38 lines
719 B
C
38 lines
719 B
C
|
#include <stdio.h>
|
||
|
|
#include <sys/types.h>
|
||
|
|
#include <sys/stat.h>
|
||
|
|
#include <unistd.h>
|
||
|
|
#include <stdlib.h>
|
||
|
|
#include <stdint.h>
|
||
|
|
|
||
|
|
struct stat sbuf;
|
||
|
|
|
||
|
|
int main(int argc, char** argv) {
|
||
|
|
FILE* fd = fopen(argv[1], "r+");
|
||
|
|
fstat(fileno(fd), &sbuf);
|
||
|
|
if (sbuf.st_size & 0x1F) {
|
||
|
|
fprintf(stderr, "Filesize is not a multiple of 512 bytes\n");
|
||
|
|
exit(1);
|
||
|
|
}
|
||
|
|
|
||
|
|
// Fill out filesize flag
|
||
|
|
fseek(fd, 2, SEEK_SET);
|
||
|
|
fputc(sbuf.st_size >> 9, fd);
|
||
|
|
|
||
|
|
// Calculate checksum
|
||
|
|
fseek(fd, 0, SEEK_SET);
|
||
|
|
off_t i;
|
||
|
|
uint8_t s;
|
||
|
|
for (i=0; i<sbuf.st_size; i++) {
|
||
|
|
s+=fgetc(fd);
|
||
|
|
}
|
||
|
|
|
||
|
|
// Edit last byte so that checksum is 0
|
||
|
|
fseek(fd, -1, SEEK_END);
|
||
|
|
s=fgetc(fd)-s;
|
||
|
|
fseek(fd, -1, SEEK_END);
|
||
|
|
fputc(s, fd);
|
||
|
|
|
||
|
|
fclose(fd);
|
||
|
|
}
|