create tun/tap device and read/write

gistfile.c

cat tunclient.c 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <net/if.h>
#include <linux/if_tun.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <arpa/inet.h> 
#include <sys/select.h>
#include <sys/time.h>
#include <errno.h>
#include <stdarg.h>

/* buffer for reading from tun/tap interface, must be >= 1500 */
#define BUFSIZE 2000   
#define CLIENT 0
#define SERVER 1
#define PORT 55555


/**************************************************************************
 * tun_alloc: allocates or reconnects to a tun/tap device. The caller     *
 *            must reserve enough space in *dev.                          *
 **************************************************************************/
int tun_alloc(char *dev, int flags) {

  struct ifreq ifr;
  int fd, err;
  char *clonedev = "/dev/net/tun";

  if( (fd = open(clonedev , O_RDWR)) < 0 ) {
    perror("Opening /dev/net/tun");
    return fd;
  }

  memset(&ifr, 0, sizeof(ifr));

  ifr.ifr_flags = flags;

  if (*dev) {
    strncpy(ifr.ifr_name, dev, IFNAMSIZ);
  }

  if( (err = ioctl(fd, TUNSETIFF, (void *)&ifr)) < 0 ) {
    perror("ioctl(TUNSETIFF)");
    close(fd);
    return err;
  }

  strcpy(dev, ifr.ifr_name);

  return fd;
}

static const char pkt_data[] =
        "\x3c\xfd\xfe\x9e\x7f\x71\xec\xb1\xd7\x98\x3a\xc0\x08\x00\x45\x00"
        "\x00\x2e\x00\x00\x00\x00\x40\x11\x88\x97\x05\x08\x07\x08\xc8\x14"
        "\x1e\x04\x10\x92\x10\x92\x00\x1a\x6d\xa3\x34\x33\x1f\x69\x40\x6b"
        "\x54\x59\xb6\x14\x2d\x11\x44\xbf\xaf\xd9\xbe\xaa";

int main() {
	/* tunclient.c */
	int tap_fd;
	char tun_name[IFNAMSIZ];
	char buffer[4096];
	int nwrite;
	unsigned long x = 0;

	/* Connect to the device */
	strcpy(tun_name, "tap0");
	tap_fd = tun_alloc(tun_name,
		 IFF_TAP | IFF_MULTI_QUEUE | IFF_NAPI);

	if (tap_fd < 0) {
		perror("Allocating interface");
		exit(1);
	}
#define SENDER 0
#if SENDER
	while(1) {
		nwrite = write(tap_fd, pkt_data, sizeof(pkt_data));
		if (nwrite < 0) {
			perror("writing data");
			// ip link set dev tap0 up
		//	exit(1);
		}
		printf("Write %d bytes\n", nwrite);
		sleep(1);
	}

#else
	/* Now read data coming from the kernel */
	while(1) {
		/* Note that "buffer" should be at least the MTU
		   size of the interface, eg 1500 bytes */
		int nread = read(tap_fd,buffer,sizeof(buffer));
		if(nread < 0) {
			perror("Reading from interface");
			close(tap_fd);
			exit(1);
		}

		/* Do whatever with the data */
		if ((x++ % 1000) == 0)
		printf("Read %d bytes from device %s\n", nread, tun_name);
	}
#endif
	return 0;
}

Hii @williamtu , I am trying to write some character buffer to created tunnel interface but i am getting errno 22 i.e invalid argument error. I am just replacing pkt_data value with the following value
static const char pkt_data[]={0x06, 0xfd, 0xd4 }
Could you please help?

Why you are using IFF_NAPI?