__very__ simple O_TMPFILE usage, took me a little to figure out the requirements. O_TMPFILE requires MAY_WRITE privileges in the kernel, which can be explicitly achieved with O_RDWR flag. Requires linux >=3.11-rc2, which introduced O_TMPFILE into the mainline.

o_tmpfile.c

#include <stdlib.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>

/*
 * There may not be a userspace definition yet since
 * at the time of this gist, 3.11 is only in rc2.
 */
#ifndef O_TMPFILE
#define __O_TMPFILE 020000000
#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
#define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)
#endif

extern int errno;

int
main(void)
{
  int fd;

	fd = open(".", O_TMPFILE | O_RDWR);
	printf("fd #: %d\n", fd);
	if(fd == -1)
		perror("open");
	else
		close(fd);

	return 0;
}

The O_TMPFILE definitions are not needed anymore, simply add #define _GNU_SOURCE 1 before the #includes. Also O_RDWR can be replaced by O_WRONLY.

Still, the value 020000000 is very helpful if you are writing assembly instructions and populating the register values manually.