[PATCH v2 0/2] MR10933: kernelbase: Add null check for str in OutputDebugStringW

[Matthias Gorzellik (＠gotzl)]

The following code crashes wine but seems to be OK in Windows:
```
#include <windows.h>
int main() {
    OutputDebugStringW(NULL);
    return 0;
}
```
The MR aligns behavior of `OutputDebugStringW` to Windows by adding a null check and bailing out of `OutputDebugStringW`.

--
  v2: kernel32/tests: Add null pointer test for OutputDebugStringW
 
https://gitlab.winehq.org/wine/wine/-/merge_requests/10933

[PATCH v2 1/2] kernelbase: Add null check for str in OutputDebugStringW

[Matthias Gorzellik]

From: Matthias Gorzellik <[e-mail]>

---
 dlls/kernelbase/debug.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/dlls/kernelbase/debug.c b/dlls/kernelbase/debug.c
index a600b6b59c9..8656859b7bb 100644
--- a/dlls/kernelbase/debug.c
+++ b/dlls/kernelbase/debug.c
@@ -281,6 +281,7 @@ void WINAPI DECLSPEC_HOTPATCH OutputDebugStringW( LPCWSTR str )
     STRING strA;
 
     WARN( "%s\n", debugstr_w(str) );
+    if (!str) return;
 
     RtlInitUnicodeString( &strW, str );
     if (!RtlUnicodeStringToAnsiString( &strA, &strW, TRUE ))
-- 
GitLab


https://gitlab.winehq.org/wine/wine/-/merge_requests/10933

[PATCH v2 2/2] kernel32/tests: Add null pointer test for OutputDebugStringW

[Matthias Gorzellik]

From: Matthias Gorzellik <[e-mail]>

---
 dlls/kernel32/tests/debugger.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/dlls/kernel32/tests/debugger.c b/dlls/kernel32/tests/debugger.c
index 478eaee9793..489cb535b72 100644
--- a/dlls/kernel32/tests/debugger.c
+++ b/dlls/kernel32/tests/debugger.c
@@ -2435,6 +2435,10 @@ static void test_OutputDebugString(void)
     pOutputDebugStringA("test");
     ok(GetLastError() == 0xdeadbeef, "got %ld.\n", GetLastError());
 
+    SetLastError(0xdeadbeef);
+    OutputDebugStringW(NULL);
+    ok(GetLastError() == 0xdeadbeef, "got %ld.\n", GetLastError());
+
     SetLastError(0xdeadbeef);
     OutputDebugStringW(L"test");
     ok(GetLastError() == 0xdeadbeef, "got %ld.\n", GetLastError());
-- 
GitLab

https://gitlab.winehq.org/wine/wine/-/merge_requests/10933

[Matthias Gorzellik (＠gotzl)]

On Mon May 18 13:00:54 2026 +0000, Jinoh Kang wrote:
> This would need tests. Is NULL string skipped entirely? Or coerced to
> empty (but valud) string and passed to the debugger regardless.
> NULL string seems to be skipped entirely, it doesn't appear in Sysinternals DebugView.
> I've updated the MR with a test case.

-- 
https://gitlab.winehq.org/wine/wine/-/merge_requests/10933#note_140364