/* chkfatflags
 * (c) Sebastian Wills 2005
 */

#include <stdio.h>

unsigned int read_twobytes(FILE *stream);


int main(int argc, char **argv)
{
   FILE *fs;
   unsigned int sector_size;
   unsigned int reserved_sectors;
   unsigned char num_fats;
   unsigned int fat_size;
   unsigned int fat_start;
   unsigned int fatentry;
   int fattype=16;
   char clean_shutdown;
   char no_errors_detected;

   unsigned char c;
   int i;
   
   if(argc<2) 
     {
	fprintf(stderr,"Usage: %s <device>\n",argv[0]);
	exit(3);
     }
   
   if((fs=fopen(argv[1],"rb"))==NULL) 
     {
	fprintf(stderr,"Error opening %s\n",argv[1]);
	exit(3);
     }
   
   safe_fseek(fs, 11, SEEK_SET);
   sector_size=read_twobytes(fs);
   printf("Sector size: %d\n",sector_size);
   
   safe_fseek(fs, 14, SEEK_SET);
   reserved_sectors=read_twobytes(fs);
   printf("Number of reserved sectors: %d\n", reserved_sectors);
   
   safe_fseek(fs,16, SEEK_SET);
   fread(&num_fats,1,1,fs);
   printf("Number of FATs: %d\n",num_fats);
   
   safe_fseek(fs,22, SEEK_SET);
   fat_size=read_twobytes(fs);
   if(fat_size==0) /* FAT32 */
     {
	fattype=32;
	safe_fseek(fs,36,SEEK_SET);
	fread(&fat_size,4,1,fs);
     }
   printf("FAT size: %d sectors\n", fat_size);
   
   fat_start=sector_size*reserved_sectors;
   printf("FAT starts at offset %d (%x)\n",fat_start,fat_start);

   if(fattype & 0xfff != 0) 
     {
	fprintf(stderr,"Only FAT16 or FAT32 supported, not FAT12\n");
	exit(3);
     }
   
   /* seek to start of fat plus skip over first FAT entry (cluster 0) to get to cluster 1. Each entry is fat_type bits) */
   fseek(fs, fat_start+fattype/8, SEEK_SET);
   fread(&fatentry, fattype/8, 1, fs);
   printf("FAT entry for cluster 1: 0x%2x\n",fatentry);
   
   clean_shutdown= (fatentry & (0x8 << (fattype/8-1))) != 0;
   no_errors_detected=(fatentry & (0x4 << (fattype/8-1))) != 0;
   printf("Clean shutdown: %s\n",clean_shutdown ? "yes":"no");
   printf("Errors detected: %s\n",!no_errors_detected ? "yes":"no");
	
}

unsigned int read_twobytes(FILE *stream) 
{
   unsigned char c;
   unsigned int result;
   fread(&c,1,1,stream);
   result=c;
   fread(&c,1,1,stream);
   result=result | ((unsigned int) c << 8);
   return result;
}


int safe_fseek(FILE *stream, long offset, int whence) 
{
   int retval;
   if(retval=fseek(stream,offset,whence)==-1)
     {
	fprintf(stderr,"Error seeking to offset %ld. Exciting.\n",offset);
	exit(3);
     }
   return retval;
}