#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <memory.h>

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

#include <pthread.h>

#define BUF_ALIGN	1024

struct writer_data {
	int fd;
	size_t blksize;
	char *buf;
};

static void *writer(void *arg)
{
	struct writer_data *data = (struct writer_data *)arg;
	int ret;

	ret = write(data->fd, data->buf, data->blksize);
	if (ret < 0)
		fprintf(stderr, "write file failed: %s\n", strerror(errno));

	return NULL;
}

int main(int argc, char *argv[])
{
	pthread_t tid;
	struct writer_data wdata;
	size_t max_blocks = 10 * 1024;
	size_t blksize = 1 * 1024 * 1024;
	char *rbuf, *wbuf;
	int readfd, writefd;
	int i, j;
	int k;

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

	writefd = open(argv[1], O_CREAT|O_DIRECT|O_WRONLY|O_APPEND|O_TRUNC, S_IRWXU);
	if (writefd < 0) {
		fprintf(stderr, "failed to open wfile: %s\n", strerror(errno));
		exit(1);
	}
	readfd = open(argv[1], O_DIRECT|O_RDONLY, S_IRWXU);
	if (readfd < 0) {
		fprintf(stderr, "failed to open rfile: %s\n", strerror(errno));
		exit(1);
	}

	if (posix_memalign((void **)&wbuf, BUF_ALIGN, blksize)) {
		fprintf(stderr, "failed to alloc memory: %s\n", strerror(errno));
		exit(1);
	}

	if (posix_memalign((void **)&rbuf, 4096, blksize)) {
		fprintf(stderr, "failed to alloc memory: %s\n", strerror(errno));
		exit(1);
	}

	memset(wbuf, 'a', blksize);

	wdata.fd = writefd;
	wdata.blksize = blksize;
	wdata.buf = wbuf;

	for (i = 0; i < max_blocks; i++) {
		void *retval;
		int ret;

		ret = pthread_create(&tid, NULL, writer, &wdata);
		if (ret) {
			fprintf(stderr, "create thread failed: %s\n", strerror(errno));
			exit(1);
		}

		memset(rbuf, 'b', blksize);
		do {
			ret = pread(readfd, rbuf, blksize, i * blksize);
		} while (ret <= 0);

		if (ret < 0) {
			fprintf(stderr, "read file failed: %s\n", strerror(errno));
			exit(1);
		}

		if (pthread_join(tid, &retval)) {
			fprintf(stderr, "pthread join failed: %s\n", strerror(errno));
			exit(1);
		}

		if (ret >= 0) {
			for (j = 0; j < ret; j++) {
				if (rbuf[j] != 'a') {
					char c = rbuf[j];
					fprintf(stderr, "encounter an error: offset %ld, j=%ld, ret=%ld rbuf[j]=0x%02x\n",
						i, j, ret, rbuf[j]);
					for(k = j; k < ret; k++) {
						if (rbuf[k] != c)
							break;
					}
					fprintf(stderr, "0x%02x continues for %d bytes, next=0x%02x\n", c, k - j, rbuf[k]);
					goto err;
				}
			}
		}
	}

err:
	free(wbuf);
	free(rbuf);

	return 0;
}