[Lldb-commits] [lldb] [lldb][windows] print stop reason if MSVC's runtime check fails (PR #185473)

[Charles Zablit via lldb-commits]

https://github.com/charles-zablit created https://github.com/llvm/llvm-project/pull/185473

This patch extracts the `msg` value of the `failwithmessage` error and uses it as the stop reason if the MSVC Runtime fails while debugging.

# Before
```
lldb.exe C:\Users\charleszablit\Developer\testing\uninit.exe -b -o 'r'
(lldb) target create "C:\\Users\\charleszablit\\Developer\\testing\\uninit.exe"
Current executable set to 'C:\Users\charleszablit\Developer\testing\uninit.exe' (x86_64).
(lldb) r
Process 9400 launched: 'C:\Users\charleszablit\Developer\testing\uninit.exe' (x86_64)
Process 9400 stopped
* thread #1, stop reason = Exception 0x80000003 encountered at address 0x7ff96516c96a
       frame #0: 0x00007ff77efe20ba uninit.exe`failwithmessage(retaddr=0x00007ff77efe150f, crttype=1, errnum=3, msg="The variable 'x' is being used without being initialized.") at error.cpp:210
```

# After
```
lldb.exe C:\Users\charleszablit\Developer\testing\uninit.exe -b -o 'r'
(lldb) target create "C:\\Users\\charleszablit\\Developer\\testing\\uninit.exe"
Current executable set to 'C:\Users\charleszablit\Developer\testing\uninit.exe' (x86_64).
(lldb) r
Process 9400 launched: 'C:\Users\charleszablit\Developer\testing\uninit.exe' (x86_64)
Process 9400 stopped
* thread #1, stop reason = Run-time check failure: The variable 'x' is being used without being initialized.
       frame #0: 0x00007ff77efe20ba uninit.exe`failwithmessage(retaddr=0x00007ff77efe150f, crttype=1, errnum=3, msg="The variable 'x' is being used without being initialized.") at error.cpp:210
```


fix https://github.com/llvm/llvm-project/issues/184990.

>From ac5a4fa7a2544233d13a31c95aaf9e1a38371892 Mon Sep 17 00:00:00 2001
From: Charles Zablit <c_zablit at apple.com>
Date: Mon, 9 Mar 2026 17:34:49 +0000
Subject: [PATCH] [lldb][windows] print stop reason if MSVC's runtime check
 fails

---
 .../Process/Windows/Common/ProcessWindows.cpp | 50 +++++++++++++++++++
 lldb/test/Shell/Process/MSVCRTCException.cpp  | 16 ++++++
 2 files changed, 66 insertions(+)
 create mode 100644 lldb/test/Shell/Process/MSVCRTCException.cpp

diff --git a/lldb/source/Plugins/Process/Windows/Common/ProcessWindows.cpp b/lldb/source/Plugins/Process/Windows/Common/ProcessWindows.cpp
index c61a41cd00444..16bbb636fad0c 100644
--- a/lldb/source/Plugins/Process/Windows/Common/ProcessWindows.cpp
+++ b/lldb/source/Plugins/Process/Windows/Common/ProcessWindows.cpp
@@ -27,6 +27,7 @@
 #include "lldb/Host/windows/ConnectionConPTYWindows.h"
 #include "lldb/Host/windows/HostThreadWindows.h"
 #include "lldb/Symbol/ObjectFile.h"
+#include "lldb/Symbol/VariableList.h"
 #include "lldb/Target/DynamicLoader.h"
 #include "lldb/Target/MemoryRegionInfo.h"
 #include "lldb/Target/StopInfo.h"
@@ -34,6 +35,7 @@
 #include "lldb/Utility/LLDBLog.h"
 #include "lldb/Utility/Log.h"
 #include "lldb/Utility/State.h"
+#include "lldb/ValueObject/ValueObject.h"
 
 #include "llvm/Support/ConvertUTF.h"
 #include "llvm/Support/Format.h"
@@ -79,6 +81,49 @@ std::string GetProcessExecutableName(DWORD pid) {
   }
   return file_name;
 }
+
+std::optional<std::string> GetMSVCRTCFailureDescription(Thread &thread) {
+  const uint32_t kMaxFrames = 8;
+
+  for (uint32_t i = 0; i < kMaxFrames; ++i) {
+    StackFrameSP frame = thread.GetStackFrameAtIndex(i);
+    if (!frame)
+      break;
+
+    SymbolContext sc = frame->GetSymbolContext(eSymbolContextSymbol);
+    if (!sc.symbol)
+      continue;
+
+    const char *fn_name = frame->GetFunctionName();
+    if (!fn_name)
+      continue;
+    llvm::StringRef name(fn_name);
+
+    if (!name.contains("failwithmessage"))
+      continue;
+    VariableListSP vars = frame->GetInScopeVariableList(false);
+    if (!vars)
+      continue;
+    for (size_t j = 0; j < vars->GetSize(); ++j) {
+      VariableSP var = vars->GetVariableAtIndex(j);
+      if (!var || var->GetName() != ConstString("msg"))
+        continue;
+      ValueObjectSP val =
+          frame->GetValueObjectForFrameVariable(var, eNoDynamicValues);
+      if (!val)
+        break;
+      uint64_t ptr = val->GetValueAsUnsigned(0);
+      if (!ptr)
+        break;
+      std::string msg;
+      Status err;
+      thread.GetProcess()->ReadCStringFromMemory(ptr, msg, err);
+      if (err.Success() && !msg.empty())
+        return "Run-time check failure: " + msg;
+    }
+  }
+  return std::nullopt;
+}
 } // anonymous namespace
 
 namespace lldb_private {
@@ -488,6 +533,11 @@ void ProcessWindows::RefreshStateAfterStop() {
       }
       stop_thread->SetStopInfo(stop_info);
       return;
+    } else if (auto rtc_desc = GetMSVCRTCFailureDescription(*stop_thread)) {
+      stop_info = StopInfo::CreateStopReasonWithException(*stop_thread,
+                                                          rtc_desc->c_str());
+      stop_thread->SetStopInfo(stop_info);
+      return;
     } else {
       // The thread hit a hard-coded breakpoint like an `int 3` or
       // `__debugbreak()`.
diff --git a/lldb/test/Shell/Process/MSVCRTCException.cpp b/lldb/test/Shell/Process/MSVCRTCException.cpp
new file mode 100644
index 0000000000000..3238aaa24c8d2
--- /dev/null
+++ b/lldb/test/Shell/Process/MSVCRTCException.cpp
@@ -0,0 +1,16 @@
+// clang-format off
+
+// Test that lldb prints MSVC's runtime checks exceptions as stop reasons.
+
+// REQUIRES: msvc
+
+// RUN: %msvc_cl /nologo /Od /Zi /MDd /RTC1 -o %t.exe %s
+// RUN: %lldb -f %t.exe -b -o 'r' 2>&1 | FileCheck %s
+// CHECK: thread #1, stop reason = Run-time check failure: The variable 'x' is being used without being initialized.
+
+#include <iostream>
+
+int main() {
+    int x;
+    printf("%d\n", x);
+}
\ No newline at end of file