CS50 PSET4 RECOVER: fread() not populating buffer array - c

Question

Hello and thank you for taking a look.
I'm working through CS50x and am struggling with Recover. The aim is to open a .raw file, read its contents in 512-byte blocks, check the initial four bytes for .jpg headers, and then write each JPEG data to a new file.
I have a body of code written, and the file compiles. The debugger tells me that my buffer[512] variable remains empty/zeroed. This then means the program skips if/else conditions and the program exits.
While my logic within the While loop may be flawed, I haven't been able to step far enough into the program to consider this.
I looked up my issue before posting. Some sources like to use fread(buffer, 512, 1, input), but CS50 itself uses fread(buffer, 1, 512, input). Also, when initialising the filename, I have tried both char *filename = malloc(8 * sizeof(char)); and char filename[8];. For both lines I have tried each method and am still missing something.
My code is below. Thank you in advance for your time.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
typedef uint8_t BYTE;
int main(int argc, char *argv[])
{
// First check the number of arguments is correct.
if (argc != 2)
{
printf("Correct Usage: ./recover.c [filename]\n");
return 1;
}
// Open the file.
FILE *inputFile = fopen(argv[1], "r");
if (inputFile == NULL)
{
printf("File not found.\n");
return 1;
}
// Create counter of number of files.
int counter = 0;
// Create filename variable
char *filename = malloc(8 * sizeof(char)); // 7 + 1 for \0
// Create a 512-size array buffer.
BYTE buffer[512];
// Initialise img file for scope access.
FILE *img = NULL;
while (fread(buffer, sizeof(BYTE), 512, inputFile))
{
// If start of new JPEG:
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
{
if (counter == 0) // If the FIRST JPEG
{
// Make new file:
sprintf(filename, "%03i.jpg", counter);
img = fopen(filename, "w");
fwrite(buffer, sizeof(BYTE), 512, img);
}
else // If not the first JPEG
{
fclose(img); // Close previous file.
counter++;
// Make new file:
sprintf(filename, "%03i.jpg", counter); // Update filename.
img = fopen(filename, "w");
fwrite(buffer, sizeof(BYTE), 512, img);
}
}
else if (counter > 0) // buffer is continuation of previous.
{
fwrite(buffer, sizeof(BYTE), 512, img);
}
else
{
printf("I exited with no images.\n");
return 2;
}
}
free(filename);
fclose(img);
fclose(inputFile);
return 0;
}

Answer 1

The program exits (returns) after the first line in the raw file is read (assuming it's not a jpeg header, which is the case with the distro raw file). else if (counter > 0) evaluates to false, so the else branch executes.

Answer 2

Thank you everyone for your response. The issue is now fixed!
#DinoCoderSaurus (sorry, can't upvote yet) prompted me to realise that I had assumed (wrongly) that the data in the raw file would immediately begin with a .jpeg header (in fact it looks like the data begins with a hidden message, "surprise").
The Else condition was initially put there to avoid errors but of course it was prematurely exiting the While loop. The buffer was populated correctly after a couple of loops.
I then encountered the second problem (pointed out by #Some_programmer_dude) that counter++; was in the wrong place, which meant after the first new JPEG, no others could be written.
I'll also take your comments about best practice into consideration.