, and the fourth is a position number (exact, positive integer) or `#f`.

When the procedure returns a syntax object, then the syntax object is used directly in the result of ``read-syntax``, and converted with ``syntax-object->datum`` for the result of ``read``. If the result is not a syntax object, then the result is used directly in the result for ``read``, and converted with ``datum->syntax-object`` for the result of ``read-syntax``. In either case, structure sharing that occurs only as a the result of multiple non-character results is not preserved as syntax sharing.

Instead of returning two values, the procedure can raise the `exn:special-comment` exception to indicate that the special result is a comment, and therefore produces no `read` result. When called by `read` and `read-syntax`, the exception is caught. The exception's `width` field indicates the width of the special object in port positions, like the second return value for a non-comment result.

Examples:

    (define /dev/null (make-custom-input-port (lambda (s) eof) #f void))
    (read-char /dev/null) ; => eof

    ;; This port produces a stream of 1s:
    (define infinite-ones
      (make-custom-input-port
       (lambda (s) (string-set! s 0 #\1) 1)
       #f
       void))
    (read-string 5 infinite-ones) ; => "11111"

    ;; To peek ahead 25000 characters, we have to
    ;;  buffer all 25000 preceding characters:
    (peek-string 5 (expt 2 5000) infinite-ones)  ; Out of memory!

    ;; With a peek procedure, we can support arbitrary lookahead:
    (define better-infinite-ones
      (make-custom-input-port
       (lambda (s) (string-set! s 0 #\1) 1)
       (lambda (s offset) (string-set! s 0 #\1) 1)
       void))
    (peek-string 5 (expt 2 5000) better-infinite-ones) ; => "11111"

    ;; This port produces 0, 1, 2, 0, 1, 2, etc:
    (define mod3-cycle/not-thread-safe
      (let ([n 0])
        (make-custom-input-port
         (lambda (s) (string-set! s 0 (integer->char (+ 48 n)))
                     (set! n (modulo (add1 n) 3))
                     1)
         #f
         void)))
    (read-string 5 mod3-cycle/not-thread-safe) ; => "01201"

    ;; Same thing, but safe for concurrent access:
    (define mod3-cycle
      (let ([n 0]
            ;; for guarding use of n:
            [lock (make-semaphore 1)])
        (make-custom-input-port
         (lambda (s)
           (if (semaphore-try-wait? lock)
               (begin
                 (string-set! s 0 (integer->char (+ 48 n)))
                 (set! n (modulo (add1 n) 3))
                 (semaphore-post lock)
                 1)
               ;; another thread must have the lock;
               ;; instead of spinning, wait until the
               ;; lock looks available:
               (make-semaphore-peek lock)))
         #f
         void)))
    (let ([result1 #f]
          [result2 #f])
      (let ([t1 (thread (lambda ()
                          (set! result1 (read-string 5 mod3-cycle))))]
            [t2 (thread (lambda ()
                          (set! result2 (read-string 5 mod3-cycle))))])
        (thread-wait t1)
        (thread-wait t2)
        (string-append result1 "," result2))) ; => "0212,10201", maybe

    ;; Non-character port results:
    (define infinite-voids
      (make-custom-input-port
       (lambda (s)
         ;; Return a procedure that produces void:
         (lambda (where line col pos) (values (void) 1)))
       #f
       void))
    (read-char infinite-voids) ; => error: non-char in an unsupported context
    (read-char-or-special infinite-voids) ; => void

#### 11.1.6.2 Custom Output

`(make-custom-output-port``` `waitable-or-false write-string-proc flush-proc close-proc``)` creates an output port. The port is immediately open for writing. If `close-proc` procedure has no side effects, then the port need not be explicitly closed. The port can buffer data within its `write-string-proc`.

- `waitable-thunk-or-false` -- `#f` or a procedure of no arguments that returns a waitable and object to be used with `object-wait-multiple` (e.g., a semaphore or another port); see section 7.6 for more information on waitables.

  If a waitable-generating procedure is supplied, it is called by the system under certain circumstances to determine when the port is ready for writing at least one character without blocking, or ready to make progress in flushing an internal buffer without blocking. For example, the procedure is called to implement `object-wait-multiple` for the output port. A returned waitable must not unblock unless the port is ready for writing. Otherwise, the guarantees of ``object-wait-multiple`` will be broken for the output port.

  The waitable-generating procedure will not always be used before a call to the writing procedure (e.g., for a non-blocking write). Even after the waitable-generating procedure is called, its result might be ignored, or the wait may be terminated arbitrarily.

  If `waitable-thunk-or-false` is `#f`, the system assumes that writes to the port will always succeed. In other words, supplying `#f` is the same as supplying `(lambda () (`make-semaphore` 1))`. If the waitable-generating procedure returns a non-waitable value, it is also treated like a `(`make-semaphore` 1)` result.

- `write-string-proc` -- a procedure of four arguments:

  - an immutable string containing characters to write;

  - a non-negative exact integer for a starting offset (inclusive) into the string,

  - a non-negative exact integer for an ending offset (exclusive) into the string,

  - a boolean; `#t` indicates that the port is allowed to keep the written characters in a buffer, and that it is allowed to block indefinitely; `#f` indicates that the write should not block, and that the port should attempt to flush its buffer and completely write new characters instead of buffering them.

  The procedure returns a non-negative exact integer representing the number of characters written and buffered, or `#f` if no characters could be written because the internal buffer could not be completely flushed. If the returned integer is larger than the supplied string, the `exn:application:mismatch` exception is raised. If the start and end indices are the same (i.e., no characters are to be written), then the final boolean argument will be `#f`, and the procedure should return `0` only if the buffer is completely flushed.

  From a user's perspective, the difference between buffered and completely written data is (1) buffered data can be lost in the future due to a failed write, and (2) `flush-output` forces all buffered data to be completely written. Under no circumstances is buffering required.

  If the writing procedure raises an exception, due either to write or commit operations, it must not have committed any characters (though it may have committed previously buffered characters).

  A port's writing procedure may be called in multiple threads simultaneously (if the port is accessible in multiple threads). The port is responsible for its own internal synchronization. Note that improper implementation of such synchronization mechanisms might cause the writing procedure to block for a non-blocking write.

- `flush-proc` -- a procedure of zero arguments that is called to flush the port's buffer, if any, in response to `flush-output`. The flushing operation can block. The flush and writing procedures can be called simultaneously in multiple threads (if the port is accessible in multiple threads). If the flushing procedure is called while another thread is flushing the buffer, the call should not return until the flush has completed.

- `close-proc` -- a procedure of zero arguments that is called to close the port. The port is not considered closed until the closing procedure returns. The port's waitable, writing procedure, flushing procedure, and closing procedure will never be used again via the port is closed. However, the closing procedure can be called simultaneously in multiple threads (if the port is accessible in multiple threads), and it may be called during a call to the writing for flushing procedures by another thread; in the latter case, the write or flush must be terminated immediately with an error.

