Can someone explain why fread() is not working? - c

Question

After a couple hours working on the recover exercise of cs50 i've been stumbling in the segmentention error problem. After running the debbuger i've discovered that the cause of the segmentation error is the malfuction of fread(memory, 512, 1, file), even after calling the function the memory[] array keeps empty, thus, the segmentation error.
i've tried to work with malloc(512) instead of an unsigned char array but the error persists. Can someone explain why is this happening and how to solve it?
(PS. Sorry for my bad english)
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
int main(int argc, char *argv[])
{
// making sure the only user input is the name of the file
if (argc != 2)
{
printf("Usage: ./recover image\n");
return 1;
}
// open the file and check if it works
FILE *file = fopen("card.raw", "r");
if (file == NULL)
{
printf("Could not open card.raw.\n");
return 2;
}
int ending = 1000;
int count = 0;
char img = '\0';
FILE *picture = NULL;
unsigned char memory[512];
do
{
//creating buffer and reading the file into the buffer
fread(memory, 512, 1, file);
//checking if the block is a new jpg file
if (memory[0] == 0xff && memory[1] == 0xd8 && memory[2] == 0xff && (memory[3] & 0xf0) == 0xe0)
{
//if it's the first jpg file
if (count == 0)
{
sprintf(&img, "000.jpg");
picture = fopen(&img, "w");
fwrite(&memory, 512, 1, picture);
}
//closing previous jpg file and writing into a new one
else
{
fclose(picture);
img = '\0';
sprintf(&img, "%03i.jpg", count + 1);
picture = fopen(&img, "w");
fwrite(&memory, 512, 1, picture);
}
}
//continue writing into the file
else
{
picture = fopen(&img, "a");
fwrite(&memory, 512, 1, picture);
}
count++;
}
while(ending >= 512);
fclose(file);
fclose(picture);
return 0;
}

Answer 1

if you're using C or C++ then you should be fully aware of the memory model being used. For example, declaring a character local variable, means allocating from 1 to 4 bytes of memory in the stack, depending on memory alignment and architecture being used (16-bit? 32-bit? 64-bit?). And guess what happens when you do sprintf of more than 4 characters on such character local variable. It will overrun to whatever variable occupying the space after the img variable. So you must prepare a buffer large enough to hold characters that are needed to create the filename.
In C, if you make a mistake, there are several possibilities :
sometime you get segmentation fault after you do a mistake
sometime you didn't get any error but the data silently corrupted
sometime error happens long after the mistake is done
There are other problems with your code, which has been pointed out by Weather Vane and Jabberwocky in the comments above. I would like to add that reopening 'img' file and discarding previous file handle is not a good thing either (you already said continue writing? why need to reopen? ). You might get a dangling file handle or needlessly create many file handles during the iteration. C will not help you check such things, it assumes you really know what are you doing. Even mixing types will not cause compile error nor identifiable runtime error. It will just do one of the three things I said above.
You might want to use other modern language with more memory safety features in order to learn programming, like C#, Java or Python.