194 lines
7.0 KiB
HTML
194 lines
7.0 KiB
HTML
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<head>
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<!-- This file has been generated by unroff 1.0, 03/21/96 19:29:45. -->
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<!-- Do not edit! -->
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<link rev="made" href="mailto:net@informatik.uni-bremen.de">
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<title>unroff Programmer's Manual, section 9.</title>
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</head><body>
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<h2><a name="section9">9.</a> <tt> </tt>Streams
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Input, output, and storage of text lines in
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<i>unroff
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</i>are centered around a new Scheme data type named
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<i>stream
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</i>and a set of primitives that work on streams.<tt> </tt>
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A stream can act as a source (input stream) or as a sink (output
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stream) for lines of text.<tt> </tt>
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Streams not only serve as the basis for input and output operations
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and for the exchange of text with shell commands, but can also be used
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to temporarily buffer lines of text (e.g. footnotes or tables of
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contents) and to implement user-defined macros in a simple way.<tt> </tt>
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Each input or output stream can be connected to one of the
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following three types of
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<i>targets</i>:
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<ul>
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<li>
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a file, or the program's standard input or standard output
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<li>
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a UNIX pipe connected to a shell running a shell command
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<li>
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an internal
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<i>buffer
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</i>whose lifetime is limited to that of the current invocation of
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<i>unroff</i>.<tt> </tt>
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</ul>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Buffers act similar to (initially empty) files, except that
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they are not visible from the outside and that they are destroyed
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automatically on exit of the program.<tt> </tt>
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Once a buffer has been filled with text through an output stream,
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it can be reopened and read through an input stream multiple times.<tt> </tt>
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However, if a buffer is currently written through an output stream,
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no more streams may refer to the same buffer.<tt> </tt>
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As the contents of buffers kept in memory, input and output operations
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on buffers are fast.<tt> </tt>
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The sample implementation of user-defined macros utilizes buffers
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to store the macro bodies; a macro can then be expanded simply
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by redirecting the current input source to the corresponding buffer
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temporarily.<tt> </tt>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Both the parser and all input and output primitives operate on a
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<i>current input stream
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</i>and a
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<i>current output stream</i>;
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input and output is always performed using these two streams.<tt> </tt>
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On startup,
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<i>unroff
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</i>initializes the current output stream to either point to
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standard output or to a newly created output file (usually depending on
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the value of the
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<b>document
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</b>option).<tt> </tt>
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If the current output stream is assigned the literal #f,
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output is sent to standard output<a href="m-notes.html#footnote2">[note 2]</a>
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.<tt> </tt>
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Likewise, for each input file mentioned in the command line,
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a stream pointing to that file is created and assigned to
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the current input stream before the parser starts processing
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the file.<tt> </tt>
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The rest of this section lists the Scheme primitives operating
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on streams.<tt> </tt>
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<h2>
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(stream? <i>obj</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>The type predicate for the new data type.<tt> </tt>
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It returns #t if
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<i>obj
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</i>is a member of the type
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<i>stream</i>,
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otherwise #f.<tt> </tt>
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<h2>
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(input-stream)
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<br>
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(output-stream)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Returns the current input stream, or output stream respectively.<tt> </tt>
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<h2>
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(open-input-stream <i>target</i>)
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<br>
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(open-output-stream <i>target</i>)
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<br>
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(append-output-stream <i>target</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>These primitives create a new input stream or output stream pointing
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to the specified target.<tt> </tt>
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The argument
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<i>target
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</i>is a string or a symbol.<tt> </tt>
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If the target is enclosed in square brackets, it names a buffer;
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if it begins with the pipe symbol `|', a pipe to a shell running
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the rest of the target as a shell command is established; otherwise
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<i>target
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</i>is interpreted as a file name.<tt> </tt>
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<i>append-output-stream
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</i>rewinds to the end of the specified output buffer or file before
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the first output operation; it acts like
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<i>open-output-stream
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</i>in case of a pipe.<tt> </tt>
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Examples:
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<dl><dt><dd>
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<pre>
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(let* ((buffer (open-output-stream '[temp]))
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(pipe (open-input-stream "|ls -l /usr/lib/tmac"))
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(file (open-input-stream "/etc/passwd")))
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...)
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</pre>
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</dl>
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<h2>
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(set-input-stream! <i>stream</i>)
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<br>
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(set-output-stream! <i>stream</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>These primitives make the specified stream the
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<i>current
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</i>input stream (or output stream respectively).<tt> </tt>
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<i>stream
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</i>must be the result of a call to one of the three primitives that
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open a stream, or #f.<tt> </tt>
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An error is signaled if
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<i>set-input-stream!
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</i>is applied to an output stream or vice versa, or if the stream
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has been closed in the meantime.<tt> </tt>
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<h2>
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(close-stream <i>stream</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Closes the specified stream.<tt> </tt>
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An error is signaled if the stream is still the current input
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stream or current output stream.<tt> </tt>
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Once an output stream pointing to a buffer has been closed, the
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buffer can be reopened for reading.<tt> </tt>
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A stream that is no longer reachable is closed automatically
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during the next run of the garbage collector.<tt> </tt>
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<h2>
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(stream-buffer? <i>stream</i>)
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<br>
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(stream-file? <i>stream</i>)
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<br>
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(stream-pipe? <i>stream</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>These predicates return #t if the specified stream points to a
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buffer, a file, or a pipe respectively, otherwise #f.<tt> </tt>
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<h2>
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(stream-target <i>stream</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>This primitive returns the target to which the specified stream
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points.<tt> </tt>
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The return value is a string.<tt> </tt>
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In case of a pipe, the target is truncated at the first space,
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that is, only the command name is included.<tt> </tt>
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The target of the current input stream (together with the current
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line number) is displayed as a prefix of error messages and
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can also be obtained through the primitive
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<a href="m-11.html#.substitute"><i>substitute</i></a>
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described below.<tt> </tt>
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<h2>
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(stream-position <i>stream</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>Returns the current character position of the specified output stream,
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that is, the offset at which the next character will be written.<tt> </tt>
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The return value for input streams is currently always zero.<tt> </tt>
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This primitive is useful in conjunction with
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<a href="m-14.html#.file-insertions"><i>file-insertions</i></a>
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(described below).<tt> </tt>
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<h2>
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(stream-string <i>target</i>)
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</h2>
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<p>
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 <tt> </tt> <tt> </tt> <tt> </tt>This primitive opens an input string to the specified target,
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reads from the stream until end-of-stream is reached, closes
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the stream, and returns the concatenation of all the lines that
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have been read as a string<a href="m-notes.html#footnote3">[note 3]</a>
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.<tt> </tt>
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<p><hr>
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Markup created by <em>unroff</em> 1.0, <tt> </tt> <tt> </tt>March 21, 1996, <tt> </tt> <tt> </tt>net@informatik.uni-bremen.de</body>
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</html>
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