protoc-c: add const_strings option

Keep default as false to preserve backwards compatibility.

Fixes #239

protobuf-c/protobuf-c.proto

@@ -45,6 +45,9 @@ message ProtobufCFileOptions {
 
     // Generate helper init message functions?
     optional bool gen_init_helpers = 3 [default = true];
+
+    // Use const char * instead of char * for string fields
+    optional bool const_strings = 4 [default = false];
 }
 
 extend google.protobuf.FileOptions {

protoc-c/c_string_field.cc

@@ -93,13 +93,19 @@ StringFieldGenerator::~StringFieldGenerator() {}
 
 void StringFieldGenerator::GenerateStructMembers(io::Printer* printer) const
 {
+  const ProtobufCFileOptions opt = descriptor_->file()->options().GetExtension(pb_c_file);
+
   switch (descriptor_->label()) {
     case FieldDescriptor::LABEL_REQUIRED:
     case FieldDescriptor::LABEL_OPTIONAL:
+      if (opt.const_strings())
+        printer->Print(variables_, "const ");
       printer->Print(variables_, "char *$name$$deprecated$;\n");
       break;
     case FieldDescriptor::LABEL_REPEATED:
       printer->Print(variables_, "size_t n_$name$$deprecated$;\n");
+      if (opt.const_strings())
+        printer->Print(variables_, "const ");
       printer->Print(variables_, "char **$name$$deprecated$;\n");
       break;
   }
@@ -123,10 +129,13 @@ std::string StringFieldGenerator::GetDefaultValue(void) const
 void StringFieldGenerator::GenerateStaticInit(io::Printer* printer) const
 {
   std::map<std::string, std::string> vars;
+  const ProtobufCFileOptions opt = descriptor_->file()->options().GetExtension(pb_c_file);
   if (descriptor_->has_default_value()) {
     vars["default"] = GetDefaultValue();
   } else if (FieldSyntax(descriptor_) == 2) {
     vars["default"] = "NULL";
+  } else if (opt.const_strings()) {
+    vars["default"] = "(const char *)protobuf_c_empty_string";
   } else {
     vars["default"] = "(char *)protobuf_c_empty_string";
   }

t/generated-code2/common-test-arrays.h

@@ -56,7 +56,7 @@ TEST_ENUM_TYPE_NAME enum_random[] = {
    T(0), T(2097152), T(268435455), T(127), T(16383), T(16384) };
 #undef T
 
-char *repeated_strings_0[] = { (char*)"onestring" };
-char *repeated_strings_1[] = { (char*)"two", (char*)"string" };
-char *repeated_strings_2[] = { (char*)"many", (char*)"tiny", (char*)"little", (char*)"strings", (char*)"should", (char*)"be", (char*)"handled" };
-char *repeated_strings_3[] = { (char*)"one very long strings XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX" };
+const char *repeated_strings_0[] = { "onestring" };
+const char *repeated_strings_1[] = { "two", "string" };
+const char *repeated_strings_2[] = { "many", "tiny", "little", "strings", "should", "be", "handled" };
+const char *repeated_strings_3[] = { "one very long strings XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX" };

t/generated-code2/test-generated-code2.c

@@ -1274,7 +1274,7 @@ static void test_repeated_string (void)
 {
 
 #define DO_TEST(static_array, example_packed_data) \
-  DO_TEST_REPEATED(test_string, (char **), \
+  DO_TEST_REPEATED(test_string, (const char **), \
                    static_array, example_packed_data, \
                    STRING_EQUALS)
 
@@ -2109,8 +2109,8 @@ test_message_check(void)
   Foo__TestMessageCheck__SubMessage sm = FOO__TEST_MESSAGE_CHECK__SUB_MESSAGE__INIT;
   Foo__TestMessageCheck__SubMessage sm2 = FOO__TEST_MESSAGE_CHECK__SUB_MESSAGE__INIT;
   Foo__TestMessageCheck m = FOO__TEST_MESSAGE_CHECK__INIT;
-  char *null = NULL;
-  char *str = "";
+  const char *null = NULL;
+  const char *str = "";
   Foo__TestMessageCheck__SubMessage *sm_p;
   ProtobufCBinaryData bd;
 

t/test-full.proto

@@ -2,6 +2,8 @@ package foo;
 
 import "protobuf-c/protobuf-c.proto";
 
+option (pb_c_file).const_strings = true;
+
 message SubMess {
   required int32 test = 4;