Silly zero-copy minecraft protocol parser idea!

Makefile

CFLAGS += -std=c99 -Wall -Werror -Wextra -pedantic -O0 -g

all: test

parser.o: parser.c
	$(CC) $(CFLAGS) $(LDFLAGS) -c -o parser.o parser.c

test.o: test.c
	$(CC) $(CFLAGS) $(LDFLAGS) -c -o test.o test.c

test: parser.o test.o
	$(CC) $(CFLAGS) $(LDFLAGS) -o test test.o parser.o

clean:
	rm -f *.o test

packets.h

#ifndef PARSER_PACKETS_H
#define PARSER_PACKETS_H

#include <stdint.h>

#define PACKET_LENGTH 1
typedef struct protocol_packet_s {
  uint8_t type;
} protocol_packet_t;

#define PACKET_LENGTH_00 5
typedef struct protocol_packet_00_s {
  uint8_t type;
  int32_t id;
} protocol_packet_00_t;

#endif

parser.c

#include <stdlib.h>
#include <sys/types.h>
#define __USE_BSD
#include <endian.h>

#include "parser.h"
#include "packets.h"

#define UNUSED(x) (void)(x)
#define PARSER_CASE(x) case 0x##x: { if (data_len < PACKET_LENGTH_##x) { return 0; } else { return protocol_parser_parse_##x(parser, settings, data, data_len); } }

size_t protocol_parser_execute(protocol_parser_t* parser, protocol_parser_settings_t* settings, char* data, size_t data_len) {
  if (data_len < 1) {
    return 0;
  }

  switch (data[0]) {
    PARSER_CASE(00);

    default: {
      if (settings->on_error != NULL) {
        settings->on_error(parser, -1);
      }

      return 0;
    }
  }

  return 0;
}

size_t protocol_parser_parse_00(protocol_parser_t* parser, protocol_parser_settings_t* settings, char* data, size_t data_len) {
  protocol_packet_00_t packet;

  UNUSED(data_len);

  packet.type = data[0];
  packet.id = htobe32(*((int32_t*)(data + 1)));

  if (settings->on_packet != NULL) {
    settings->on_packet(parser, (protocol_packet_t*)&packet);
  }

  return 5;
}

parser.h

#ifndef PARSER_H
#define PARSER_H

#include "packets.h"

typedef struct protocol_parser_s {
  void* data;
} protocol_parser_t;

typedef void (*packet_cb)(protocol_parser_t* parser, protocol_packet_t* packet);
typedef void (*error_cb)(protocol_parser_t* parser, int err);

typedef struct protocol_parser_settings_s {
  packet_cb on_packet;
  error_cb on_error;
} protocol_parser_settings_t;

size_t protocol_parser_execute(protocol_parser_t* parser, protocol_parser_settings_t* settings, char* data, size_t data_len);

size_t protocol_parser_parse_00(protocol_parser_t* parser, protocol_parser_settings_t* settings, char* data, size_t data_len);

#endif

test.c

#include <stdio.h>
#include <stdlib.h>

#include "parser.h"
#include "packets.h"

void on_packet(protocol_parser_t* parser, protocol_packet_t* packet) {
  printf("[%p] packet type: %02x\n", parser->data, packet->type);

  if (packet->type == 0x00) {
    protocol_packet_00_t* packet_00 = (protocol_packet_00_t*)packet;
    printf("-> id: %d\n", packet_00->id);
  }
}

void on_error(protocol_parser_t* parser, int err) {
  printf("[%p] error %d\n", parser->data, err);
}

int main() {
  protocol_parser_t parser = { .data = NULL };
  protocol_parser_settings_t settings = { .on_packet = on_packet, .on_error = on_error };

  char data[] = { 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x02 };

  size_t nparsed = 0, offset = 0;

  while ((nparsed = protocol_parser_execute(&parser, &settings, data + offset, 10 - offset)) != 0) {
    printf("parsed %d bytes\n", nparsed);
    offset += nparsed;
  }

  return 0;
}