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Date: Sat, 02 Dec 2006 21:24:01 -0500
From: CBFalconer <cbfalconer AT yahoo DOT com>
Organization: Ched Research http://cbfalconer.home.att.net
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Subject: Multi level ungetc
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

A few days ago it struck me that most systems would have no problem
implementing multi-level ungetc, provided it didn't try to cross
line ending boundaries.  Without that complication it will usually
simply have to adjust an internal pointer to the input buffer.  So
I wrote a little program to test it, and lo and behold, DJGPP does
just that.  Congratulations DJ.  That means strtod() can have the
proper semantics in the presence of input such as "123.4e-x", and I
have finally seen a reason for making strtod() a system routine.

#include <stdio.h>
#include <stdlib.h>
#define MAXLN 10

int main(void) {
   char line[MAXLN + 1];
   int  ix, ch;

   puts("Test ability to ungetc for multiple chars in one line");
   fputs("Enter no more than 10 chars:", stdout); fflush(stdout);
   ix = 0;
   while ((EOF != (ch = getchar())) && ('\n' != ch)) {
      if (MAXLN <= ix) break;
      line[ix++] = ch;
   }
   line[ix] = '\0';
   if ('\n' != ungetc('\n', stdin)) {
      puts("Can't unget a '\\n'");
      return(EXIT_FAILURE);
   }
   puts(line);
   puts("Trying to push back the whole line");
   while (ix > 0) {
      ch = ungetc(line[--ix], stdin);
      if (ch == line[ix]) putchar(ch);
      else {
         putchar(line[ix]);
         puts(" failed to push back");
         return(EXIT_FAILURE);
      }
   }
   puts("\nTrying to reread the whole line");
   while ((EOF != (ch = getchar())) && ('\n' != ch)) {
      if (ix++ == MAXLN) break;
      putchar(ch);
   }
   return 0;
} /* main tungetc.c */

[1] c:\c\junk>tungetc
Test ability to ungetc for multiple chars in one line
Enter no more than 10 chars:12345
12345
Trying to push back the whole line
54321
Trying to reread the whole line
12345

-- 
Chuck F (cbfalconer at maineline dot net)
   Available for consulting/temporary embedded and systems.
   <http://cbfalconer.home.att.net>