How to serialize in C++ for parsing in Python

[Randy Nuss]

I am building a C++ library with protobuf messages for consumption by a Python program.  The issue is SerializeToString() takes std::string, but only c_types (like char*) can be exported by the dll, which results in numerical zeros being recognized as string terminators and truncating the ParseFromString() process.

I tried using SerializeToArray() but get the following error in Python:

google.protobuf.message.DecodeError: Field number 0 is illegal.

I would appreciate any suggestions.  Thanks.

[Adam Cozzette]

I think if you have to work with C types then you will need to provide both a char* and an integer length. That Python error makes it sound like the Python code is probably trying to parse beyond the end of the serialized message.

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[Randy Nuss]

I solved this.  It did require the Python caller obtaining the string size from the C++ DLL.  The key part is to use ctypes.string_at() to then define the string at the Python end.

lib.fun.argtypes = [c_void_p, c_void_p]
lib.fun.restype = c_void_p

size = c_int()

return_ptr = lib.fun(self.obj, byref(size))

cstring = string_at(return_ptr, size)