#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>
#include <stdio.h>
#include <string.h>

int main() {
    int page_size = getpagesize();
#define SIZE 512
    int rwfd;
    int res, res1, res2;
    char *buf;

    rwfd = open("rwfile", O_RDWR); assert(rwfd != -1);
    res = ftruncate(rwfd, SIZE); assert(res == 0);

    /* Map rwfile with PROT_WRITE */
    buf = mmap(0, page_size, PROT_WRITE, MAP_SHARED, rwfd, 0); assert(buf != (char *)-1);

    /* Try to make the kernel read from buf */
    res1 = write(rwfd, buf, SIZE);
    printf("write(rwfd, buf, SIZE), res1 = %d, %s\n", res1, res1 < 0 ? strerror(-res1) : "OK");

    /* Try to make the kernel write into buf */
    res = read(rwfd, buf, SIZE);
    printf("read(rwfd, buf, SIZE), res = %d, %s\n", res, res < 0 ? strerror(-res) : "OK");

    /* Try to make kernel read from buf again */
    res2 = write(rwfd, buf, SIZE);
    printf("write(rwfd, buf, SIZE), res2 = %d, %s\n", res2, res2 < 0 ? strerror(-res2) : "OK");

    printf("result: res1 %s res2\n", res1 == res2 ? "==" : "!=");
}