Bus error problem of MMAP and its solution

Preface

In the new trading system , adopt mmap To achieve local persistence of positions and orders , Not used before mmap, There are some problems in this implementation , Record only Bus error problem ;

Realization

Basic logic

OM perhaps PM When it starts , First recover the order or position through the local persistent file ：

• If it is the first normal start of the day , There is no persistent file , This is the time ,PM You need to get the initialized position from other places （ Trading hasn't started that day , No orders ,OM No need to deal with ）;
• If it is not the first normal start of the day , There are persistent files ,OM perhaps PM You need to obtain orders and positions from local persistent files , Initialization complete ;

problem

Persistence is through mmap Mapping local files , Yes Bus error The problem of , What's the reason ？
Because at the first normal start of the day , There is no persistent file （ For each strategy , Persistent files are created and recorded in days ）, At this time, you need to create a persistent file first , And then through mmap Write the record information after mapping ; The problem is After creating a new file mmap Mapping modification , For specific reasons, please refer to ：
Use mmap Bus error encountered bus error
The reason for the error is mmap Cannot extend a new file with empty content , Because the size is 0, There is no corresponding legal physical page , Can't expand .

solve

The solution is 2 individual ：
One is the scheme in the link above ： Just write some data in the newly created empty file
The other is adopt ftruncate Map and modify the newly created file after expansion

int fd = open("/workspace/delete/data.txt", O_RDWR | O_CREAT, 777);
  if(!fd) {
    
    printf("!fd\n");
    return -1;
  }
  // programme 1: write(fd,"1",1);
  ftruncate(fd, 4096*10);   //  programme 2

  void* m_mmap =
    mmap(NULL, 4096*10, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  if(!m_mmap){
    
    printf("!m_mmap\n");
    return -1;
  }
  printf("come here\n");

  Position cur_pos;
  char buf[1024];
  for (int i = 1; i < 10; i++) {
        
    cur_pos.symbol = i*2 + 15;
    cur_pos.pos_long = i*3 + 15;
    cur_pos.pos_short = i*4 + 15;
    cur_pos.long_pending = i*5 + 15;
    cur_pos.short_pending = i*6 + 15;
    printf("%d: %lld,%d,%d,%d,%d\n", i-1, cur_pos.symbol, cur_pos.pos_long, cur_pos.pos_short, cur_pos.long_pending, cur_pos.short_pending);
    memcpy( (void*)((char*)m_mmap + sizeof(Position)*(i-1)), &cur_pos, sizeof(cur_pos));
    //memcpy( (buf + sizeof(Position)*(i-1)), &cur_pos, sizeof(cur_pos));
  }