Recovering partial deleted data on memory card using C - c

Question

I have been working on an assignment that recovers deleted data from the memory card so that the image is viewable. My code compiles and runs fine and produces viewable jpeg images. The program also automatically titles the jpeg image by assigning a number to it. However, when I run it, it is only able produce about half of the images. The other half were unviewable, with random numbers as titles and the error 'unsupported image format' is shown. I'm not too sure what went wrong with my code and I'm fairly new to the intricacies but I suspect it has something to do with memory. Here is my code, any help would be really nice:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
bool function(unsigned char arr[], FILE *pointer);
int main(int argc, char *argv[])
{
if (argc != 2)
{
fprintf(stderr,"Usage: ./recover image");
return 1;
}
// opening the memory card
FILE *memorycard = fopen(argv[1], "r");
if (memorycard == NULL)
{
fprintf(stderr, "Could not open %s.\n", argv[1]);
return 2;
}
unsigned char buffer[512];
int i = 0;
char filename[8];
while(fread(buffer,1, 512, memorycard) == 512)
{
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
{
//create a new file
sprintf(filename, "%03i.jpg", i);
//write the values currently stored in the buffer to the file
FILE *img = fopen(filename, "w");
fwrite(buffer, sizeof(char), 512, img);
memset (buffer, 0, 512);
//add one to the title of the next jpeg file
i++;
do
{
//read the next chunk of 512 bytes
fread(buffer,1, 512, memorycard);
}
while (function(buffer, img) == true);
}
}
}
bool function(unsigned char arr[], FILE *pointer)
{
if (arr[0] == 0xff && arr[1] == 0xd8 && arr[2] == 0xff && (arr[3] & 0xf0) == 0xe0)
{
//rewind by 512 bytes
fseek(pointer, -512, SEEK_CUR);
//close file being written to
fclose(pointer);
memset (arr, 0, 512);
return false;
}
else
{
//write values into the currently opened file
fwrite(arr, sizeof(char), 512, pointer);
memset(arr, 0, 512);
return true;
}
}

Answer 1

I changed a couple things in your code. In general try to always keep the file "cursor" moving in one direction (try not to seek backwards too much) because this can ruin time complexity especially for large sd cards. This is the modified code I came up with let me know if it works or might clear anything up for you. I haven't actually run this because I don't have anything to test with; however it should work in theory.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <stdint.h>
struct _imgblock{
// The first 4 bytes have a magic sequence and it is just easier to view them all as 1 integer
int32_t magicTag;
char buffer[508];
}__attribute__((packed));
typedef union{
struct _imgblock imageBlock;
unsigned char dataBuffer[512];
} Buffer;
// make sure this function matches the sequence sepcified in the original code (that the low order byte & 0xf0 is 0xe0 and the other bytes match 0xffd8ff)
bool buffer_starts_with_magic(Buffer* buff){
return (buff->imageBlock.magicTag & 0xf0) == 0xe0 && (buff->imageBlock.magicTag >> 8) & 0xffd8ff;
}
int main(int argc, char *argv[])
{
if (argc != 2)
{
fprintf(stderr,"Usage: ./recover image");
return 1;
}
// opening the memory card
FILE *memorycard = fopen(argv[1], "r");
if (memorycard == NULL)
{
fprintf(stderr, "Could not open %s.\n", argv[1]);
return 2;
}
Buffer custom_buff;
int i = 0;
char filename[8];
FILE* openFile = NULL;
// Start reading 512 byte buffers from the file
while(fread(&custom_buff,1, sizeof(Buffer), memorycard) == sizeof(Buffer))
{
// If the buffer starts with the magic sequence found in the original code and a file is open
if(buffer_starts_with_magic(&custom_buff) && openFile){
// then end the current file and start a new one
fclose(openFile);
openFile = NULL;
}else if(openFile){
// else if the buffer doesnt start with the magic sequence append data and keep reading
goto appendBuffer;
}
//create the new file
sprintf(filename, "%03i.jpg", i++);
openFile = fopen(filename,"w");
appendBuffer: fwrite(custom_buff.dataBuffer,1,512,openFile);
}
if(openFile)
fclose(openFile);
openFile = NULL;
fclose(memorycard);
}