#include <pthread.h>
#include <stdio.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define nthreads 32
#define pagecount 16384
#define nrounds 10000
#define nchunks 10
#define PAGE_SIZE 4096

int fd;
unsigned long v;

void *threadf(void *ptr)
{
	unsigned long *addr;
	int i, j;

	for (j = 0; j < nrounds; j++) {
		for (i = 0; i < nchunks; i++) {
			addr = mmap(NULL, PAGE_SIZE * pagecount, PROT_READ, MAP_SHARED, fd, i * PAGE_SIZE);
			if (addr == MAP_FAILED) {
				fprintf(stderr, "mmap\n");
				exit(1);
			}
			v += *addr;
			munmap(addr, PAGE_SIZE * pagecount);
		}
	}
}

int main(int argc, char *argv[]) {
	pthread_t thr[nthreads];
	int i;

	if (argc < 2) {
		fprintf(stderr, "usage: %s <some-big-file>\n", argv[0]);
		exit(1);
	}

	fd = open(argv[1], O_RDONLY);
	if (fd < 0) {
		fprintf(stderr, "open\n");
		exit(1);
	}
	
	for (i = 0; i < nthreads; i++) {
		if(pthread_create(&thr[i], NULL, threadf, NULL)) {
			fprintf(stderr, "pthread_create\n");
			exit(1);
		}
	}

	for (i = 0; i < nthreads; i++) {
		if(pthread_join(thr[i], NULL)) {
			fprintf(stderr, "pthread_join\n");
			exit(1);
		}
	}

	return 0;
}