Add support for "optional" field for proto3 files

[Puneethgr]

When there is a proto3 file which uses optional field, then protoc command throws an error.

example.proto:

syntax = "proto3";

message Example {
  string name = 1;
  optional int32 age = 2; // optional field is used here
}

Command:

protoc --c_out=. example.proto

Error:

example.proto: is a proto3 file that contains optional fields, 
but code generator protoc-gen-c hasn't been updated to support optional fields in proto3. 
Please ask the owner of this code generator to support proto3 optional.

Solution:

Open /protobuf-c-1.4.0/protoc-c/c_generator.h and add the following lines inside CGenerator class: (Refer this https://chromium.googlesource.com/external/github.com/protocolbuffers/protobuf/+/refs/heads/master/docs/implementing_proto3_presence.md)

  uint64_t GetSupportedFeatures() const override {
    // Indicate that this code generator supports proto3 optional fields.
    return FEATURE_PROTO3_OPTIONAL;
  }

Finally,

the CGenerator class inside c_generator.h should look like this:

class PROTOC_C_EXPORT CGenerator : public CodeGenerator {
 public:
  CGenerator();
  ~CGenerator();

  // implements CodeGenerator ----------------------------------------
  bool Generate(const FileDescriptor* file,
                const std::string& parameter,
                OutputDirectory* output_directory,
                std::string* error) const;
 
  uint64_t GetSupportedFeatures() const override {
    // Indicate that this code generator supports proto3 optional fields.
    return FEATURE_PROTO3_OPTIONAL;
  }

 private:
  GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(CGenerator);
};

At last,

you need to re-install protobuf-c,
go to protobuf-c-1.4.0 directory and run the installation command:
./autogen.sh && ./configure && make && make install

Please add this support into the source code.

[acozzette]

Before updating the generator to have GetSupportedFeatures() return FEATURE_PROTO3_OPTIONAL, it is a good idea to make sure the generator has proper support for handling proto3 optional fields. Otherwise, protoc will run successfully but proto3 optional fields will be treated like single-element oneofs, which probably wouldn't be the ideal API. The oneof trick is a backward-compatibility measure but it's better to add proper support if possible.

[devangvyas90]

Hi,
Any progress/update on this?

1. Is above advised solution good to go or will it have long term compatibility issues?

2. I am observing below issue while building following the suggested solution. (Note that i am building against protobuf version 3.6.1-4)

In file included from protoc-c/c_generator.cc:63:0:
./protoc-c/c_generator.h:97:12: error: ‘uint64_t google::protobuf::compiler::c::CGenerator::GetSupportedFeatures() const’ marked override, but does not override
uint64_t GetSupportedFeatures() const override {
^
./protoc-c/c_generator.h: In member function ‘uint64_t google::protobuf::compiler::c::CGenerator::GetSupportedFeatures() const’:
./protoc-c/c_generator.h:99:12: error: ‘FEATURE_PROTO3_OPTIONAL’ was not declared in this scope
return FEATURE_PROTO3_OPTIONAL;
^
make: *** [Makefile:1609: protoc-c/protoc_c_protoc_gen_c-c_generator.o] Error 1

[samjgarnham]

Any progress?
It seems others have attempted this:
master...fluent:protobuf-c:master

[gsstark]

Hi, I just want to mention I ran into this problem trying to compile the opentelemetry protobufs. Specifically it seems it's the metrics protobufs that trigger this:

I assume it's this section:

  // min is the minimum value over (start_time, end_time].
  optional double min = 11;

  // max is the maximum value over (start_time, end_time].
  optional double max = 12;

[sudhank29]

Any progress? It seems others have attempted this: master...fluent:protobuf-c:master

Incorporating this change fixes the issue of not being able to use "optional" with "proto3" syntax but the generated C structs don't have the "has_field" members.

[AxelLin]

Hi @edmonds

Looks like v1.5.0 does not support "optional" field for proto3 files
This issue has been reported for more than 2 years, just wondering if
you have plan to fix this issue?

[luisGallet]

Any progress on adding support for optional fields for proto3?