size of OpenBSD disk partition

disksize.c

#include <sys/types.h>
#include <sys/disklabel.h>
#include <sys/dkio.h> /* DIOCGDINFO */
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>

/*
 * Shows the size in bytes of a disk partition in OpenBSD.
 */
int main(int argc, char **argv) {
	struct disklabel label;
	struct partition *part;
	struct stat st;
	unsigned long long size;
	int fd;
	char *path;

	if (argc != 2) {
		fprintf(stderr, "usage: %s special\n", argv[0]);
		return 1;
	}
	path = argv[1];
	fd = open(path, O_RDONLY);
	if (fd < 0 || ioctl(fd, DIOCGDINFO, &label) < 0 ||
	    fstat(fd, &st) < 0) {
		perror(path);
		return 1;
	}
	part = &label.d_partitions[DISKPART(st.st_rdev)];
	size = label.d_secsize * DL_GETPSIZE(part);
	printf("%s: %llu bytes\n", path, size);
	return 0;
}