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I'm doing the CS50 Recover problem and after executing ./recover card.raw, the program runs but I don't see any pictures.

What I have tried:

Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[])
{
    if(strcmp(argv[1], "card.raw")!=0){
        printf("Usage: ./recover card.raw\n");
        return 1;
    }

    FILE *input=fopen(argv[1], "r");
    if (input==NULL){
        printf("Could not open input file: %s\n", argv[1]);
        return 1;
    }

    unsigned int buffer[512];
    char filename[8];
    int counter=0;

    FILE *file=NULL;




    while(fread(buffer, 1, sizeof(buffer), input)){

     //if it's the start of a JPEG file
      if(buffer[0]==0xff && buffer[1]==0xd8 && buffer[2]==0xff && (buffer[3] & 0xf0)==0xe0){

        if(counter==0){             //if first JPEG

            sprintf(filename, "%03i.jpg", counter);
            filename[7]='\0';
            file=fopen(filename, "a");

            fwrite(buffer, 1, sizeof(buffer), file);
            counter++;

        }else{                  //else if not first JPEG
            fclose(file);

            sprintf(filename, "%03i.jpg", counter);
            filename[7]='\0';
            file=fopen(filename, "a");

            fwrite(buffer, 1, sizeof(buffer), file);
            counter++;
        }
    }
    else{
        if(file!=NULL){
            fwrite(buffer, 1, sizeof(buffer), file);
            }
        }

    }

    if (file != NULL) { // close the file only if it was opened successfully
    fclose(file);
    }

    fclose(input);
}
Posted
Updated 3-Mar-23 9:37am

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:
C
int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!
 
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I just ran your code with some dummy data and it works OK. So you need to get with yur debugger to see exactly what happens when you run it. The chances are that the problem lies around the part of the code starting at the line:
C++
if(buffer[0]==0xff && buffer[1]==0xd8 && buffer[2]==0xff && (buffer[3] & 0xf0)==0xe0){
 
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It is not likely that you ever will see any pictures with this code. You are reading 512 bytes from a file called card.raw. You then write that same data to a different file that has a .JPG extension. In general, a .RAW file will not have a valid JPG header so that data can not be interpreted as being a JPG file. That means you will not see a picture.

To see a picture, you have to write a valid header for a JPG image file and write data that is appropriate for that header in terms of width and height. I can't give details for all of that since I don't know them because I always use the CxImage library that I found here[^].
 
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Richard MacCutchan 4-Mar-23 3:52am    

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