#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>


int print_error(const int error)
{
   printf("%s\n", strerror(error));
   return error;
}
struct Message
{
   int pid;
   int index;
};

int main()
{
   const char* name = "/tmp/my_pipe";
   const int result = mkfifo(name, 0666);
   if (result)
      return print_error(errno);
   const int writers{5};
   const int messages{5};
   printf("open parent pipe\n");
   const int rfd = open(name, O_RDONLY | O_NONBLOCK);
   const int wfd = open(name, O_WRONLY);
   if (rfd < 0)
      return print_error(errno);
   const int block_alternation[] = {0, O_NONBLOCK};
   int pids[writers];
   for (auto idx = 0; idx < writers; ++idx)
   {
      const auto pid = fork();
      if (pid < 0)
         return print_error(errno);
      if (pid == 0)
      {
         printf("child %d\n", getpid());
         for (int index = 0; index < messages; ++index)
         {
            const int wfd = open(name, O_WRONLY | block_alternation[index % 2]);
            if (wfd < 0)
               return print_error(errno);
            Message message{getpid(), index};
            short attempts{10000};
            while (write(wfd, &message, sizeof(message)) == -1 && errno == EAGAIN && attempts > 0)
               --attempts;
            if(attempts == 0)
               printf("child %d write - pipe busy\n", getpid());
            close(wfd);
         }
         return 0;
      }
      pids[idx] = pid;
   }
   {
      printf("parent read\n");
      for (int idx = 0; idx < writers * messages; ++idx)
      {
         short attempts{10000};
         Message message{};
         while (::read(rfd, &message, sizeof(message)) == -1 && errno == EAGAIN && attempts > 0)
            --attempts;

         if (attempts == 0)
            printf("parent %d read - pipe empty\n", getpid());
         else
            printf("message %d %d\n", message.index, message.pid);
      }
      // wait for children.
      for (int idx = 0; idx < writers; ++idx)
      {
         int status = 0;
         waitpid(pids[idx], &status, 0);
         if (WIFEXITED(status))
            printf("child exited %d\n", pids[idx]);
      }
      close(wfd);
      close(rfd);
   }
   if (unlink(name) < 0)
      return print_error(errno);
   return 0;
}