#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <signal.h>
#include <errno.h>
#include <sys/mman.h>
#include <sys/stat.h>

volatile int stop = 0, signalled = 0;

void handle_usr1(int x)
{ signalled = 1; }

void handle_ctrlc(int x)
{ stop = 1; }

void setup(int sig, void (* handler)(int))
{ struct sigaction sa;
  sa.sa_handler = handler;
  sa.sa_flags = 0;
  sigemptyset(& sa.sa_mask);
  int r = sigaction(sig, & sa, NULL);
  if (r < 0)
  { perror("sigaction failed");
    exit(1); } }

void P(long long int * mutexptr)
{ long long int val = 1;
  while (1)
  { asm volatile
    ( "mov $1, %%rax             \n"
      "mov %[mutexptr], %%rbx    \n"
      "lock xchg %%rax, (%%rbx)  \n"
      "mov %%rax, %[val]         \n"
      :
        [val] "=m" (val)
      :
        [mutexptr] "m" (mutexptr)
      :
        "rax", "rbx", "cc", "memory" );
    if (val)
      usleep(1000);
    else
      break; } } 

void V(long long int * mutex)
{ * mutex = 0; }

int main(int argc, char * argv[])
{ int otherpid, clear = 0;
  printf("my pid is %d\n", getpid());
  if (argc == 1 || argc == 2 && strcmp(argv[1], "clear") == 0)
  { if (argc == 2)
      clear = 1;
    printf("other process' pid? ");
    scanf("%d", & otherpid); }
  else
    otherpid = atol(argv[1]);

  setup(SIGUSR1, handle_usr1);
  setup(SIGINT, handle_ctrlc);

  int f = shm_open("/home/students/sratbag/421/shmem/shm.02", O_RDWR | O_CREAT | O_EXCL, 0644);
  if (f >= 0)
    clear = 1;
  else
  { f = shm_open("/home/students/sratbag/421/shmem/shm.02", O_RDWR, 0644);
    if (f < 0)
    { perror("shm_open failed");
      exit(1); } }
  ftruncate(f, 1024);
  char * p = mmap(NULL, 1024, PROT_READ | PROT_WRITE, MAP_SHARED, f, 0);
  if (clear)
    bzero(p, 1024);

  long long int * mutex_ptr = (long long int *) p;
  long long int * message_ready_ptr = (long long int *) (p + sizeof(long long int));
  char * message = p + 2 * sizeof(long long int);

  char typed[128];
  while (! stop)
  { int n = read(0, typed, 127);

    if (n < 0)
    { if (! signalled)
      { perror("read failed");
        usleep(10000);
        continue; }
      signalled = 0;
      if (stop)
        break;
      P(mutex_ptr);
      if (* message_ready_ptr)
      { printf("%s", message);
        * message_ready_ptr = 0; }
      V(mutex_ptr);
      continue; }

    typed[n] = '\0';
    if (strcmp(typed, "exit\n") == 0)
      break;

    int done = 0;
    while (! done)
    { P(mutex_ptr);
      if (* message_ready_ptr)    
        done = 0;
      else
      { strcpy(message, typed);
        * message_ready_ptr = 1;
        done = 1; }
      V(mutex_ptr);
      if (! done)
        usleep(1000); }
    int r = kill(otherpid, SIGUSR1);
    if (r < 0)
      perror("kill failed"); }

  f = shm_unlink("/home/students/sratbag/421/shmem/shm.02");
  if (f < 0)
  { perror("shm_unlink failed");
    exit(1); } }