CS471/571 - Operating Systems

Displaying ./code/Semaphores/reader.c

#include <stdio.h>
#include <sys/stat.h>
#include <semaphore.h>
#include <sys/mman.h>
#include <fcntl.h>

#define PIPESIZE	10

// Compile: gcc -o reader reader.c -lpthread

struct pipe {
  sem_t lock, block;
  char data[PIPESIZE];
  int wp, rp, initialized;
};

int main(void)
{
  int fd = shm_open("/my_pipe", O_RDWR, 0666);
  if (fd < 0) {
    perror("shm_open");
    return 1;
  }

  struct pipe *pipe = mmap(NULL, sizeof(struct pipe),
       PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

  if (pipe == MAP_FAILED) {
    perror("mmap");
    return 1;
  }

  while (pipe->initialized == 0);

  sem_wait(&(pipe->lock));
  while(1) {
    while (pipe->rp == pipe->wp) {
      sem_post(&(pipe->block));
      fprintf(stderr,"blocking...\n");
      sem_wait(&(pipe->lock));
    }
    fprintf(stderr, "%c", pipe->data[pipe->rp]);
    pipe->rp = (pipe->rp+1) % PIPESIZE;
  }
  return 0;
}