Examples:

    (define /dev/null (make-custom-output-port
                       #f
                       (lambda (s start end buffer-ok?) (- end start))
                       void void))
    (display "hello" /dev/null)

    (define accum-list null)
    (define accumulator/not-thread-safe
      (make-custom-output-port
       #f
       (lambda (s start end buffer-ok?)
         (set! accum-list
               (append accum-list
                       (string->list (substring s start end))))
         (- end start))
       void void))
    (display "hello" accumulator/not-thread-safe)
    accum-list ; => '(#\h #\e #\l #\l #\o)

    (define accum-list null)
    (define accumulator
      (let ([lock (make-semaphore 1)])
        (make-custom-output-port
         (lambda () (make-semaphore-peek lock))
         (lambda (s start end buffer-ok?)
           (if (semaphore-try-wait? lock)
               (begin
                 (set! accum-list
                       (append accum-list
                               (string->list (substring s start end))))
                 (semaphore-post lock)
                 (- end start))
               0))
         void
         void)))
    (display "hello" accumulator)
    accum-list ; => '(#\h #\e #\l #\l #\o)

    (define (make-capitalize port)
      (make-custom-output-port
       ;; This port is ready when the original is ready:
       (lambda () port)
       ;; Writing procedure:
       (lambda (s start end buffer-ok?)
         (let ([s (list->string
                   (map char-upcase
                        (string->list (substring s start end))))])
           (if buffer-ok?
               (begin
                 (display s port)
                 (string-length s))
               (write-string-avail* s port))))
       ;; Flush procedure --- flush original port:
       (lambda () (flush-output port))
       ;; Close procedure --- close original port:
       (lambda () (close-output-port port))))
    (define orig-port (open-output-string))
    (define cap-port (make-capitalize orig-port))
    (display "Hello" cap-port)
    (get-output-string orig-port) ; => "HELLO"

## 11.2 Reading and Writing

### 11.2.1 Reading

In addition to the standard reading procedures, MzScheme provides block reading procedures such as ``read-line``, ``read-string``, and ``peek-string``:

- `(read-line``` [`input-port mode-symbol`]`)` returns a string containing the next line of characters from `input-port`. If `input-port` is omitted, the current input port is used.

  Characters are read from `input-port` until a line separator or an end-of-file is read. The line separator is not included in the result string (but it is removed from the port's stream). If no characters are read before an end-of-file is encountered, ``eof`` is returned.

  The `mode-symbol` argument determines the line separator(s). It must be one of the following symbols:

  - `'linefeed` breaks lines on linefeed characters; this is the default.

  - `'return` breaks lines on return characters.

  - `'return-linefeed` breaks lines on return-linefeed combinations. If a return character is not followed by a linefeed character, it is included in the result string; similarly, a linefeed that is not preceded by a return is included in the result string.

  - `'any` breaks lines on any of a return character, linefeed character, or return-linefeed combination. If a return character is followed by a linefeed character, the two are treated as a combination.

  - `'any-one` breaks lines on either a return or linefeed character, without recognizing return-linefeed combinations.

  Return and linefeed characters are detected after the conversions that are automatically performed when reading a file in text mode. For example, reading a file in text mode under Windows automatically changes return-linefeed combinations to a linefeed. Thus, when a file is opened in text mode, `'linefeed` is usually the appropriate ``read-line`` mode.

- `(read-string``` `k``` [`input-port`]`)` returns a string containing the next `k` characters from `input-port`. The default value of `input-port` is the current input port.

  If `k` is `0`, then the empty string is returned. Otherwise, if fewer than `k` characters are available before an end-of-file is encountered, then the returned string will contain only those characters before the end-of-file (i.e., the returned string's length will be less than `k`). 20 If no characters are available before an end-of-file, then ``eof`` is returned.

  If an error occurs during reading, some characters may be lost (i.e., if ``read-string`` successfully reads some characters before encountering an error, the characters are dropped.)

- `(read-string-avail!``` `string``` [`input-port start-k end-k`]`)` reads characters from `input-port` and puts them into `string` starting from index `start-k` (inclusive) up to `end-k` (exclusive). The default value of `input-port` is the current input port. The default value of `start-k` is `0`. The default value of `end-k` is the length of the `string`. Like ``substring``, the `exn:application:mismatch` exception is raised if `start-k` or `end-k` is out-of-range for `string`.

  If the difference between `start-k` and `end-k` is `0`, then `0` is returned and the string is not modified. If no characters are available before an end-of-file, then ``eof`` is returned. Otherwise, the return value is the number of characters read. If `m` characters are read and `m` < `end-k` `-` `start-k`, then `string` is not modified at indices `start-k` + `m` though `end-k`.

  Unlike ``read-string``, ``read-string-avail!`` returns without blocking after reading immediately-available characters. It blocks only if no characters are yet available. Also unlike ``read-string``, ``read-string-avail!`` never drops characters; if ``read-string-avail!`` successfully reads some characters and then encounters an error, it suppresses the error (treating it roughly like an end-of-file) and returns the read characters. (The error will be triggered by future reads.) If an error is encountered before any characters have been read, an exception is raised.

- `(read-string-avail!*``` `string``` [`input-port start-k end-k`]`)` is like ``read-string-avail!``, except that it returns `0` immediately if no characters are available for reading and the end-of-file is not reached.

- `(read-string-avail!/enable-break``` `string``` [`input-port start-k end-k`]`)` is like ``read-string-avail!``, except that breaks are enabled during the read. The procedure provides a guarantee about the interaction of reading and breaks: if breaking is disabled when ``read-string-avail!/enable-break`` is called, and if the `exn:break` exception is raised as a result of the call, then no characters will have been read from `input-port`. See also section 6.6.

- `(peek-string``` `k skip-k``` [`input-port`]`)` is similar to `read-string`, except that the returned characters are preserved in the port for future reads. The `skip-k` argument indicates a number of characters in the input stream to skip before collecting characters to return; thus, in total, the next `k` + `skip-k` characters are inspected.

  For most kinds of ports, inspecting `k` + `skip-k` characters requires `k` + `skip-k` bytes of memory overhead associated with the port, at least until the characters are read. No such overhead is required when peeking into a string port (see section 11.1.4), a pipe port (see section 11.1.3), or a custom port with a specific peek procedure (depending on how the peek procedure is implemented; see section 11.1.6).

- `(peek-string-avail!``` `string skip-k``` [`input-port start-k end-k`]`)` is like ``read-string-avail!``, but for peeking, and with a `skip-k` argument like ``peek-string``. When skipping characters, ``peek-string-avail!`` blocks until finding the end-of-file or at least one character past the skipped characters.

