protoc-c: c_message: Order oneof union members from largest to smallest

This commit changes the code generator to output the members of a oneof
union from largest to smallest, rather than in field descriptor order.

This is necessary on certain compilers such as gcc >= 15 which do not
guarantee that initializing a union with the universal zero initializer
actually initializes all the bits of the object representation of the
members of the union, unless the largest union member is the first
member of the union.

protoc-c/c_message.cc

@@ -32,7 +32,8 @@
 //  Based on original Protocol Buffers design by
 //  Sanjay Ghemawat, Jeff Dean, and others.
 
-// Copyright (c) 2008-2013, Dave Benson.  All rights reserved.
+// Copyright (c) 2008-2025, Dave Benson and the protobuf-c authors.
+// All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
@@ -63,6 +64,9 @@
 #include <algorithm>
 #include <map>
 #include <memory>
+#include <string_view>
+#include <tuple>
+#include <vector>
 #include <protoc-c/c_message.h>
 #include <protoc-c/c_enum.h>
 #include <protoc-c/c_extension.h>
@@ -211,15 +215,28 @@ GenerateStructDefinition(io::Printer* printer) {
 
     printer->Print("union {\n");
     printer->Indent();
+
+    std::vector<std::tuple<int, std::string_view, const FieldDescriptor *>> sorted_fds;
+
     for (int j = 0; j < oneof->field_count(); j++) {
       const FieldDescriptor *field = oneof->field(j);
+      std::string_view name = field->name();
+      int order = MessageGenerator::GetOneofUnionOrder(field);
+      sorted_fds.push_back({order, name, field});
+    }
+
+    std::sort(sorted_fds.begin(), sorted_fds.end());
+
+    for (const auto& tuple : sorted_fds) {
+      const auto& [order, name, field] = tuple;
       SourceLocation fieldSourceLoc;
       field->GetSourceLocation(&fieldSourceLoc);
 
-      PrintComment (printer, fieldSourceLoc.leading_comments);
-      PrintComment (printer, fieldSourceLoc.trailing_comments);
+      PrintComment(printer, fieldSourceLoc.leading_comments);
+      PrintComment(printer, fieldSourceLoc.trailing_comments);
       field_generators_.get(field).GenerateStructMembers(printer);
     }
+
     printer->Outdent();
     printer->Print(vars, "};\n");
   }
@@ -627,6 +644,60 @@ GenerateMessageDescriptor(io::Printer* printer, bool gen_init) {
       "};\n");
 }
 
+int MessageGenerator::GetOneofUnionOrder(const FieldDescriptor *fd)
+{
+  auto cpp_type = fd->cpp_type();
+  auto pb_type = fd->type();
+
+  switch (cpp_type) {
+    case FieldDescriptor::CPPTYPE_STRING:
+      if (pb_type == FieldDescriptor::TYPE_BYTES) {
+        return 1;
+      } else if (pb_type == FieldDescriptor::TYPE_STRING) {
+        return 4;
+      } else {
+        GOOGLE_LOG(FATAL)
+          << fd->full_name()
+          << ": Unhandled combination of CPPTYPE ("
+          << cpp_type
+          << ") and protobuf type ("
+          << pb_type
+          << ")";
+        return -1;
+      }
+
+    case FieldDescriptor::CPPTYPE_DOUBLE:
+      return 2;
+
+    case FieldDescriptor::CPPTYPE_INT64:
+    case FieldDescriptor::CPPTYPE_UINT64:
+      return 3;
+
+    case FieldDescriptor::CPPTYPE_MESSAGE:
+      return 5;
+
+    case FieldDescriptor::CPPTYPE_FLOAT:
+      return 6;
+
+    case FieldDescriptor::CPPTYPE_INT32:
+    case FieldDescriptor::CPPTYPE_UINT32:
+      return 7;
+
+    case FieldDescriptor::CPPTYPE_ENUM:
+      return 8;
+
+    case FieldDescriptor::CPPTYPE_BOOL:
+      return 9;
+
+    default:
+      GOOGLE_LOG(FATAL)
+        << fd->full_name()
+        << ": Unhandled CPPTYPE "
+        << cpp_type;
+      return -1;
+  }
+}
+
 }  // namespace c
 }  // namespace compiler
 }  // namespace protobuf

protoc-c/c_message.h

@@ -32,7 +32,8 @@
 //  Based on original Protocol Buffers design by
 //  Sanjay Ghemawat, Jeff Dean, and others.
 
-// Copyright (c) 2008-2013, Dave Benson.  All rights reserved.
+// Copyright (c) 2008-2025, Dave Benson and the protobuf-c authors.
+// All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
@@ -128,7 +129,7 @@ class MessageGenerator {
 
  private:
 
-  std::string GetDefaultValueC(const FieldDescriptor *fd);
+  int GetOneofUnionOrder(const FieldDescriptor *fd);
 
   const Descriptor* descriptor_;
   std::string dllexport_decl_;