Segmentation Fault: CS50 recover.c - c

Question

This program accepts a filename as input and should recover all jpegs on that file. It reads 512 bytes at a time, checking for the start of a new jpeg.
The program compiles when I run it, but it gives a segmentation fault. Please advise me how I can go about fixing this.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
// check for proper usage
if (argc != 2)
{
printf("Usage: 1 command line argument\n");
return 1;
}
// check if file can be opened
FILE *file = fopen(argv[1], "r");
if (file == NULL)
{
printf("Cannot be opened\n");
return 2;
}
// read 512 bytes into buffer until end of card
int buffer[128];
int counter;
counter = 0;
char filename[8];
FILE *img = NULL;
while(fread(buffer, 4, 128, file) == 128)
{
//check if start of new JPEG
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
{
//check if first JPEG
if (counter == 0)
{
sprintf(filename, "%03i.jpg", counter);
img = fopen(filename, "w");
fwrite(buffer, 4, 128, img);
counter += 1;
}
else if (counter > 0)
{
fclose(img);
sprintf(filename, "%03i.jpg", counter);
img = fopen(filename, "w");
fwrite(buffer, 4, 128, img);
counter += 1;
}
}
else if (counter > 0)
{
fwrite(buffer, 4, 128, img);
}
}
fclose(img);
fclose(file);
return 0;
}

Answer 1

The call fread(buffer, 4, 128, file) reads (as you rightly say) 512 bytes into the array of 128 integers. However, when you then test for the start of a new JPEG file in this code:
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
{
//...
you are inspecting the low bytes of each of the first four integers in the array, rather than (as you should be) checking the first four bytes of the array. These four bytes will all be in the first integer element (buffer[0]).
Thus, your program will never find the start of a new JPEG and, consequently, the img file will never be opened ... and you are then calling fclose with a NULL file pointer, which is undefined behaviour and likely to cause the segmentation fault you see.
Instead (assuming the correct 'endianness'), do the following check:
if ((buffer[0] & 0xfffffff0) == 0xffd8ffe0)
{
//...
To allow for the 'wrong endianness', you could check for either byte order in buffer[0]:
if ((buffer[0] & 0xfffffff0) == 0xffd8ffe0 || (buffer[0] & 0xf0ffffff) == 0xe0ffd8ff)
{
//...
Better still, just read (and write) the buffer for what it is - an array of 512 bytes, with code like this:
unsigned char buffer[512];
//...
while(fread(buffer, 1, 512, file) == 512)
{
//...
This way, you can keep your 'JPEG start' test as it is.

Answer 2

fclose(img) is the culprit. img is NULL. In order to find out why, you can insert some printf after your fopen. Here is the link to debug any segfaults in the future.
$ ./recover card.raw
Memory access error: invalid parameter; abort execution.
# fclose's parameter (0x0) is not a valid FILE pointer.
# Stack trace (most recent call first) of the error.
# [0] file:/recover.c::55, 5
# [1] [libc-start-main]