- `(peek-string-avail!*``` `string skip-k``` [`input-port start-k end-k`]`)` is like ``read-string-avail!*``, but for peeking, and with a `skip-k` argument like ``peek-string``. Since this procedure never blocks, it may return before even `skip-k` characters are available from the port.

- `(peek-string-avail!/enable-break``` `string skip-k``` [`input-port start-k end-k`]`)` is the peeking version of ``read-string-avail!/enable-break``, with a `skip-k` argument like ``peek-string``.

- `(read-char-or-special``` `input-port``)` is the same as ``read-char``, except that if the input port returns a non-character value (through a value-generating procedure in a custom port; see section 11.1.6 for details), the non-character value is returned. If the input port generates `exn:special-comment`, the exception is propagated after adjusting the port's position information based on the exception's `width` field.

- `(peek-char``` `input-port``` [`skip-k`]`)` extends the standard ``peek-char`` with an optional argument (defaulting to `0`) that represents the number of characters to skip. If the input port generates a non-character value (through a value-generating procedure in a custom port; see section 11.1.6 for details) before `skip-k` characters, the `exn:application:mismatch` exception is raised.

- `(peek-char-or-special``` `input-port``` [`skip-k`]`)` is the same as ``peek-char``, except that if the input port returns a non-character value after `skip-k` characters, the symbol `'special` is returned.

### 11.2.2 Writing

In addition to the standard printing procedures, MzScheme provides ``print``, which outputs values to a port by calling the port's print handler (see section 11.2.5), plus the block-writing procedures such as ``write-string-avail``:

- `(print``` `v``` [`output-port`]`)` outputs `v` to `output-port`. The default value of `output-port` is the current output port.

  The ``print`` procedure is used to print Scheme values in a context where a programmer expects to see a Scheme value. The rationale for providing ``print`` is that ``display`` and ``write`` both have standard output conventions, and this standardization restricts the ways that an environment can change the behavior of these procedures. No output conventions should be assumed for ``print`` so that environments are free to modify the actual output generated by ``print`` in any way. Unlike the port display and write handlers, a global port print handler can be installed through the `global-port-print-handler` parameter (see section 7.7.1.2).

- `(write-string-avail``` `string``` [`output-port start-k end-k`]`)` write characters to `output-port` from `string` starting from index `start-k` (inclusive) up to `end-k` (exclusive). The default value of `output-port` is the current output port. The default value of `start-k` is `0`. The default value of `end-k` is the length of the `string`. Like ``substring``, the `exn:application:mismatch` exception is raised if `start-k` or `end-k` is out-of-range for `string`.

  The result is the number of characters written and flushed to `output-port`. The ``write-string-avail`` procedure returns without blocking after writing as many characters as it can immediately flush. It blocks only if no characters can be flushed immediately.

  The ``write-string-avail`` procedure never drops characters; if ``write-string-avail`` successfully writes some characters and then encounters an error, it suppresses the error and returns the number of written characters. (The error will be triggered by future writes.) If an error is encountered before any characters have been written, an exception is raised.

- `(write-string-avail*``` `string``` [`output-port start-k end-k`]`)` is like ``write-string-avail``, except that it never blocks, it returns `#f` if the port contains buffered data that cannot be written immediately, and it returns `0` if the port's internal buffer (if any) is flushed but no additional characters can be written immediately.

- `(write-string-avail/enable-break``` `string``` [`input-port start-k end-k`]`)` is like ``write-string-avail``, except that breaks are enabled during the write. The procedure provides a guarantee about the interaction of writing and breaks: if breaking is disabled when ``write-string-avail/enable-break`` is called, and if the `exn:break` exception is raised as a result of the call, then no characters will have been written to `output-port`. See also section 6.6.

The ``fprintf``, ``printf``, and ``format`` procedures create formatted output:

- `(fprintf``` `output-port format-string v``` `···``)` prints formatted output to `output-port`, where `format-string` is a string that is printed; `format-string` can contain special formatting tags:

  - `~`n or `~`% prints a newline

  - `~`a or `~`A ``display``s the next argument among the `v`s

  - `~`s or `~`S ``write``s the next argument among the `v`s

  - `~`v or `~`V ``print``s the next argument among the `v`s

  - `~`e or `~`E outputs the next argument among the `v`s using the current error value conversion handler (see section 7.7.1.7) and current error printing width

  - `~`c or `~`C ``write-char``s the next argument in `v`s; if the next argument is not a character, the `exn:application:mismatch` exception is raised

  - `~`b or `~`B prints the next argument among the `v`s in binary; if the next argument is not an exact number, the `exn:application:mismatch` exception is raised

  - `~`o or `~`O prints the next argument among the `v`s in octal; if the next argument is not an exact number, the `exn:application:mismatch` exception is raised

  - `~`x or `~`X prints the next argument among the `v`s in hexadecimal; if the next argument is not an exact number, the `exn:application:mismatch` exception is raised

  - `~`~ prints a tilde (~)

  - `~``w`, where `w` is a whitespace character, skips characters in `format-string` until a non-whitespace character is encountered or until a second end-of-line is encountered (whichever happens first). An end-of-line is either `#\return`, `#\newline`, or `#\return` followed immediately by `#\newline` (on all platforms).

  The return value is void.

- `(printf``` `format-string v``` `···``)` same as ``fprintf`` with the current output port.

- `(format``` `format-string v``` `···``)` same as ``fprintf`` with a string output port where the final string is returned as the result.

When an illegal format string is supplied to one of these procedures, the `exn:application:type` exception is raised. When the format string requires more additional arguments than are supplied, the `exn:application:fprintf:mismatch` exception is raised. When more additional arguments are supplied than are used by the format string, the `exn:application:mismatch` exception is raised.

For example,

    (fprintf port "~a as a string is ~s.~n" '(3 4) "(3 4)")

prints this message to `port`:21

    (3 4) as a string is "(3 4)".

followed by a newline.

### 11.2.3 Counting Positions, Lines, and Columns

MzScheme keeps track of the position in a port as the number of characters that have been read from any input port (independent of the read/write position, which is accessed or changed with ``file-position``). In addition, MzScheme can track line locations and column locations when specifically enabled for a port via ``port-count-lines!`` or the ``port-count-lines-enabled`` parameter (see section 7.7.1.2). Position, line, and column locations for a port are used by ``read-syntax`` (see section 12.2 for more information). Position and line locations are numbered from 1; column locations are numbered from 0.

- `(port-count-lines!``` `input-port``)` turns on line and column counting for a port. Counting can be turned on at any time, though generally it is turned on before any data is read from a port. When an input port is created, if the value of the `port-count-lines-enabled` parameter is true (see section 7.7.1.2), then line counting is automatically enabled for the port. Line counting cannot be disabled for a port after it is enabled.

