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< h1 id = "title-non-xs" > < a href = "index.html" > The Common Lisp Cookbook< / a > – Input/Output< / h1 >
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< p > < a name = "redir" > < / a > < / p >
< h2 id = "redirecting-the-standard-output-of-your-program" > Redirecting the Standard Output of your Program< / h2 >
< p > You do it like this:< / p >
< pre > < code class = "language-lisp" > (let ((*standard-output* < some form generating a stream> ))
...)
< / code > < / pre >
< p > Because
< a href = "http://www.lispworks.com/documentation/HyperSpec/Body/v_debug_.htm" > < code > *STANDARD-OUTPUT*< / code > < / a >
is a dynamic variable, all references to it during execution of the body of the
< code > LET< / code > form refer to the stream that you bound it to. After exiting the < code > LET< / code >
form, the old value of < code > *STANDARD-OUTPUT*< / code > is restored, no matter if the exit
was by normal execution, a < code > RETURN-FROM< / code > leaving the whole function, an
exception, or what-have-you. (This is, incidentally, why global variables lose
much of their brokenness in Common Lisp compared to other languages: since they
can be bound for the execution of a specific form without the risk of losing
their former value after the form has finished, their use is quite safe; they
act much like additional parameters that are passed to every function.)< / p >
< p > If the output of the program should go to a file, you can do the following:< / p >
< pre > < code class = "language-lisp" > (with-open-file (*standard-output* "somefile.dat" :direction :output
:if-exists :supersede)
...)
< / code > < / pre >
< p > < a href = "http://www.lispworks.com/documentation/HyperSpec/Body/m_w_open.htm" > < code > WITH-OPEN-FILE< / code > < / a >
opens the file - creating it if necessary - binds < code > *STANDARD-OUTPUT*< / code > , executes
its body, closes the file, and restores < code > *STANDARD-OUTPUT*< / code > to its former
value. It doesn’ < a name = "faith" > < / a > < / p >
< h2 id = "faithful-output-with-character-streams" > Faithful Output with Character Streams< / h2 >
< p > By < em > faithful output< / em > I mean that characters with codes between 0 and 255 will be
written out as is. It means, that I can < code > (PRINC (CODE-CHAR 0..255) s)< / code > to a
stream and expect 8-bit bytes to be written out, which is not obvious in the
times of Unicode and 16 or 32 bit character representations. It does < em > not< / em >
require that the characters ä, ß, or þ must have their
< a href = "http://www.lispworks.com/documentation/HyperSpec/Body/f_char_c.htm" > < code > CHAR-CODE< / code > < / a >
in the range 0..255 - the implementation is free to use any code. But it does
require that no < code > #\Newline< / code > to CRLF translation takes place, among others.< / p >
< p > Common Lisp has a long tradition of distinguishing character from byte (binary)
I/O,
e.g. < a href = "http://www.lispworks.com/documentation/HyperSpec/Body/f_rd_by.htm" > < code > READ-BYTE< / code > < / a >
and
< a href = "http://www.lispworks.com/documentation/HyperSpec/Body/f_rd_cha.htm" > < code > READ-CHAR< / code > < / a >
are in the standard. Some implementations let both functions be called
interchangeably. Others allow either one or the other. (The
< a href = "https://www.cliki.net/simple-stream" > simple stream proposal< / a > defines the
notion of a < em > bivalent stream< / em > where both are possible.)< / p >
< p > Varying element-types are useful as some protocols rely on the ability to send
8-Bit output on a channel. E.g. with HTTP, the header is normally ASCII and
ought to use CRLF as line terminators, whereas the body can have the MIME type
application/octet-stream, where CRLF translation would destroy the data. (This
is how the Netscape browser on MS-Windows destroys data sent by incorrectly
configured Webservers which declare unknown files as having MIME type
text/plain - the default in most Apache configurations).< / p >
< p > What follows is a list of implementation dependent choices and behaviours and some code to experiment.< / p >
