81 lines
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81 lines
6.2 KiB
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Function READ-CHAR-NO-HANG</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="f_rd_cha.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_stream.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="f_terpri.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<HR>
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<A NAME="read-char-no-hang"><I>Function</I> <B>READ-CHAR-NO-HANG</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>read-char-no-hang</B> <I><TT>&optional</TT> input-stream eof-error-p eof-value recursive-p</I> => <I>char</I><P>
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<P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>input-stream</I> -- an <A REL=DEFINITION HREF="26_glo_i.htm#input"><I>input</I></A> <A REL=DEFINITION HREF="26_glo_s.htm#stream_designator"><I>stream designator</I></A>. The default is <A REL=DEFINITION HREF="26_glo_s.htm#standard_input"><I>standard input</I></A>. <P>
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<I>eof-error-p</I>---a <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>. The default is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>. <P>
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<I>eof-value</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. The default is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<I>recursive-p</I>---a <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>. The default is <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>. <P>
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<I>char</I>---a <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A> or <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> or the <I>eof-value</I>. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#read-char-no-hang"><B>read-char-no-hang</B></A> returns a character from <I>input-stream</I> if such a character is available. If no character is available, <A REL=DEFINITION HREF="#read-char-no-hang"><B>read-char-no-hang</B></A> returns <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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If <I>recursive-p</I> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, this call is expected to be embedded in a higher-level call to <A REL=DEFINITION HREF="f_rd_rd.htm#read"><B>read</B></A> or a similar <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> used by the <A REL=DEFINITION HREF="26_glo_l.htm#lisp_reader"><I>Lisp reader</I></A>. <P>
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If an <A REL=DEFINITION HREF="26_glo_e.htm#end_of_file"><I>end of file</I></A>[2] occurs and <I>eof-error-p</I> is <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>, <I>eof-value</I> is returned. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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;; This code assumes an implementation in which a newline is not
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;; required to terminate input from the console.
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(defun test-it ()
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(unread-char (read-char))
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(list (read-char-no-hang)
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(read-char-no-hang)
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(read-char-no-hang)))
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=> TEST-IT
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;; Implementation A, where a Newline is not required to terminate
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;; interactive input on the console.
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(test-it)
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>> a
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=> (#\a NIL NIL)
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;; Implementation B, where a Newline is required to terminate
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;; interactive input on the console, and where that Newline remains
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;; on the input stream.
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(test-it)
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>> a<NEWLINE>
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=> (#\a #\Newline NIL)
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</PRE>
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</TT> <P>
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<P><B>Affected By:</B><P>
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<P>
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<A REL=DEFINITION HREF="v_debug_.htm#STstandard-inputST"><B>*standard-input*</B></A>, <A REL=DEFINITION HREF="v_termin.htm#STterminal-ioST"><B>*terminal-io*</B></A>. <P>
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<P><B>Exceptional Situations:</B><P>
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<P>
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If an <A REL=DEFINITION HREF="26_glo_e.htm#end_of_file"><I>end of file</I></A>[2] occurs when <I>eof-error-p</I> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_end_of.htm#end-of-file"><B>end-of-file</B></A> is signaled . <P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="f_listen.htm#listen"><B>listen</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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<A REL=DEFINITION HREF="#read-char-no-hang"><B>read-char-no-hang</B></A> is exactly like <A REL=DEFINITION HREF="f_rd_cha.htm#read-char"><B>read-char</B></A>, except that if it would be necessary to wait in order to get a character (as from a keyboard), <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> is immediately returned without waiting. <P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issue</A>, <I>not part of the specification</I>, applies to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss013.htm">ARGUMENTS-UNDERSPECIFIED:SPECIFY</A><P></UL><HR>
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