Error: "free(): double free detected in tcache 2" – CS50 PSET 4: Recover

So this program attempts to recover the file data of some deleted JPEG images using fread() and fwrite(). Going through my code, I can’t seem to find an error, but when I run the program with the raw data file, I get the error " free(): double free detected in tcache 2 Aborted (core dumped)"

Looking online this seems the be a memory allocation / freeing issue where some piece of memory is being freed multiple times, but I can’t see how that’s being done.

Any help would be really appreciated!

Here is my code:

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>
#include <stdint.h>

typedef uint8_t BYTE;

int main(int argc, char *argv[])
{
    // ensure proper usage of program
    if (argc != 2)
    {
        printf("Usage: ./recover img\n");
        return 1;
    }

    // rename and open the memory card
    char* inputf = argv[1];
    FILE *file = fopen(inputf, "r");

    // check that the memory card file is valid
    if(file == NULL)
    {
        printf("Unable to Read Memory Card\n");
        return 1;
    }

    // set up buffer array
    BYTE buffer[512];

    // initialize jpegcounter
    int jpeg = 0;

    // allocate memory for filename
    char filename[9];

    // initialize a file for the bytes to be read into when needed
    FILE *JPEGPTR = NULL;

    while (fread(buffer, 512, 1, file) == 1)
    {
        if(buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
        {

            if (jpeg != 0)
            {
                fclose(JPEGPTR);
            }

            else
            {
                sprintf(filename, "%03i.jpeg", jpeg);
                JPEGPTR = fopen(filename, "w");
                if (JPEGPTR == NULL)
                {
                    printf("Could not create image. \n");
                    fclose(file);
                    return 1;
                }

                jpeg++;
                if (jpeg != 0)
                {
                    fwrite(buffer, 512, 1, JPEGPTR);
                }
            }
        }

    }
    fclose(JPEGPTR);
    fclose(file);
    return 0;
}

>Solution :

The else should not be there.

This code block will only execute once, and the third and subsequent loops will attempt to close a file that was not open.

It’s worth noting too that fwrite will be called only when the header was detected.

So re-examine the loop logic. Proper formatting helps with understanding the flow of control.