When counting lines, MzScheme treats linefeed, return, and return-linefeed combinations as a line terminator and as a single position (on all platforms). Each tab advances the column count to one before the next multiple of 8.

A position is known for any port as long as its value can be expressed as a fixnum (which is more than enough tracking for realistic applications in, say, syntax-error reporting). If the position for a port exceeds the value of the largest fixnum, then the position for the port becomes unknown, and line and column tacking is disabled. Return-linefeed combinations are treated as a single character position only when line and column counting is enabled.

- `(port-next-location``` `input-port``)` returns three values: a positive exact integer or `#f` for the line number of the next read character, a non-negative exact integer or `#f` for the character's column, and a positive exact integer or `#f` for the character's position.

### 11.2.4 Customizing Read

Each input port has its own port read handler. This handler is invoked to read S-expressions or syntax objects from the port when the built-in ``read`` or ``read-syntax`` procedure is applied to the port. A port read handler must accept both a single argument or three arguments:

- A single argument is supplied when the port is used with ``read``; the argument is the port being read. The return value is the value that was read from the port.

- Three arguments are supplied when the port is used with ``read-syntax``; the first argument is the port being read, the second argument is a value indicating the source, and the third argument is a list of three non-negative, exact integers (see section 12.2 for more information). The return value is a syntax object that was read from the port.

A port's read handler is configured with ``port-read-handler``:

- `(port-read-handler``` `input-port``)` returns the current port read handler for `input-port`.

- `(port-read-handler``` `input-port proc``)` sets the handler for `input-port` to `proc`.

The default port read handler reads standard Scheme expressions with MzScheme's built-in parser (see section 14.3).

### 11.2.5 Customizing Display, Write, and Print

Each output port has its own port display handler, port write handler, and port print handler. These handlers are invoked to output S-expressions to the port when the standard ``display``, ``write`` or ``print`` procedure is applied to the port. A port display/write/print handler takes a two arguments: the value to be printed and the destination port. The handler's return value is ignored.

- `(port-display-handler``` `output-port``)` returns the current port display handler for `output-port`.

- `(port-display-handler``` `output-port proc``)` sets the display handler for `output-port` to `proc`.

- `(port-write-handler``` `output-port``)` returns the current port write handler for `output-port`.

- `(port-write-handler``` `output-port proc``)` sets the write handler for `output-port` to `proc`.

- `(port-print-handler``` `output-port``)` returns the current port print handler for `output-port`.

- `(port-print-handler``` `output-port proc``)` sets the print handler for `output-port` to `proc`.

The default port display and write handlers print Scheme expressions with MzScheme's built-in printer (see section 14.4). The default print handler calls the global port print handler (the value of the `global-port-print-handler` parameter; see section 7.7.1.2); the default global port print handler is the same as the default write handler.

## 11.3 Filesystem Utilities

Additional filesystem utilities are in MzLib; see Chapter 18 in PLT MzLib: Libraries Manual.

### 11.3.1 Pathnames

File and directory paths are specified as strings. Since the syntax for pathnames can vary across platforms (e.g., under Unix, directories are separated by ``/'' while Mac OS Classic uses ``:''), MzScheme provides tools for portably constructing and deconstructing pathnames.

Most MzScheme primitives that take pathnames perform an expansion on the pathname before using it. (Procedures that build pathnames or merely check the form of a pathname do not perform this expansion.) Under Unix and Mac OS X, a user directory specification using ``~'' is expanded.22 Under Mac OS Classic, file and folder aliases are resolved to real pathnames.23 Under Windows, multiple slashes are converted to single slashes (except at the beginning of a shared folder name), and a slash is inserted after the colon in a drive specification (if it is missing). In a Windows pathname, slash and backslash are always equivalent (and can be mixed together in the same pathname).

A pathname string cannot be empty or contain a null character (`#\nul`). When an empty string or a string containing a null character is provided as a pathname to any procedure except ``absolute-path?``, ``relative-path?``, ``complete-path?``, or ``normal-case-path``, the `exn:i/o:filesystem` exception is raised.

The pathname utilities are:

- `(build-path``` `base-path sub-path``` `···``)` creates an pathname given a base pathname and any number of sub-pathname extensions. If `base-path` is an absolute pathname, the result is an absolute pathname; if `base` is a relative pathname, the result is a relative pathname. Each `sub-path` must be either a relative pathname, a directory name, the symbol `'up` (indicating the relative parent directory), or the symbol `'same` (indicating the relative current directory). Under Windows, if `base-path` is a drive specification (with or without a trailing slash) the first `sub-path` can be an absolute (driveless) path. The last `sub-path` can be a filename.

  Each `sub-path` and `base-path` can optionally end in a directory separator. If the last `sub-path` ends in a separator, it is included in the resulting pathname.

  Under Mac OS Classic, if a `sub-path` argument does not begin with a colon, one is added automatically. This means that `sub-path` arguments are never interpreted as absolute paths under Mac OS Classic. For other platforms, if an absolute path is provided for any `sub-path`, then the `exn:i/o:filesystem` exception is raised. On all platforms, if `base-path` or `sub-path` is an illegal path string (e.g., it contains a null character), the `exn:i/o:filesystem` exception is raised.

  The ``build-path`` procedure builds a pathname without checking the validity of the path or accessing the filesystem.

  The following examples assume that the current directory is `/home/joeuser` for Unix examples and `My Disk:Joe's Files` for Mac OS Classic examples.

      (define p1 (build-path (current-directory) "src" "scheme"))
        ; Unix: p1 => "/home/joeuser/src/scheme"
        ; Mac OS Classic: p1 => "My Disk:Joe's Files:src:scheme"
      (define p2 (build-path 'up 'up "docs" "MzScheme"))
        ; Unix: p2 => "../../docs/MzScheme"
        ; Mac OS Classic: p2 => ":::docs:MzScheme"
      (build-path p2 p1)
        ; Unix: raises exn:i/o:filesystem:path because p1 is absolute
        ; Mac OS Classic: => ":::docs:MzScheme:My Disk:Joe's Files:src:scheme"
      (build-path p1 p2)
        ; Unix: => "/home/joeuser/src/scheme/../../docs/MzScheme"
        ; Mac OS Classic: => "My Disk:Joe's Files:src:scheme:::docs:MzScheme"

- `(absolute-path?``` `path``)` returns `#t` if `path` is an absolute pathname, `#f` otherwise. If `path` is not a legal pathname string (e.g., it contains a null character), `#f` is returned. This procedure does not access the filesystem.

- `(relative-path?``` `path``)` returns `#t` if `path` is a relative pathname, `#f` otherwise. If `path` is not a legal pathname string (e.g., it contains a null character), `#f` is returned. This procedure does not access the filesystem.

