CS50 PSET4 recover.c - 我只恢复了 25 jpg

Question

Im doing CS50 course and in stuck on PSET4. I have written a code that recovers JPG's from card.raw. Problem is that i only get 25 picks (from 000.jpg to 024.jpg) after running my program, and there should be 50 of them. I dont have a clue why is this happening. Could You give me a pointer:)

My code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

char *name[9] = {"card.raw"};
int i;
int a = -1;
char recovered[sizeof "050.jpg"];


uint8_t buffer[512];

int main(int argc, char *argv[])
{
    //Check for valid quantity of command line arguments
    if (argc != 2 || (strcmp (argv[1], *name) != 0))
    {
    printf("Usage: ./recover image\n");
    return 1;
    }

    FILE *card = fopen(argv[1], "r");

    //Check if file card.raw exists
    if (card == NULL)
    {
        printf("Could not open file\n");
        return 1;
    }

    while(fread(&buffer, 512, 1, card))
    {
        if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && ((buffer[3] & 0xf0) == 0xe0))
        {
            a++;
            sprintf(recovered, "%03d.jpg", a);
            FILE *output = fopen(recovered,"w");
            if (output == NULL)
            {
                printf("Could not create file %03d/n",a);
                return 1;
            }
            fwrite(buffer, 512, 1, output);


            while(fread(&buffer, 512, 1, card))
            {
                 if ( !(buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && ((buffer[3] & 0xf0) == 0xe0)))
                 {
                     fwrite(buffer, 512, 1, output);
                 }
                 else
                 {
                     break;
                 }

            }
            fclose(output);

        }
    }
    fclose(card);
    return 0;
}

Answer 1

There are several issues.

You have two nested loops. You want only one .

When the inner loop detects a header, you break out of the loop.

But, only the outer loop opens the output file.

It does a new read, so it will never see the header that the inner loop detected, and will not open the next output file.

So, you only get half as many output files. And, they're all corrupt because they have the header and data from the first file and have only the data from the second one. This repeats alternately throughout.

FILE *output is loop scoped. It should be function scoped.

recovered is defined in an unconventional way. Better to just define it with a large amount. This prevents the compiler from flagging it as too small. The compiler assumes that a could be (eg) 1000000000 and the sprintf would overflow the size of recovered

name is also defined in an unorthodox manner.

i is never used.

---

Here's a refactored and annotated version. It compiles but I've not tested it:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

const char *name = "card.raw";

int a = -1;
char recovered[1000];

uint8_t buffer[512];

int
main(int argc, char **argv)
{

    // Check for valid quantity of command line arguments
    if (argc != 2 || (strcmp(argv[1], name) != 0)) {
        printf("Usage: ./recover image\n");
        return 1;
    }

    FILE *card = fopen(argv[1], "r");

    // Check if file card.raw exists
    if (card == NULL) {
        printf("Could not open file\n");
        return 1;
    }

    FILE *output = NULL;

    while (fread(buffer, sizeof(buffer), 1, card)) {
        // detect new input header
        if ((buffer[0] == 0xff) && (buffer[1] == 0xd8) &&
            (buffer[2] == 0xff) && ((buffer[3] & 0xf0) == 0xe0)) {

            // close previous output stream
            if (output != NULL)
                fclose(output);

            // open new output stream
            ++a;
            sprintf(recovered, "%03d.jpg", a);
            output = fopen(recovered, "w");
            if (output == NULL) {
                printf("Could not create file %03d/n", a);
                return 1;
            }
        }

        // write data to output stream
        // this writes both the header and data for a given output file
        if (output != NULL)
            fwrite(buffer, sizeof(buffer), 1, output);
    }

    // close the last output stream
    if (output != NULL)
        fclose(output);

    fclose(card);

    return 0;
}