cs50 Prblem set 4 recover. Can someone please look at my code and tell me why I'm getting a segmentation fault?

Question

/** * recover.c * * Computer Science 50 * Problem Set 4 * * Recovers JPEGs from a forensic image. */

include

include

include

int main(int argc, char* argv[]) { // Ensure proper usage if(argc != 2) { printf("You need ./recover and an infile\n"); return 1; }

//remember file name
char* infile = argv[1];

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

//define buffer
unsigned char buffer[512] = {0};
//jpegs
char jpg[8] = {0};
//keep track of the filenumbers
int filenumber = 0;
//pointer for outfile
FILE* outptr; 

//repeat until end of file
while(fread(&buffer, sizeof(buffer), 1, inptr) == 1)
{//read 512 bytes into a buffer

    //Finding the beginning of a jpg
    if(buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] == 0xe0 || buffer[3] == 0xe1))
    {
        //opens new jpg; creates file name for jpgs
        sprintf(jpg, "%03d.jpg", filenumber);

        //opens a new file
        outptr = fopen(jpg, "w");
        if (NULL == outptr)
        {
            printf("Cannot open jpg\n");
            return 1;
        }

        //write into outfile
        fwrite(buffer, sizeof(buffer), 1, outptr);

        //increment jpgs
        filenumber++;
    }

    else
    {
        if(NULL != outptr)
        {
            fwrite(buffer, sizeof(buffer), 1, outptr);
        }
    }
}

fclose(inptr);

fclose(outptr);

return 0;

}

Answer 1

Here's the problem:

FILE* outptr; 
...
if(NULL != outptr)
    {
        fwrite(buffer, sizeof(buffer), 1, outptr);

When created, outptr contains garbage data. It isn't automatically set to NULL, so when the first block doesn't have a signature and no file is opened, it drops down to the IF statement. Since outptr contains garbage data, it doesn't == NULL and the fwrite() is called, which triggers the seg fault.

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Answer 2

Also, you need to check the 4th byte for 0xe0 to 0xef and not only 0xe1 and 0xe0.