- `(complete-path?``` `path``)` returns `#t` if `path` is a completely determined pathname (not relative to a directory or drive), `#f` otherwise. Note that under Windows, an absolute path can omit the drive specification, in which case the path is neither relative nor complete. If `path` is not a legal pathname string (e.g., it contains a null character), `#f` is returned. This procedure does not access the filesystem.

- `(path->complete-path``` `path``` [`base-path`]`)` returns `path` as a complete path. If `path` is already a complete path, it is returned as the result. Otherwise, `path` is resolved with respect to the complete path `base-path`. If `base-path` is omitted, `path` is resolved with respect to the current directory. If `base-path` is provided and it is not a complete path, the `exn:i/o:filesystem` exception is raised. This procedure does not access the filesystem.

- `(resolve-path``` `path``)` expands `path` and returns a pathname that references the same file or directory as `path`. Under Unix and Mac OS X, if `path` is a soft link to another pathname, then the referenced pathname is returned (this may be a relative pathname with respect to the directory owning `path`) otherwise `path` is returned (after expansion).

- `(expand-path``` `path``)` returns the expanded version of `path` (as described at the beginning of this section). The filesystem might be accessed, but the source or expanded pathname might be a non-existent path.

- `(simplify-path``` `path``)` eliminates up-directory (``..'' in Unix, Mac OS X, and Windows) and same-directory (``.'') indicators in `path`. If no indicators are in `path`, then `path` is returned. Otherwise, a complete path is returned; if `path` is relative, it is resolved with respect to the current directory. Up-directory indicators are dropped when they refer to the parent of a root directory. The filesystem might be accessed, but the source or expanded pathname might be a non-existent path. If `path` cannot be simplified due to a cycle of links, the `exn:i/o:filesystem` exception is raised (but a successfully simplified path may still involve a cycle of links if the cycle did not inhibit the simplification).

- `(normal-case-path``` `string``)` is deprecated, and should not be used.24 It returns `string` with ``normalized'' case letters. Under Unix and Mac OS X, this procedure always returns the input path. Under Windows and Mac OS Classic, the resulting string uses only lowercase letters. Under Windows, all forward slashes (``/'') are converted to backward slashes (```\`''), and trailing spaces are removed. This procedure does not access the filesystem or guarantee that the output string is a legal pathname (i.e., `string` and the result may contain a null character).

- `(split-path``` `path``)` deconstructs `path` into a smaller pathname and an immediate directory or file name. Three values are returned (see section 2.2):

  - `base` is either

    - a string pathname,

    - `'relative` if `path` is an immediate relative directory or filename, or

    - `#f` if `path` is a root directory.

  - `name` is either

    - a string directory name,

    - a string file name,

    - `'up` if the last part of `path` specifies the parent directory of the preceding path (e.g., ``..'' under Unix), or

    - `'same` if the last part of `path` specifies the same directory as the preceding path (e.g., ``.'' under Unix).

  - `must-be-dir?` is `#t` if `path` explicitly specifies a directory (e.g., with a trailing separator), `#f` otherwise. Note that `must-be-dir?` does not specify whether `name` is actually a directory or not, but whether `path` syntactically specified a directory.

  If `base` is `#f`, then `name` cannot be `'up` or `'same`. All strings returned for `base` and `name` are newly allocated. This procedure does not access the filesystem.

- `(find-executable-path``` `program-sub-path related-sub-path``)` finds a pathname for the executable `program-sub-path`, returning `#f` if the pathname cannot be found.

  If `related-sub-path` is not `#f`, then it must be a relative path string, and the pathname found for `program-sub-path` must be such that the file or directory `related-sub-path` exists in the same directory as the executable. The result is then the full path for the found `related-sub-path`, instead of the path for the executable.

  This procedure is used by MzScheme (as a stand-alone executable) to find the standard library collection directory (see Chapter 16). In this case, `program` is the name used to start MzScheme and `related` is `"collects"`. The `related-sub-path` argument is used because, under Unix and Mac OS X, `program-sub-path` may involve to a sequence of soft links; in this case, `related-sub-path` determines which link in the chain is relevant.

  If `program-sub-path` has a directory path, exists as a file or link to a file, and `related-sub-path` is not `#f`, ``find-executable-path`` determines whether `related-sub-path` exists relative to the directory of `program-sub-path`. If so, the complete path for `program-sub-path` is returned. Otherwise, if `program-sub-path` is a link to another file path, the destination directory of the link is checked for `related-sub-path`. Further links are inspected until `related-sub-path` is found or the end of the chain of links is reached.

  If `program-sub-path` is a pathless name, ``find-executable-path`` gets the value of the `PATH` environment variable; if this environment variable is defined, ``find-executable-path`` tries each path in `PATH` as a prefix for `program-sub-path` using the search algorithm described above for path-containing `program-sub-path`s. If the `PATH` environment variable is not defined, `program-sub-path` is prefixed with the current directory and used in the search algorithm above. (Under Windows, the current directory is always implicitly the first item in `PATH`, so ``find-executable-path`` checks the current directory first under Windows.)

- `(find-system-path``` `kind-symbol``)` returns a machine-specific path for a standard type of path specified by `kind-symbol`, which must be one of the following:

  - `'home-dir` -- the current user's home directory. (See below for information on the user's home directory in Windows and Mac OS Classic.)

  - `'pref-dir` -- the standard directory for storing the current user's preferences. Under Unix and Windows, this is the user's home directory. Under Mac OS Classic, it is the Preferences subdirectory of the System Folder. Under Mac OS X, it is the subdirectory `Library/Preferences` of the user's home directory. (See below for information on the user's home directory in Windows.)

  - `'pref-file` -- a file that contains a symbol-keyed association list of preference values; the file's directory path always matches the result returned for `'pref-dir`. Under Unix and Mac OS X, the file is `.plt-prefs.ss`, and under Windows and Mac OS Classic, the file is `plt-prefs.ss`. See also `get-preference` in Chapter 18 in PLT MzLib: Libraries Manual.

  - `'temp-dir` -- the standard directory for storing temporary files. Under Unix and Mac OS X, this is the directory specified by the `TMPDIR` environment variable, if it is defined.

  - `'init-dir` -- the directory containing the initialization file used by stand-alone MzScheme application. It is the same as the current user's home directory. (See below for information on the user's home directory in Windows and Mac OS Classic.)

  - `'init-file` -- the file loaded at start-up by the stand-alone MzScheme application. The directory part of the path is the same path as returned for `'init-dir`. The file name is platform-specific:

    - Unix and Mac OS X: `.mzschemerc`

    - Windows and Mac OS Classic: `mzschemerc.ss`

  - `'addon-dir` -- a directory for installing PLT Scheme extensions. The directory is typically user-specific. Under Unix, the directory is `.plt-scheme` in the user's home directory. Under Windows, it is `PLT Scheme` in the user's `Application Data` folder. Under Mac OS X, it is `Library/PLT Scheme` in the user's home directory. Under Mac OS Classic, it is `PLT Scheme` in the preferences folder.

