A little method builder

main.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <sys/mman.h>
#include <limits.h>
#ifndef PAGESIZE
#define PAGESIZE 4096
#endif

struct state {
  int32_t test;
};

#define M_BEGIN \
  if(ofs != ~ofs) goto __end;   \
  __start: {       \

#define M_END \
  } \
  __end: { \
  void *_start = &&__start; \
  void *_end = &&__end; \
  memcpy(&program[ofs], _start, _end - _start); \
  return ofs + (_end - _start); }

uint32_t op1(struct state *state, void *program, size_t ofs) {
  M_BEGIN;
  state->test = 50;
  M_END;
}

uint32_t op2(struct state *state, void *program, size_t ofs) {
  M_BEGIN;
  state->test += 5;
  M_END;
}

size_t add_ops(struct state *state, void *ptr) {
  size_t ofs = 0;
  ofs = op1(state, ptr, ofs);
  ofs = op2(state, ptr, ofs);
  ofs = op2(state, ptr, ofs);
  return ofs;
}

void create_and_execute(struct state *state) {
  size_t max = PAGESIZE * 10;
  void *ptr = malloc(max);
  memset(ptr, 0xc3, max);
  size_t ofs = add_ops(state, ptr);

  size_t size = PAGESIZE * ((ofs / PAGESIZE) + 1);
  void *run = mmap(0, size, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_SHARED | MAP_ANON, -1, 0);
  memset(run, 0xc3, size);
  memcpy(run, ptr, ofs);
  free(ptr);

  void(*method)(void) = run;
  method();
}

int main(void) {
  struct state state;
  state.test = 0;
  create_and_execute(&state);
  printf("state.test  = %d\n", state.test);
  return 0;
}