Fix a bug in wcsnrtombs

__d is a char * destination buffer, so __b is already the byte capacity.
Dividing by sizeof(wchar_t) makes no sense here, it was likely copy-pasted
from mbsnrtowcs (where the destination is wchar_t *). The first branch also
fails to limit __n (the byte write cap) to __b, so overflows are possible when
a wide character produces multi-byte output. The second branch (else) correctly
limits __n to __b.

This commit replaces the broken two-branch logic with the simple correct
pattern matching wcsrtombs, and adds two tests two prove that nothing broke.

3 files changed, 56 insertions, 0 deletions

diff --git a/tests/Makefile b/tests/Makefile
index 71fb930..adea381 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -94,6 +94,8 @@ RUNTIME_TARGETS= \
         test_vsnprintf_static         \
         test_vsprintf                 \
         test_wcrtomb                  \
+        test_wcsnrtombs_dynamic       \
+        test_wcsnrtombs_static        \
         test_wcscat_static_write      \
         test_wcscpy_static_write      \
         test_wcsncat_static_write     \
diff --git a/tests/test_wcsnrtombs_dynamic.c b/tests/test_wcsnrtombs_dynamic.c
new file mode 100644
index 0000000..808c9c8
--- /dev/null
+++ b/tests/test_wcsnrtombs_dynamic.c
@@ -0,0 +1,28 @@
+#include "common.h"
+
+#include <wchar.h>
+#include <string.h>
+
+int main(int argc, char** argv) {
+  char buffer[8] = {0};
+  const wchar_t src[] = L"ABCD";
+  const wchar_t *srcp = src;
+  mbstate_t st;
+  memset(&st, 0, sizeof(st));
+
+  /* Safe: convert up to 4 wide chars, write at most 4 bytes */
+  srcp = src;
+  wcsnrtombs(buffer, &srcp, 4, 4, &st);
+
+  /* Unsafe: ask to write argc (10) bytes into 8-byte buffer.
+   * Before the fix, the first branch incorrectly divided the byte-sized
+   * buffer capacity by sizeof(wchar_t), making the check too permissive. */
+  CHK_FAIL_START
+  srcp = src;
+  memset(&st, 0, sizeof(st));
+  wcsnrtombs(buffer, &srcp, 4, argc, &st);
+  CHK_FAIL_END
+
+  puts(buffer);
+  return ret;
+}
diff --git a/tests/test_wcsnrtombs_static.c b/tests/test_wcsnrtombs_static.c
new file mode 100644
index 0000000..7f2883f
--- /dev/null
+++ b/tests/test_wcsnrtombs_static.c
@@ -0,0 +1,26 @@
+#include "common.h"
+
+#include <wchar.h>
+#include <string.h>
+
+int main(int argc, char** argv) {
+  char buffer[4] = {0};
+  const wchar_t src[] = L"ABCDEFGHIJ";
+  const wchar_t *srcp = src;
+  mbstate_t st;
+  memset(&st, 0, sizeof(st));
+
+  /* Safe: convert up to 2 wide chars, write at most 2 bytes */
+  srcp = src;
+  wcsnrtombs(buffer, &srcp, 2, 2, &st);
+
+  /* Unsafe: ask to write 16 bytes into 4-byte buffer */
+  CHK_FAIL_START
+  srcp = src;
+  memset(&st, 0, sizeof(st));
+  wcsnrtombs(buffer, &srcp, 10, 16, &st);
+  CHK_FAIL_END
+
+  puts(buffer);
+  return ret;
+}