pset4 recover.c. I have no clue on how to start

Question

I get how to solve it. I have the pseudocode in mind on how to solve it. But i cannot understand how do I go about solving it though because I cannot address the most fundamental problem. How do I go about reading the file from the start to the very end

so far I only understood how to read the file to find the start of the jpeg. However I don't really understand how I should proceed from here to keep moving.

FILE* card = fopen("card.raw", "r");
if(card == NULL)
{
    printf("did not open card.raw");
    return 1;
}

int* buffer = malloc(sizeof(int) * 1024);
fread(buffer, 512, 1, card);
for(int c = 0; c != EOF; c++)
{
    if(c > 3)
    {
        if(buffer[c - 3] == 0xff)
        {
            if(buffer[c - 2] == 0xd8)
            {
                if(buffer[c - 1] == 0xff)
                {
                    if(buffer[c] == 0xe1)
                    {
                        printf("found one");
                    }
                }
            }
        }
    }
}

Answer 1

Your logic is wrong so far, so let's start from the beginning. You read from the file, 512 bytes every time, and you save them in buffer, which is an array of bytes, and 512 places of them. So you can check the buffer bytes by

• buffer[0]
• buffer[1]
• buffer[2]
• buffer[3]
• ...
• buffer[511]

The beginning of each image, is in the first 4 bytes of each chunk of 512 bytes. So you should check only buffer[0], buffer[1], buffer[2] and buffer[3] for the appropriate values. Not every 4 adjacent bytes. So the nested ifs are wrong in your code and I would advise you rewatch the tutorials and try again. I think this will help you get started.

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Sidenote

Also 512 bytes are enough for your buffer. You don't need 1024 of them.

Sidenote 2

Also it would be better to use uint8_t instead of int, to make sure that each BYTE you read is the same on every machine your code is compiled. That is that it's an unsigned int of 1 byte.