I am trying to use mmap to map a uio device for user-space usage.
This is what I use in my code:

if (map == MAP_FAILED) {
    perror("mmap");
    close(fd);
    return -1;
}

fd is dev/uio0 device
In the address value, I use NULL, which, to the best of my knowledge lets the kernel find the right address according to the device opened.
It fails, and the map pointer value is 0xffffffff.
I hope yuo guys can help me figure out what’s wrong with my implementation. I’d add more details if necessary.