[Bug c/41698] New: "\uFFFF" converts incorrectly to two-byte character

[chasonr at newsguy dot com]

GCC 4.4.1 incorrectly parses the code point U+FFFF when generating a two-byte
character.  It mistakes this code point for a supplemental one, and generates
an improper surrogate pair U+D7FF U+DFFF.  This bug is present as far back as
GCC 3.4.6.

Here is a test program that demonstrates the bug, and could function as a
regression test.  This program uses char16_t, but GCC 3.4.5 as shipped with
MinGW also shows this bug when wchar_t is used.

--CUT HERE--CUT HERE--CUT HERE--CUT HERE--CUT HERE--CUT HERE--CUT HERE--
/* gcc-utf16-test.c -- demonstrate a bug in GCC 4.4.1, that causes the code
   point U+FFFF to convert incorrectly to UTF-16.
   Compile on GCC 4.4.1 with -std=gnu99. */

#include <stdio.h>
#include <stdlib.h>

int
main(void)
{
    static const __CHAR16_TYPE__ teststr1[] = u"\uFFFF";
    static const __CHAR16_TYPE__ teststr2[] = u"\U00010000";
    size_t i;

    printf("The string \"\\uFFFF\" converts as:");
    for (i = 0; teststr1[i] != 0; i++)
        printf(" U+%04X", teststr1[i]);
    printf("\n");
    if (teststr1[0] != 0xFFFF || teststr1[1] != 0)
    {
        printf("This conversion is INCORRECT.  It should be U+FFFF.\n");
        return EXIT_FAILURE;
    }

    printf("The string \"\\U00010000\" converts as:");
    for (i = 0; teststr2[i] != 0; i++)
        printf(" U+%04X", teststr2[i]);
    printf("\n");
    if (teststr2[0] != 0xD800 || teststr2[1] != 0xDC00 || teststr2[2] != 0)
    {
        printf("This conversion is INCORRECT.  It should be U+D800 U+DC00.\n");
        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;
}
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The problem is a simple off-by-one error in the function one_utf8_to_utf16 in
libcpp/charset.c .  The following patch against the GCC 4.4.1 source corrects
the bug:

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--- gcc-4.4.1/libcpp/charset.c.old      2009-04-09 19:23:07.000000000 -0400
+++ gcc-4.4.1/libcpp/charset.c  2009-10-12 04:06:25.000000000 -0400
@@ -354,7 +354,7 @@
       return EILSEQ;
     }

-  if (s < 0xFFFF)
+  if (s <= 0xFFFF)
     {
       if (*outbytesleftp < 2)
        {
--CUT HERE--CUT HERE--CUT HERE--CUT HERE--CUT HERE--CUT HERE--CUT HERE--


-- 
           Summary: "\uFFFF" converts incorrectly to two-byte character
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: chasonr at newsguy dot com
 GCC build triplet: x86_64-unknown-linux
  GCC host triplet: x86_64-unknown-linux
GCC target triplet: x86_64-unknown-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41698