The devname function returns a pointer to the name of the block or character device in /dev with a device number of dev, and a file type matching the one encoded in type which must be one of S_IFBLK or S_IFCHR. To find the right name, devname asks the kernel via the kern.devname sysctl. If it is unable to come up with a suitable name, it will format the information encapsulated in dev and type in a human-readable format.
devname returns the name stored in a static buffer which will be overwritten on subsequent calls. devname_r takes a buffer and length as argument to avoid this problem.
struct stat buf;
fd = open("/dev/tun");
printf("devname is /dev/%s\n", devname(buf.st_rdev, S_IFCHR));