Getting segmenatation fault even though i freed memory - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
typedef uint8_t BYTE;
int main(int argc, char *argv[])
{
// Only allow 2 command line arguments
if (argc != 2)
{
printf("Usage: ./recover FILENAME\n");
return 1;
}
// Open the file
FILE *input_file = fopen(argv[1], "r");
// Check if the input_file file is a valid ifle
if (input_file == NULL)
{
printf("No existing file");
return 1;
}
// Make a buffer for storing 512 bytes, !unsigned because our values MUSTN'T be negative!
unsigned char buffer[512];
// Count the number of images we found
int image_count = 0;
// Make a pointer for recovered images
FILE *output_file = NULL;
// Use malloc for memory allocation
char *filename = malloc(8 * sizeof(char));
// Read the blocks of our array
while (fread(buffer, sizeof(char), 512, input_file))
{
// Check for JPEG header hex values
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0 )
{
// Set the names of the JPEG files
sprintf(filename, "%03i.jpg", image_count);
// Open the file we will write to
output_file = fopen(filename, "w");
// Count the number of images we found
image_count++;
}
// Check validity of input output_file input
if (output_file != NULL)
{
fwrite(buffer, sizeof(char), 512, output_file);
}
// Free the memory becuase we used malloc (to prevent leaking/segmentation fault)
free(filename);
// Close both the input and output files
fclose(output_file);
fclose(input_file);
return 0;
}
}
I am trying to solve this CS50 problem set and i don't get why I'm getting segmentation fault even though i freed memory. I obviously did something wrong, but i can't wrap my head around what exactly because I am only a begginer in c, especially with all this malloc stuff. Also, I can compile my code, it's only that I get an error when running the program.

Answer 1

You're freeing inside the loop. i.e. you allocate once but try to free it several times.
You want to free it just once. Put it outside the loop.
OTOH, since you know the filenames are only 7 bytes long, you might as well use a char array instead of mallocing.
char filename[256];
Then use snprintf:
snprintf(filename, sizeof filename, "%03i.jpg", image_count);