mbstowcs() returns bytes not elements

[komh]

Hi/2.

POSIX says in https://pubs.opengroup.org/onlinepubs/9799919799/functions/mbstowcs.html:

Otherwise, mbstowcs() shall return the number of the array elements modified [XSI] (or required if pwcs is null), not including a terminating 0 code, if any. The array shall not be zero-terminated if the value returned is n.

Here is a test code.

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

int main( void )
{
    const char *s = "한글!";
    wchar_t ws[ 4 ];
    int rc;
    int wlen;
    int errors = 0;
    int failed;

    printf("Testing mbstowcs() returns in wide-characters...\n");

    setlocale( LC_ALL, "ko_KR.CP949");

    rc = mbstowcs( ws, s, sizeof( ws ));
    wlen = wcslen( ws );
    failed = rc != wlen;

    printf("%s: non-NULL: rc = %d(%d)\n",
           failed ? "FAILED" : "PASSED", rc, wlen );

    if( failed )
        errors++;

    rc = mbstowcs( NULL, s, 0 );
    failed = rc != wlen;

    printf("%s: NULL: rc = %d(%d)\n",
           failed ? "FAILED" : "PASSED", rc, wlen );

    if( failed )
        errors++;

    return !!errors;
}

[komh]

Here is the patch.

0001-Fix-mbstowcs-returns-the-result-in-bytes-not-in-char.patch

From 4b6947bc0725bef6dacb81b446a35a15608a6686 Mon Sep 17 00:00:00 2001
From: KO Myung-Hun <komh78@gmail.com>
Date: Fri, 9 Jan 2026 19:51:07 +0900
Subject: [PATCH] Fix mbstowcs() returns the result in bytes not in chars

---
 src/emx/src/lib/locale/mb_libuni.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/emx/src/lib/locale/mb_libuni.c b/src/emx/src/lib/locale/mb_libuni.c
index 62ad00c..8e33174 100644
--- a/src/emx/src/lib/locale/mb_libuni.c
+++ b/src/emx/src/lib/locale/mb_libuni.c
@@ -300,7 +300,7 @@ size_t  __libc_localeFuncsGeneric_mbsnrtowcs(size_t  (*pfnmbrtowc)(__wchar_t * _
                                              __wchar_t * __restrict dst, const char * __restrict * __restrict src, size_t nms, size_t len, __mbstate_t * __restrict ps)
 {
     const char *pch = *src;
-    size_t      cbIn = nms;
+    size_t      nchr = 0;
     if (dst == NULL)
     {
         __wchar_t wc;
@@ -312,12 +312,13 @@ size_t  __libc_localeFuncsGeneric_mbsnrtowcs(size_t  (*pfnmbrtowc)(__wchar_t * _
                 if (cb == (size_t)-1)
                     return (size_t)-1;    /* Invalid sequence - mbrtowc() sets errno. */
                 else /* if (cb == 0 || cb == (size_t)-2) */
-                    return cbIn - nms;
+                    return nchr;
             }
 
             /* advance */
             pch += cb;
             nms -= cb;
+            nchr++;
         }
         /* (won't ever get here.) */
     }
@@ -331,12 +332,12 @@ size_t  __libc_localeFuncsGeneric_mbsnrtowcs(size_t  (*pfnmbrtowc)(__wchar_t * _
                 if (!cb)
                 {
                     *src = NULL;
-                    return cbIn - nms;
+                    return nchr;
                 }
                 else if (cb == (size_t)-2)
                 {
                     *src = pch + nms;
-                    return cbIn - nms;
+                    return nchr;
                 }
                 else /*if (cb == (size_t)-1) */
                 {
@@ -349,10 +350,11 @@ size_t  __libc_localeFuncsGeneric_mbsnrtowcs(size_t  (*pfnmbrtowc)(__wchar_t * _
             pch += cb;
             nms -= cb;
             dst++;
+            nchr++;
         }
 
         *src = pch;
-        return cbIn - nms;
+        return nchr;
     }
 }
 
-- 
2.50.1