New subject: [Bug 56994] mbstowcs for UTF8, with an exact (not overallocated) output buffer size fails

22 Jul 2024

      https://bugs.winehq.org/show_bug.cgi?id=56994

            Bug ID: 56994
           Summary: mbstowcs for UTF8, with an exact (not overallocated)
                    output buffer size fails
           Product: Wine
           Version: unspecified
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: msvcrt
          Assignee: wine-bugs(a)winehq.org
          Reporter: martin(a)martin.st
                CC: piotr(a)codeweavers.com
      Distribution: ---

This issue can be illustrated with the following test snippet:

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

int main() {
  const char *locname = "en_US.UTF-8";
  if (!setlocale(LC_ALL, locname)) {
    fprintf(stderr, "locale %s failed\n", locname);
    return 1;
  }

  wchar_t wcsbuf[10] = { 0xe9, 0 }; // LATIN SMALL LETTER E WITH ACUTE
  char mbsbuf[10] = { 0 };
  size_t ret;
  ret = wcstombs(NULL, wcsbuf, 0);
  printf("initial wcstombs returned %d bytes\n", (int) ret);
  ret = wcstombs(mbsbuf, wcsbuf, ret);
  printf("wcstombs returned %d bytes\n", (int) ret);
  if (ret == -1)
    return 1;
  for (size_t i = 0; i < ret; i++)
    printf("%02x ", (unsigned char)mbsbuf[i]);
  printf("\n");
  ret = mbstowcs(NULL, mbsbuf, 0);
  printf("initial mbstowcs returned %d wchars\n", (int) ret);
  if (ret > sizeof(wcsbuf)/sizeof(wcsbuf[0]))
    return 1;
  ret = mbstowcs(wcsbuf, mbsbuf, ret);
  printf("mbstowcs returned %d wchars\n", (int) ret);
  if (ret == -1)
    return 1;
  for (size_t i = 0; i < ret; i++)
    printf("%02x ", (unsigned int)wcsbuf[i]);
  printf("\n");

  return 0;
}

Compiled with mingw tools targeting UCRT.

On native Windows, this outputs:
initial wcstombs returned 2 bytes
wcstombs returned 2 bytes
c3 a9 
initial mbstowcs returned 1 wchars
mbstowcs returned 1 wchars
e9 

With Wine, it outputs:
initial wcstombs returned 2 bytes
wcstombs returned 2 bytes
c3 a9
initial mbstowcs returned 1 wchars
mbstowcs returned -1 wchars

Thus, the actual mbstowcs conversion fails, whereas it succeds on native
Windows.

The problem here is that the output buffer size, given to mbstowcs, is a
tightly allocated 1.

When scanning the (null terminated) input buffer, to calculate the input size,
at
https://gitlab.winehq.org/wine/wine/-/blob/wine-9.13/dlls/msvcrt/mbcs.c?ref_...,
we limit the loop to "i<count". As we've passed count=1, as we know the output
is going to be 1 wchar, this loop terminates after one iteration.

The check "_isleadbyte_l((unsigned char)mbstr[size], locale)" seems to fail for
the leading UTF8 byte, which probably is a bug in itself. But if mbstowcs is
given an overallocated output size (passing a larger count parameter), that
issue isn't visible here.

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[Bug 56994] New: mbstowcs for UTF8, with an exact (not overallocated) output buffer size fails

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