PSET 4 Recover SEGMENTATION FAULT CS50

问题

I'm struggling a bit with reading files and writing into files. I've been working on this problem for a few days and I just can't seem to get it. The program is supposed to recover some jpeg files from a memory card. With the following code I keep getting a segmentation fault. Can you guys see where I went wrong? Here is my code below:

Hope you guys can help!

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>

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

    //Ensure only one command line argument//
    if (argc != 2)
    {
        printf("Usage: ./recover filename\n");
        return 1;
    }

    //Open File//
    FILE *file = fopen(argv[1], "r");
    if (file == NULL)
    {
        printf("File not working\n");
        return 1;
    }

    //Create buffer of size 512 bytes to store data from file that is being read//
    unsigned char buffer[512];
    //Set Bool value of already found first jpeg to false//
    bool already_found_jpeg = false; 
    //Create filename length of 8 that will be of new jpegs
    char filename[8];
    FILE *img = NULL;
    //num_of_imgs counter//
    int num_of_imgs = 0;
    while(fread(buffer, 512, 1, file) == 1)
    {
        //If start of new jpeg//
        if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
        {
            //If first jpeg, start writing the very first file// 
            if (already_found_jpeg == false) 
            {
                sprintf(filename, "%03i.jpg", num_of_imgs);
                img = fopen(filename, "w");
                if (img == NULL)
                {
                    return 3;
                }
                //Write from buffer into new file//
                fwrite(&buffer, 512, 1, img);
                num_of_imgs ++;
                already_found_jpeg = true;
            }
            //Else if already found a jpeg meaning it's not the first jpeg then close file, so you can open up a new file that can be written too
            else if (already_found_jpeg == true)
            {
                fclose(img);
                sprintf(filename, "%03i.jpg", num_of_imgs);
                img = fopen(filename, "w");
                if (img == NULL)
                {
                    return 3;
                }
                //Write from buffer into new file//
                fwrite(&buffer, 512, 1, img);
                num_of_imgs ++;
            }
        }
        //else if not found the jpeg headers and already found first jpeg keep writing to file
        else 
        {
            if (already_found_jpeg == true)
            {
                fwrite(&buffer, 512, 1, img);
            }
        }
        //Close all files//
        fclose(file);
        fclose(img);
    }
}

回答1:

Problem

At the end of first iteration of while loop, you are closing file and img. This means the value of those pointers become NULL. In the next iteration, buffer is reading from a NULL pointer (because we closed file, it is pointing to nothing). Basically, dereferencing a null pointer results in segmentation fault.

Solution

If you move this piece of code outside of the while loop, it will work. (Tested with CS50 IDE)

//Close all files//
fclose(file);
fclose(img);

This was a great practice for me to review what I did in CS50 - recover. :)

来源：https://stackoverflow.com/questions/60923224/pset-4-recover-segmentation-fault-cs50

标签

segmentation-fault

cs50