QDM2 c code implementation

#include<stdio.h>
 


// QDM2 header structure
struct header
{
	unsigned char id;
	unsigned char length[2];
	unsigned char checksum[2];
};

struct unknown_structure
{
	unsigned char id;
	unsigned char length;
	unsigned char offset;
};


main(int argc, char **argv){
	FILE *fd;		//filehandle
	struct header h;
	int i , tmp, chunklength, headersum, checksum = 0;
	unsigned char packetbuffer[16000];

	struct unknown_structure us[100];	//make sure there is enough memory
	int us_number = 0;			//amount of us's
	int us_offset = 0;			//offset

	if ((fd = fopen(argv[1], "r")) == NULL){
		fprintf(stderr, "Cannot open %s\n", argv[1]);
		exit(1);
	}

	// read the header
	fread(&h,sizeof(struct header),1,fd);

	// calculate how many more bytes to read in this packet
	chunklength = (h.length[1] + (h.length[0] << 8)) - 2; 
	
	// read the rest of the packet
	fread(packetbuffer,sizeof(char),chunklength,fd);


	
	// checksum from header
	headersum = (h.checksum[1] + (h.checksum[0] << 8));
	
	// calculate checksum
	for (i=0 ; i < chunklength ; i++){
		checksum = checksum + packetbuffer[i];
	}
	checksum = checksum + h.id + h.length[0] + h.length[1];
	checksum = (checksum & 0x0000FFFF);
	

	// print out packet data
	fprintf(stdout,"Packet ID = %x \nChunk size =  %d bytes\n",h.id,chunklength);
	fprintf(stdout,"Header checksum = %X , calculated checksum = %X\n",headersum,checksum);


	
	// calculate chunkstructures
	while ( (packetbuffer[us_offset] != 0) && (packetbuffer[us_offset+1] != 0))
	{
		us[us_number].offset = us_offset;
		us[us_number].id = packetbuffer[us_offset];
		us[us_number].length = packetbuffer[us_offset+1];
		us_offset = us_offset + us[us_number].length + 2;
		fprintf(stdout,"ID[%d] = %X , length = %X = %d bytes [%X]\n", us_number, us[us_number].id , us[us_number].length, us[us_number].length, us[us_number].offset+5);
		us_number ++;
	}

	tmp = 0;
	// calculate the total length of the different us_structures
	for (i=0 ; i < us_number ; i++){
		tmp = tmp + us[i].length;
		//fprintf(stdout,"us[%d].length = %X tmp = %X\n",i,us[i].length,tmp);
	}
	fprintf(stdout,"Total us length: %d bytes\n",tmp);
	

			
	//fprintf(stdout , "sizeof int = %d,sizeof char = %d \n" , sizeof(int) , sizeof(char));
	
	
	/* /printf("ID=%X\n",preheader); */
	fclose(fd);
	exit(1);	
}