  - `'sys-dir` -- the directory containing the operating system for Windows or Mac OS Classic. Under Unix and Mac OS X, the result is `"/"`.

  - `'exec-file` -- the pathname of the MzScheme executable as provided by the operating system for the current invocation.25 Under Mac OS Classic, in the stand-alone MzScheme (or MrEd) application, it is always a complete path. In the stand-alone MzScheme application, this path is also bound initially to `program`.

  Under Windows, the user's home directory is the one specified by the `HOMEDRIVE` and `HOMEPATH` environment variables. If those environment variables are not defined, or if the indicated directory does not exist, the directory containing the MzScheme executable is used as the home directory. Under Mac OS Classic, the user's ``home directory'' is the preferences directory.

- `(path-list-string->path-list``` `string default-path-list``)` parses a string containing a list of paths, and returns a list of path strings. Under Unix and Mac OS X, paths in a path list are separated by a colon (``:''); under Windows and Mac OS Classic, paths are separated by a semi-colon (``;''). Whenever the path list contains an empty path, the list `default-path-list` is spliced into the returned list of paths. Parts of `string` that do not form a valid path are not included in the returned list. (The content of the list `default-path-list` is not inspected.)

### 11.3.2 Files

The file management utilities are:

- `(file-exists?``` `path``)` returns `#t` if a file (not a directory) `path` exists, `#f` otherwise. Unlike some other procedures that take a path argument, this procedure never raises the `exn:i/o:filesystem` exception.26

- `(link-exists?``` `path``)` returns `#t` if a link `path` exists (Unix, Mac OS X, and Mac OS Classic), `#f` otherwise. Note that the predicates ``file-exists?`` or ``directory-exists?`` work on the final destination of a link or series of links, while ``link-exists?`` only follows links to resolve the base part of `path` (i.e., everything except the last name in the path). This procedure never raises the `exn:i/o:filesystem` exception.

- `(delete-file``` `path``)` deletes the file with pathname `path` if it exists, returning void if a file was deleted successfully, otherwise the `exn:i/o:filesystem` exception is raised. If `path` is a link, the link is deleted rather than the destination of the link.

- `(rename-file-or-directory``` `old-path new-path``` [`exists-ok?`]`)` renames the file or directory with pathname `old-path` -- if it exists -- to the pathname `new-path`. If the file or directory is renamed successfully, void is returned, otherwise the `exn:i/o:filesystem` exception is raised.

  This procedure can be used to move a file/directory to a different directory (on the same disk) as well as rename a file/directory within a directory. Unless `exists-ok?` is provided as a true value, `new-path` cannot refer to an existing file or directory. Even if `exists-ok?` is true, `new-path` cannot refer to an existing file when `old-path` is a directory, and vice versa. (If `new-path` exists and is replaced, the replacement is atomic in the filesystem, except under Windows 95, 98, or Me. However, the check for existence is not included in the atomic action, which means that race conditions are possible when `exists-ok?` is false or not supplied.)

  If `old-path` is a link, the link is renamed rather than the destination of the link, and it counts as a file for replacing any existing `new-path`.

- `(file-or-directory-modify-seconds``` `path``` [`secs-n`]`)` returns the file or directory's last modification date as platform-specific seconds (see also section 15.1) when `secs-n` is not provided.27 If `secs-n` is provided, the access and modification times of `path` are set to the given time. On error (e.g., if no such file exists), the `exn:i/o:filesystem` exception is raised.

- `(file-or-directory-permissions``` `path``)` returns a list containing `'read`, `'write`, and/or `'execute` for the given file or directory path. On error (e.g., if no such file exists), the `exn:i/o:filesystem` exception is raised.

- `(file-size``` `path``)` returns the (logical) size of the specified file. (Under Mac OS Classic, this size excludes the resource-fork size.) On error (e.g., if no such file exists), the `exn:i/o:filesystem` exception is raised.

- `(copy-file``` `src-path dest-path``)` creates the file `dest-path` as a copy of `src-path`. If the file is successfully copied, void is returned, otherwise the `exn:i/o:filesystem` exception is raised. If `dest-path` already exists, the copy will fail. File permissions are preserved in the copy. Under Mac OS Classic, the resource fork is also preserved in the copy. If `src-path` refers to a link, the target of the link is copied, rather than the link itself.

- `(make-file-or-directory-link``` `to-path path``)` creates a link `path` to `to-path` under Unix and Mac OS X. The creation will fail if `path` already exists. The `to-path` need not refer to an existing file or directory. If the link is created successfully, void is returned, otherwise the `exn:i/o:filesystem` exception is raised. Under Windows and Mac OS Classic, the `exn:misc:unsupported` exception is raised always.

### 11.3.3 Directories

The directory management utilities are:

- `(`current-directory`)` returns the current directory and `(current-directory `path`)` sets the current directory to `path`. This procedure is actually a parameter, as described in section 7.7.1.1.

- `(current-drive``)` returns the current drive name under Windows. For other platforms, the `exn:misc:unsupported` exception is raised. The current drive is always the drive of the current directory.

- `(directory-exists?``` `path``)` returns `#t` if `path` refers to a directory, `#f` otherwise. Unlike other procedures that take a path argument, this procedure never raises the `exn:i/o:filesystem` exception.

- `(make-directory``` `path``)` creates a new directory with the pathname `path`. If the directory is created successfully, void is returned, otherwise the `exn:i/o:filesystem` exception is raised.

- `(delete-directory``` `path``)` deletes an existing directory with the pathname `path`. If the directory is deleted successfully, void is returned, otherwise the `exn:i/o:filesystem` exception is raised.

- `(`rename-file-or-directory` `old-path` `new-path` `exists-ok?`)`, as described in the previous section, renames directories.

- `(`file-or-directory-modify-seconds` `path`)`, as described in the previous section, gets directory dates.

- `(`file-or-directory-permissions` `path`)`, as described in the previous section, gets directory permissions.

- `(directory-list``` [`path`]`)` returns a list of all files and directories in the directory specified by `path`. If `path` is omitted, a list of files and directories in the current directory is returned.

- `(filesystem-root-list``)` returns a list of all current root directories.

## 11.4 Networking

Mzscheme supports networking with the TCP and UDP protocols.

### 11.4.1 TCP

For information about TCP in general, see TCP/IP Illustrated, Volume 1 by W. Richard Stevens.

