CS5Ox Pset4 Recover: code only recovers partial images - c

Question

I have some semi-working code for CS50 Pset4. If you run it you will see that it recovers 27 jpg files but only the first couple of lines are visible.
Can someone point me in the right direction?
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
typedef uint8_t BYTE;
int main (int argc, char *argv[])
{
// ensure proper usage
if (argc != 2)
{
fprintf(stderr, "Usage: ./recover infile\n");
return 1;
}
// open file to be recovered
FILE *infile = fopen(argv[1], "r");
if (infile == NULL)
{
fprintf(stderr, "Could not open infile.\n");
return 2;
}
// temp storage for blocks
BYTE buffer[512];
// variable to store filename
char filename[8];
//store number of recovered files
int n = 0;
// temp storage for outfiles
FILE* outfile = NULL;
// iterate over all blocks of memory until end of SD card is reached
while (fread(buffer, 512, 1, infile) != 0)
{
// read one block
fread(buffer, 512, 1, infile);
// check if block is start of jpeg
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
{
//close previous file if already open
if(outfile != NULL)
{
fclose(outfile);
}
// creeate new outfile
sprintf(filename, "%03i.jpg", n);
outfile = fopen(filename, "w");
// write block to outfile
fwrite(buffer, 512, 1, outfile);
n++;
}
else
{
// write block to current outfile
if(outfile != NULL)
{
fwrite(buffer, 512, 1, outfile);
}
}
}
//close last outfile
fclose(outfile);
//close infile
fclose(infile);
}

Answer 1

I see several things that could be causing that problem.
First, check the order of your n counter. The counter should be added before you actually start writing on the new file, but that's just a matter of preference, and how clean you want your code to be.
Secondly, try to substitute your else condition with the following code:
if(outfile != NULL)
{
fwrite(buffer, 512, 1, outfile);
}
Note: Please bear in mind that I've substituted your else condition for an if condition. This is due to the fact that when the first if condition is met, then it executes this condition and 'jumps out of the block'. So the else will be executed only the first if condition won't execute.
If you wanted to keep your else condition, then you should have to nest another if , as you've done in your code.
By substituting that else with an if that will always be checked regardless of the value of the first 3 bytes of your jpg (that is, regardless if they're 0x00, 0xff, 0x00), you will obtain a much clear and understandable code.
Lastly, and more important: why are you writing twice to the same file in the same operation? Pay attention to the fwrite() function below your n++ counter. Is it really necessary?
In other words: delete this line:
// read one block
fread(buffer, 512, 1, infile);
Other mistake, is that you're reading your file twice, and advance twice in every step, so you will get half information.
This is the reason you're getting half of the images (27 or so).
Erase these two lines:
// read one block
fread(buffer, 512, 1, infile);
As I've said, by reading and writing twice within your files, you get half of the information. This leads to that fuzzy way in which you're getting your images all colored in senseless colors (I guess) and half of the image files.
I've run check50 2016.recover recover.c with your code and with the fixed solutions I've just provided you and it passes all the checks from CS50's check50. Take your time and think about everything in your program, including the control-flow, which is an important part of it, as well as the pointers usage.
Starting CS50 without any prior experience can be daunting. Keep it up. You've already made it almost into Week 5.