We have an issue where if you use a C function right now that has no prototype, it isn't marked as extern "C" and the name to lookup is some C++ mangled form of the name.

Greg Clayton · Greg Clayton · commit b23c24c2a84b · 2015-04-06T17:14:02.000Z
This used to be the case for "printf" before a function prototype was added to the builtin expression prefix file. This fix makes sure that if we get a mangled name that we don't find in the current target, that we only fall back to looking up function by basename if the function isn't contained in a namespace or class (no decl context).

llvm-svn: 234178
diff --git a/lldb/source/Expression/ClangExpressionDeclMap.cpp b/lldb/source/Expression/ClangExpressionDeclMap.cpp
@@ -598,13 +598,17 @@ ClangExpressionDeclMap::GetFunctionAddress
         Mangled mangled(name, is_mangled);
                 
         CPPLanguageRuntime::MethodName method_name(mangled.GetDemangledName());
-        
-        llvm::StringRef basename = method_name.GetBasename();
-        
-        if (!basename.empty())
+
+        // the C++ context must be empty before we can think of searching for symbol by a simple basename
+        if (method_name.GetContext().empty())
         {
-            FindCodeSymbolInContext(ConstString(basename), m_parser_vars->m_sym_ctx, sc_list);
-            sc_list_size = sc_list.GetSize();
+            llvm::StringRef basename = method_name.GetBasename();
+            
+            if (!basename.empty())
+            {
+                FindCodeSymbolInContext(ConstString(basename), m_parser_vars->m_sym_ctx, sc_list);
+                sc_list_size = sc_list.GetSize();
+            }
         }
     }
 
