Pset4 Recover: Program creates segmentation fault

Question

I'm new to writing programs that read or write files and I am totally stuck as to the reason I keep getting a segmentation fault. I feel like it might have to do with one of my file pointers or newly created files, but any help by more experienced eyes would be greatly appreciated!

Another piece of information - while I was trying to debug the program, it seemed to be stuck checking the if statements around line 35 for longer than I would expect, then eventually - segmentation fault.

Here is my complete code below:

    // program to recover deleted JPEGs from memory card

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

int main(int argc, char *argv[])
{
    // ensure proper usage
    if (argc != 2)
    {
        fprintf(stderr, "Usage: recover inputFile\n");
        return 1;
    }
    // remember filename
    char *infile = argv[1];

    // open input file
    FILE *inptr = fopen(infile, "r");
    if (inptr == NULL)
    {
        fprintf(stderr, "Could not open %s.\n", infile);
        return 2;
    }

// read raw input file
    int buffer[512];
    int counter = 0;
    char filename[8];
    FILE* img;

    // repeat until EOF
    while (fread (&buffer, 1, 512, inptr) == 512)
    {
        //fread (&buffer, 1, 512, inptr);  // this might be redundant (while statement)
    // check for start of JPEG
        if (buffer[0] == 0xff &&
            buffer[1] == 0xd8 &&
            buffer[2] == 0xff &&
            (buffer[3] & 0xf0) == 0xe0)
        {
            if (counter > 0) // if this is not the first jpeg found, we need to close previous jpeg
            {
                fclose(img);
            }
            // create and open file
            sprintf(filename, "%03i.jpg", counter);
            counter ++; //keep track of current jpeg count

            img = fopen(filename, "w");
            // make sure our pointer wasn't NULL
            if (img == NULL)
            {
                fprintf(stderr, "Could not open %s.\n", filename);
                return 2;
            }
            do
            {
            // Write current buffer to JPEG
            fwrite (&buffer, 512, 1, img);
            fread (&buffer, 1, 512, inptr);
            }
            // making sure to stop at start of new JPEG
            while ((buffer[0] == 0xff &&
                    buffer[1] == 0xd8 &&
                    buffer[2] == 0xff &&
                    (buffer[3] & 0xf0) == 0xe0) == 0);
            // move cursor back one 512 byte block so it can be re-read for if-statement
            fseek (inptr, -512, SEEK_CUR);
        }
    }
    // close remaining open files
    fclose(img);
    fclose(inptr);
}

Answer 1

There are a couple of problems in the code. Ordinarily, I'd suggest that you need to identify which line is causing the seg fault, but it looks like you're stalled out on that. In this case, running debug might take too long to step through until you get to the seg fault. Inserting some carefully placed printf statements would help.

However, the seg fault is occuring at the end of the program when the code tries to close the img file. This happens because it is never opened. You could check for this in the code by looking to see if it is NULL. BUT, to do this, you need to initialize the pointer as NULL when it is declared. A file pointer should ALWAYS be set to null or assigned to a file when declared. It should never be declared without being initialized.

Now that your question is answered, here's a hint to the next problem. When you store something in a var of type int, how many bytes are you storing?

If this answers your question, please click on the check mark to accept. Let's keep up on forum mainteance. ;-)