- `(tcp-listen``` `port-k``` [`max-allow-wait-k reuse? hostname-string-or-false`]`)` creates a ``listening'' server on the local machine at the specified port number (where `port-k` is an exact integer between `1` and `65535` inclusive). The `max-allow-wait-k` argument determines the maximum number of client connections that can be waiting for acceptance. (When `max-allow-wait-k` clients are waiting acceptance, no new client connections can be made.) The default value for `max-allow-wait-k` argument is `4`.

  If the `reuse?` argument is true, then ``tcp-listen`` will create a listener even if the port is involved in a `TIME_WAIT` state. Such a use of `reuse?` defeats certain guarantees of the TCP protocol; see Stevens's book for details. The default for `reuse?` is `#f`.

  If `hostname-string-or-false` is `#f` (the default), then the listener accepts connections to all of the listening machine's IP addresses. Otherwise, the listener accepts connections only at the IP address associated with the given name. For example, providing `"127.0.0.1"` as `hostname-string-or-false` typically creates a listener that accepts only connections to `"127.0.0.1"` from the local machine.

  The return value of ``tcp-listen`` is a TCP listener value. This value can be used in future calls to ``tcp-accept``, ``tcp-accept-ready?``, and ``tcp-close``. Each new TCP listener value is placed into the management of the current custodian (see section 9.2).

  If the server cannot be started by ``tcp-listen``, the `exn:i/o:tcp` exception is raised.

- `(tcp-connect``` `hostname-string port-k``)` attempts to connect as a client to a listening server. The `hostname-string` argument is the server host's internet address name 28 (e.g., `"www.plt-scheme.org"`), and `port-k` (an exact integer between `1` and `65535`) is the port where the server is listening.

  Two values (see section 2.2) are returned by ``tcp-connect``: an input port and an output port. Data can be received from the server through the input port and sent to the server through the output port. If the server is a MzScheme process, it can obtain ports to communicate to the client with ``tcp-accept``. These ports are placed into the management of the current custodian (see section 9.2).

  Both of the returned ports must be closed to terminate the TCP connection. When both ports are still open, closing the output port with `close-output-port` sends a TCP close to the server (which is seen as an end-of-file if the server reads the connection through a port). In contrast, `tcp-abandon-port` (see below) closes the output port, but does not send a TCP close until the input port is also closed.

  If a connection cannot be established by ``tcp-connect``, the `exn:i/o:tcp` exception is raised.

- `(tcp-connect/enable-break``` `hostname-string port-k``)` is like ``tcp-connect``, but breaking is enabled (see section 6.6) while trying to connect. If breaking is disabled when ``tcp-connect/enable-break`` is called, then either ports are returned or the `exn:break` exception is raised, but not both.

- `(tcp-accept``` `tcp-listener``)` accepts a client connection for the server associated with `tcp-listener`. The `tcp-listener` argument is a TCP listener value returned by ``tcp-listen``. If no client connection is waiting on the listening port, the call to ``tcp-accept`` will block. (See also ``tcp-accept-ready?``, below.)

  Two values (see section 2.2) are returned by ``tcp-accept``: an input port and an output port. Data can be received from the client through the input port and sent to the client through the output port. These ports are placed into the management of the current custodian (see section 9.2).

  Both of the returned ports must be closed to terminate the connection. When both ports are still open, closing the output port with `close-output-port` sends a TCP close to the client (which is seen as an end-of-file if the client reads the connection through a port). In contrast, `tcp-abandon-port` (see below) closes the output port, but does not send a TCP close until the input port is also closed.

  If a connection cannot be accepted by ``tcp-accept``, or if the listener has been closed, the `exn:i/o:tcp` exception is raised.

- `(tcp-accept-ready?``` `tcp-listener``)` tests whether an unaccepted client has connected to the server associated with `tcp-listener`. The `tcp-listener` argument is a TCP listener value returned by ``tcp-listen``. If a client is waiting, the return value is `#t`, otherwise it is `#f`. A client is accepted with the ``tcp-accept`` procedure, which returns ports for communicating with the client and removes the client from the list of unaccepted clients.

  If the listener has been closed, the `exn:i/o:tcp` exception is raised.

- `(tcp-accept/enable-break``` `tcp-listener``)` is like ``tcp-accept``, but breaking is enabled (see section 6.6) while trying to accept a connection. If breaking is disabled when ``tcp-accept/enable-break`` is called, then either ports are returned or the `exn:break` exception is raised, but not both.

- `(tcp-close``` `tcp-listener``)` shuts down the server associated with `tcp-listener`. The `tcp-listener` argument is a TCP listener value returned by ``tcp-listen``. All unaccepted clients receive an end-of-file from the server; connections to accepted clients are unaffected.

  If the listener has already been closed, the `exn:i/o:tcp` exception is raised.

  The listener's port number may not become immediately available for new listeners (with the default `reuse?` argument of ``tcp-listen``). For further information, see Stevens's explanation of the `TIME_WAIT` TCP state.

- `(tcp-listener?``` `v``)` returns `#t` if `v` is a TCP listener value created by ``tcp-listen``, `#f` otherwise.

- `(tcp-abandon-port``` `tcp-port``)` is like `close-output-port` or `close-input-port` (depending on whether `tcp-port` is an input or output port), but if `tcp-port` is an output port and its associated input port is not yet closed, then then other end of the TCP connection does not receive a TCP close message until the input port is also closed.29

- `(tcp-addresses``` `tcp-port``)` returns two strings. The first string is the internet address for the local machine a viewed by the given TCP port's connection.30 The second string is the internet address for the other end of the connection.

  If the given port has been closed, the `exn:i/o:tcp` exception is raised.

- `(tcp-port?``` `v``)` returns `#t` if `v` is a port returned by `tcp-accept`, `tcp-connect`, `tcp-accept/enable-break`, or `tcp-connect/enable-break`, `#f` otherwise.

### 11.4.2 UDP

For information about UDP in general, see TCP/IP Illustrated, Volume 1 by W. Richard Stevens (which discusses UDP in addition to TCP).

- `(udp-open-socket``)` creates and returns a UDP socket to send and receive datagrams (broadcasting is allowed). Initially, the socket is not bound or connected to any address or port.

- `(udp-bind!``` `udp-socket hostname-string-or-false port-k``)` binds an unbound `udp-socket` to the local port number `port-k` (an exact integer between `1` and `65535`). The result is always void.

  If `hostname-string-or-false` is `#f`, then the socket accepts connections to all of the listening machine's IP addresses at `port-k`. Otherwise, the socket accepts connections only at the IP address associated with the given name. For example, providing `"127.0.0.1"` as `hostname-string-or-false` typically creates a listener that accepts only connections to `"127.0.0.1"` from the local machine.

  A socket cannot receive datagrams until it is bound to a local address and port. If a socket is not bound before it is used with a sending procedure `udp-send`, `udp-send-to`, etc., the sending procedure binds the socket to a random local port. The binding of a bound socket cannot be changed.

