How to Read and Write Protocol Buffer Messages with Length Prefixes in C and Java?

C Equivalents of Protocol Buffers Delimited I/O Functions in Java

Question:

In C and Java, how can Protocol Buffers messages be read and written to files using length prefixes? The Java API has "Delimited" I/O functions for this purpose, but are there equivalent functions in C ? If not, what is the underlying wire format for the size prefixes?

Answer:

As of version 3.3.0, C now includes equivalent functions in google/protobuf/util/delimited_message_util.h. These functions provide a faster and more optimized implementation than previously suggested approaches.

Original Implementation:

Before the introduction of the official equivalents, the following optimized C implementation was available:

bool writeDelimitedTo(
    const google::protobuf::MessageLite& message,
    google::protobuf::io::ZeroCopyOutputStream* rawOutput) {
  // Use a new coded stream for each message (fast).
  google::protobuf::io::CodedOutputStream output(rawOutput);

  // Write the size.
  const int size = message.ByteSize();
  output.WriteVarint32(size);

  uint8_t* buffer = output.GetDirectBufferForNBytesAndAdvance(size);
  if (buffer != NULL) {
    // Optimization: Direct-to-array serialization for messages in a single buffer.
    message.SerializeWithCachedSizesToArray(buffer);
  } else {
    // Slower path for messages across multiple buffers.
    message.SerializeWithCachedSizes(&output);
    if (output.HadError()) return false;
  }

  return true;
}

bool readDelimitedFrom(
    google::protobuf::io::ZeroCopyInputStream* rawInput,
    google::protobuf::MessageLite* message) {
  // Each message uses its own coded stream (fast).
  google::protobuf::io::CodedInputStream input(rawInput);

  // Read the size.
  uint32_t size;
  if (!input.ReadVarint32(&size)) return false;

  // Limit the stream to the size of the message.
  google::protobuf::io::CodedInputStream::Limit limit =
      input.PushLimit(size);

  // Parse the message.
  if (!message->MergeFromCodedStream(&input)) return false;
  if (!input.ConsumedEntireMessage()) return false;

  // Release the limit.
  input.PopLimit(limit);

  return true;
}

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