Trim .NDS files to be their minimum size

ndstrim.c

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

int main(int argc, char *argv[])
{
	unsigned char d[4];
	unsigned char wifi[136];
	int fd, i, j, e=0;
	unsigned int na, nb, hs;

	if (argc <= 1) {
		printf("Usage: %s files.nds\n", argv[0]);
	}
	for (i = 1; i < argc; i++) {
		fd = open(argv[i], O_RDWR);
		if (fd == -1) {
			perror(argv[i]);e++;
			continue;
		}
		/* Read the size word */
		if (lseek(fd, 128, SEEK_SET) != 128) goto BAD_NDS;
		if (read(fd, d, 4) != 4) goto BAD_NDS;
		na = d[0] + (d[1]<<8) + (d[2]<<16) + (d[3]<<24);
		/* File size */
		nb = lseek(fd, 0, SEEK_END);
		if (lseek(fd, na, SEEK_SET) != na) goto BAD_NDS;
		/* Quick test to see if there is any work to do */
		if (nb == na || ((na-nb) == 136)) {
			printf("%s: Already trimmed\n", argv[i]);
			close(fd);
			continue;
		}
		/* preserve the WIFI settings (if present) */
		if (read(fd, wifi, 136) != 136) goto BAD_NDS;
		for (j = 0; j < 136; j++) {
			if (wifi[j] == 0 || wifi[j] == 0xff) continue;
			na += 136;
			break;
		}
		if (nb < na) {errno=EINVAL;goto BAD_NDS;}
		if (nb == na) {
			printf("%s: Already trimmed\n", argv[i]);
		} else {
			/* trim */
			(void)lseek(fd,0, SEEK_SET);
			if (ftruncate(fd, na) < 0) goto BAD_NDS;
			printf("%s: %u bytes saved\n", argv[i], nb-na);
		}
		(void)close(fd);
		continue;
BAD_NDS:
		(void)close(fd);
		errno = EINVAL;
		perror(argv[i]);e++;
	}
	exit(!!e);
}