  If `udp-socket` is already bound or closed, the `exn:i/o:udp` exception is raised.

- `(udp-connect!``` `udp-socket hostname-string-or-false port-k-or-false``)` connects the socket to the indicated remote address and port if `hostname-string-or-false` is a string and `port-k-or-false` is an exact integer between `1` and `65535`. The result is always void.

  If `hostname-string-or-false` is `#f`, then `port-k-or-false` also must be `#f`, and the port is disconnected (if connected). If one of `hostname-string-or-false` or `port-k-or-false` is `#f` and the other is not, the `exn:application:mismatch` exception is raised.

  A connected socket can be used with `udp-send` (not `udp-send-to`), and it accepts datagrams only from the connected address and port. A socket need not be connected to receive datagrams. A socket can be connected, re-connected, and disconnected any number of times.

  If `udp-socket` is closed, the `exn:i/o:udp` exception is raised.

- `(udp-send-to``` `udp-socket hostname-address port-k string``` [`start-k end-k`]`)` sends `(substring `starting` `start-k` `end-k`)` as a datagram from the unconnected `udp-socket` to the socket at the remote machine `hostname-address` on the port `port-k`. The `udp-socket` need not be bound or connected; if it is not bound, `udp-send-to` binds it to a random local port. If the socket's outgoing datagram queue is too full to support the send, `udp-send-to` blocks until the datagram can be queued. The result is always void.

  The optional `start-k` argument defaults to `0`, and the optional `end-k` argument defaults to the length of `string`. If `start-k` is greater than the length of `string`, or if `end-k` is less than `start-k` or greater than the length of `string`, the `exn:application:mismatch` exception is raised.

  If `udp-socket` is closed or connected, the `exn:i/o:udp` exception is raised.

- `(udp-send``` `udp-socket string``` [`start-k end-k`]`)` is like `udp-send-to`, except that `udp-socket` must be connected, and the datagram goes to the connection target. If `udp-socket` is closed or unconnected, the `exn:i/o:udp` exception is raised.

- `(udp-send-to*``` `udp-socket hostname-address port-k string``` [`start-k end-k`]`)` is like `udp-send-to`, except that it never blocks; if the socket's outgoing queue is too full to support the send, `#f` is returned, otherwise the datagram is queued and the result is `#t`.

- `(udp-send*``` `udp-socket string``` [`start-k end-k`]`)` is like `udp-send`, except that (like `udp-send-to`) it never blocks and returns `#f` or `#t`.

- `(udp-send-to/enable-break``` `udp-socket hostname-address port-k string``` [`start-k end-k`]`)` is like `udp-send-to`, but breaking is enabled (see section 6.6) while trying to send the datagram. If breaking is disabled when `udp-send-to/enable-break` is called, then either the datagram is sent or the `exn:break` exception is raised, but not both.

- `(udp-send/enable-break``` `udp-socket string``` [`start-k end-k`]`)` is like `udp-send`, except that breaks are enabled like `udp-send-to/enable-break`.

- `(udp-receive!``` `udp-socket mutable-string``` [`start-k end-k`]`)` accepts up to `end-k` `-` `start-k` characters of `udp-socket`'s next incoming datagram into `mutable-string`, writing the datagram characters starting at position `start-k` within `mutable-string`. The `udp-socket` must be bound to a local address and port (but need not be connected). If no incoming datagram is immediately available, `udp-receive!` blocks until one is available.

  Three values are returned: an exact integer that indicates the number of received characters (between `0` and `end-k` `-` `start-k`), a hostname string indicating the source address of the datagram, and an exact integer between `1` and `65535` indicating the source port of the datagram. If the received datagram is longer than `end-k` `-` `start-k` bytes, the remainder is discarded.

  The optional `start-k` argument defaults to `0`, and the optional `end-k` argument defaults to the length of `mutable-string`. If `start-k` is greater than the length of `mutable-string`, or if `end-k` is less than `start-k` or greater than the length of `mutable-string`, the `exn:application:mismatch` exception is raised.

- `(udp-receive!*``` `udp-socket mutable-string``` [`start-k end-k`]`)` is like `udp-receive!`, except that it never blocks. If no datagram is available, the three result values are all `#f`.

- `(udp-receive!/enable-break``` `udp-socket mutable-string``` [`start-k end-k`]`)` is like `udp-receive!`, but breaking is enabled (see section 6.6) while trying to receive the datagram. If breaking is disabled when `udp-receive!/enable-break` is called, then either a datagram is received or the `exn:break` exception is raised, but not both.

- `(udp-close``` `udp-socket``)` closes `udp-socket`, discarding unreceived datagrams. If the socket is already closed, the `exn:i/o:udp` exception is raised.

- `(udp?``` `v``)` returns `#t` if `v` is a socket created by `udp-open-socket`, `#f` otherwise.

- `(udp-bound?``` `udp-socket``)` returns `#t` if `udp-socket` is bound to a local address and port, `#f` otherwise.

- `(udp-connected?``` `udp-socket``)` returns `#t` if `udp-socket` is connected to a remote address and port, `#f` otherwise.

- `(udp->send-waitable``` `udp-socket``)` returns a waitable object that is in a blocking state when `udp-send-to` on `udp-socket` would block.

- `(udp->receive-waitable``` `udp-socket``)` returns a waitable object that is in a blocking state when `udp-receive!` on `udp-socket` would block.

18 Flushing is performed by the default port read handler (see section 11.2.4) rather than by ``read`` itself.

19 More precisely, the procedure is used by the default port read handler; see also section 11.2.4.

20 A temporary string of size `k` is allocated while reading the input, even if the size of the result is less than `k` characters.

21 Assuming that the current port display and write handlers are the default ones; see section 11.2.5 for more information.

22 Under Unix and Mac OS X, expansion does not convert multiple adjacent slashes to a single slash. However, extra slashes in a pathname are always ignored.

23 Mac OS X follows the Unix behavior in its treatment of links, and Mac OS Classic aliases are simply zero-length files.

24 The problem is that the procedure ``normalizes'' based on the platform, but case sensitivity is instead a property of an individual filesystem mount, not the platform that performs the mount.

25 For MrEd, the executable path is the name of a MrEd executable.

26 Under Windows, ``file-exists?`` reports `#t` for all variations of the special filenames (e.g., `"LPT1"`, `"x:/baddir/LPT1"`).

27 For FAT filesystems under Windows, directories do not have modification dates. Therefore, the creation date is returned for a directory (but the modification date is returned for a file).

28 The name `"localhost"` generally specifies the local machine.

29 The TCP protocol does not include a ``no longer reading'' state on connections, so `tcp-abandon-port` is equivalent to `close-input-port` on input TCP ports.

30 For most machines, the answer corresponds to the current machine's only internet address. But when a machine serves multiple addresses, the result is connection-specific.