/*
 * gcc -Wall -o reader reader.c -lpthread
 */
#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <sys/sendfile.h>
#include <unistd.h>
#include <errno.h>
#include <err.h>
#include <pthread.h>

struct thread_data {
	int fd;
	size_t size;
};

static void *drop_pages(void *arg)
{
	struct thread_data *td = arg;
	int ret;
	unsigned long nr_pages = td->size / 4096;
	unsigned int seed = 0x55443322;
	off_t offset;
	unsigned long nr_drops = 0;

	while (1) {
		offset = rand_r(&seed) % nr_pages;
		offset = offset * 4096;
		ret = posix_fadvise(td->fd,  offset, 4096, POSIX_FADV_DONTNEED);
		if (ret < 0)
			err(1, "fadvise dontneed");

		/* every once and a while, drop everything */
		if (nr_drops > nr_pages / 2) {
			ret = posix_fadvise(td->fd,  0, td->size, POSIX_FADV_DONTNEED);
			if (ret < 0)
				err(1, "fadvise dontneed");
			fprintf(stderr, "+");
			nr_drops = 0;
		}
		nr_drops++;
	}
	return NULL;
}

#define READ_BUF (2 * 1024 * 1024)
static void *read_pages(void *arg)
{
	struct thread_data *td = arg;
	char buf[READ_BUF];
	ssize_t ret;
	loff_t offset;

	while (1) {
		offset = 0;
		while(offset < td->size) {
			ret = pread(td->fd, buf, READ_BUF, offset);
			if (ret < 0)
				err(1, "read");
			if (ret == 0)
				break;
			offset += ret;
		}
	}
	return NULL;
}

int main(int ac, char **av)
{
	int fd;
	int ret;
	struct stat st;
	struct thread_data td;
	pthread_t drop_tid;
	pthread_t drop2_tid;
	pthread_t read_tid;

	if (ac != 2)
		err(1, "usage: reader filename\n");

	fd = open(av[1], O_RDONLY, 0600);
	if (fd < 0)
		err(1, "unable to open %s", av[1]);

	ret = fstat(fd, &st);
	if (ret < 0)
		err(1, "stat");

	td.fd = fd;
	td.size = st.st_size;

	ret = pthread_create(&drop_tid, NULL, drop_pages, &td);
	if (ret)
		err(1, "pthread_create");
	ret = pthread_create(&drop2_tid, NULL, drop_pages, &td);
	if (ret)
		err(1, "pthread_create");
	ret = pthread_create(&read_tid, NULL, read_pages, &td);
	if (ret)
		err(1, "pthread_create");

	pthread_join(drop_tid, NULL);
	pthread_join(drop2_tid, NULL);
	pthread_join(read_tid, NULL);
}