How to use nix’s ioctl?

I want to call ioctl from Rust. I know that I should use the nix crate, but how exactly? From the documentation it’s not clear.

I have this C:

int tun_open(char *devname)
{
  struct ifreq ifr;
  int fd, err;

  if ( (fd = open("/dev/net/tun", O_RDWR)) == -1 ) {
       perror("open /dev/net/tun");exit(1);
  }
  memset(&ifr, 0, sizeof(ifr));
  ifr.ifr_flags = IFF_TUN;
  strncpy(ifr.ifr_name, devname, IFNAMSIZ);  

  /* ioctl will use if_name as the name of TUN 
   * interface to open: "tun0", etc. */
  if ( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) == -1 ) {
    perror("ioctl TUNSETIFF");close(fd);exit(1);
  }
  //..........

How would I do that same thing using the nix crate? There are no TUN* constants in the nix crate and it isn’t clear how to use the ioctl macro.

Answer

There is some example usage in rust-spidev. I will try to apply that to your code.

TUNSETIFF is defined as:

#define TUNSETIFF     _IOW('T', 202, int)

That would be this in Rust using nix:

const TUN_IOC_MAGIC: u8 = 'T' as u8;
const TUN_IOC_SET_IFF: u8 = 202;
ioctl!(write tun_set_iff with TUN_IOC_MAGIC, TUN_IOC_SET_IFF; u32);

The above macro will define the function, which you can call like this:

let err = unsafe { tun_set_iff(fd, ifr) }; // assuming ifr is an u32