< h3 id = "clisp" > CLISP< / h3 >
< p > On CLISP, faithful output is possible using< / p >
< pre > < code class = "language-lisp" > :external-format
(ext:make-encoding :charset 'charset:iso-8859-1 :line-terminator :unix)
< / code > < / pre >
< p > You can also use < code > (SETF (STREAM-ELEMENT-TYPE F) '(UNSIGNED-BYTE 8))< / code > , where the
ability to < code > SETF< / code > is a CLISP-specific extension. Using < code > :EXTERNAL-FORMAT :UNIX< / code >
will cause portability problems, since the default character set on MS-Windows
is < code > CHARSET:CP1252< / code > . < code > CHARSET:CP1252< / code > doesn’ < code > (CODE-CHAR
#x81)< / code > :< / p >
< pre > < code class = "language-lisp" > ;*** - Character #\u0080 cannot be represented in the character set CHARSET:CP1252
< / code > < / pre >
< p > Characters with code > 127 cannot be represented in ASCII:< / p >
< pre > < code class = "language-lisp" > ;*** - Character #\u0080 cannot be represented in the character set CHARSET:ASCII
< / code > < / pre >
< h3 id = "cmucl" > CMUCL< / h3 >
< p > < code > :EXTERNAL-FORMAT :DEFAULT< / code > (untested) - no unicode, so probably no problems.< / p >
< h3 id = "allegrocl" > AllegroCL< / h3 >
< p > < code > #+(AND ALLEGRO UNIX) :DEFAULT< / code > (untested) - seems enough on UNIX, but would not
work on the MS-Windows port of AllegroCL.< / p >
< h3 id = "lispworks" > LispWorks< / h3 >
< p > < code > :EXTERNAL-FORMAT '(:LATIN-1 :EOL-STYLE :LF)< / code > (confirmed by Marc Battyani)< / p >
< h3 id = "example" > Example< / h3 >
< p > Here’ < / p >
< pre > < code class = "language-lisp" > (defvar *unicode-test-file* "faithtest-out.txt")
(defun generate-256 (& key (filename *unicode-test-file*)
#+CLISP (charset 'charset:iso-8859-1)
external-format)
(let ((e (or external-format
#+CLISP (ext:make-encoding :charset charset :line-terminator :unix))))
(describe e)
(with-open-file (f filename :direction :output
:external-format e)
(write-sequence
(loop with s = (make-string 256)
for i from 0 to 255
do (setf (char s i) (code-char i))
finally (return s))
f)
(file-position f))))
;(generate-256 :external-format :default)
;#+CLISP (generate-256 :external-format :unix)
;#+CLISP (generate-256 :external-format 'charset:ascii)
;(generate-256)
(defun check-256 (& optional (filename *unicode-test-file*))
(with-open-file (f filename :direction :input
:element-type '(unsigned-byte 8))
(loop for i from 0
for c = (read-byte f nil nil)
while c
unless (= c i)
do (format t "~& Position ~D found ~D(#x~X)." i c c)
when (and (= i 33) (= c 32))
do (let ((c (read-byte f)))
(format t "~& Resync back 1 byte ~D(#x~X) - cause CRLF?." c c) ))
(file-length f)))
#| CLISP
(check-256 *unicode-test-file*)
(progn (generate-256 :external-format :unix) (check-256))
; uses UTF-8 -> 385 bytes
(progn (generate-256 :charset 'charset:iso-8859-1) (check-256))
(progn (generate-256 :external-format :default) (check-256))
; uses UTF-8 + CRLF(on MS-Windows) -> 387 bytes
(progn (generate-256 :external-format
(ext:make-encoding :charset 'charset:iso-8859-1 :line-terminator :mac)) (check-256))
(progn (generate-256 :external-format
(ext:make-encoding :charset 'charset:iso-8859-1 :line-terminator :dos)) (check-256))
|#
< / code > < / pre >
< p > < a name = "bulk" > < / a > < / p >
< h2 id = "fast-bulk-io" > Fast Bulk I/O< / h2 >
< p > If you need to copy a lot of data and the source and destination are both
streams (of the same
< a href = "http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_e.htm#element_type" > element type< / a > ),
it’
< a href = "http://www.lispworks.com/documentation/HyperSpec/Body/f_rd_seq.htm" > < code > READ-SEQUENCE< / code > < / a >
and
< a href = "http://www.lispworks.com/documentation/HyperSpec/Body/f_wr_seq.htm" > < code > WRITE-SEQUENCE< / code > < / a > :< / p >
< pre > < code class = "language-lisp" > (let ((buf (make-array 4096 :element-type (stream-element-type input-stream))))
(loop for pos = (read-sequence buf input-stream)
while (plusp pos)
do (write-sequence buf output-stream :end pos)))
< / code > < / pre >
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