Getting a segmentation fault in pset4 recover

Question

I'm getting a segmentation fault when trying to run my program for pset4 recover. I've been trying for ages to debug it, but I'm finding it really difficult because when I try to use debug50, after the first line of the while loop (while (fread(buffer, 1, sizeof(buffer), card))", it simply skips the whole loop and goes to the bottom of the program.

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

typedef uint8_t BYTE;
char filename[8];

int main (int argc, char* argv[])
{
    // Check command-line arguments
    if (argc != 2)
    {
        printf("Usage: ./recover image\n");
        return 1;
    }
    
    //Open card
    FILE *card = fopen("card.raw", "rb");
    
    //Initialize output file
    FILE *jpeg;
    
    //Create buffer
    BYTE buffer [512];
    
    //Initialize counter
    int i = 0;
    
    //Start reading contents of card
    while (fread(buffer, 1, sizeof(buffer), card))
    {
        //Check for the start of the first jpeg
        if ((buffer[0] == 0xff) && (buffer[1] == 0xd8) && (buffer[2] == 0xff) && ((buffer[3] & 0xf0) == 0xe0) && i == 0)
        {
            //Create first jpeg
            sprintf(filename, "%03i.jpg", i);
            jpeg = fopen(filename, "wb");
            
            //Start writing contents of card into first jpeg file and increment counter
            fwrite(buffer, 1, sizeof(buffer), jpeg);
            i++;
        }
        
        //Check for start of subsequent jpeg files
        else if ((buffer[0] == 0xff) && (buffer[1] == 0xd8) && (buffer[2] == 0xff) && ((buffer[3] & 0xf0) == 0xe0) && i > 0)
        {
            //Close previous file
            fclose(jpeg);
            
            //Create new file
            sprintf(filename, "%03i.jpg", i);
            FILE *out = fopen(filename, "wb");
            
            //Start writing contents of card into new jpeg file
            fwrite(buffer, 1, sizeof(buffer), jpeg);
        }
        
        //If file is larger than 512B, continue adding blocks to the file until new jpeg is found
        else if (i > 0)
        {
            fwrite(buffer, 1, sizeof(buffer),jpeg);
        }
    }
    
    //When end of the card is reached, close final jpeg and close card
    fclose(card);
    fclose(jpeg);
}

Screenshot:

Answer 1

I couldn't duplicate any seg fault. Did you remember to recompile after editing your code?

The code is almost working. Unfortunately, it only creates two output files. Maybe you should check on how file names are being created and variables incremented? ;-)

Also, if card.raw is empty, it will cause seg faults later in the code when the fclose calls are attempted at the end. No files were opened.

You might benefit from this: Do YOU know how to find a seg fault?? Advice to new programmers

BTW, why is the name of the input file hardcoded instead of using the input parameter when the program is run?

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

Answer 2

You never verify that jpeg was opened, so if there is no opened file, your line fclose(jpeg); will cause a segfault.

Be sure to always check a file is open before trying to close it.