Pset4 recover: Images are invalid or unsupported format

Question

I dont get why all of my jpeg's appear but they are all invalid. When i try to open them none of them opens and gives "invalid or unsupported Image format" error.Can someone help me to spot what am i doing wrong ? Here's my code :

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

int main(void){

    FILE* infile = fopen("card.raw","r");
    if(infile == NULL){
        printf("Could not load the memory card");
        return 1;
    }
    unsigned char memory[512] = {0};

    // array for filename is 8 for "000.jpg"
    char jpgName[7];
    int jpgcounter = 0;
    FILE* outfile = NULL;

    while(fread(memory,sizeof(memory),1,infile) == 1){
        if(memory[0] == 0xff && memory[1] == 0xd8 && memory[2] == 0xff && (memory[3]>=0xe0 || memory[3]<=0xef)){
            if(outfile != NULL){
                fclose(outfile);
            }
            sprintf(jpgName,"%0.3d.jpg",jpgcounter);
            outfile = fopen(jpgName,"a");
            fwrite(memory,sizeof(memory),1,outfile);
            jpgcounter++;
        }
        else{
            if(outfile != NULL){
                fwrite(memory,sizeof(memory),1,outfile);
            }
        }
    }
    fclose(infile);
    fclose(outfile);
}

Answer 1

It's a subtle problem and a common error for this assignment. The first byte of the first block of data is being overwritten before it is being written out to the file. This corrupts the jpeg signature and causes the error that you see.

The cause of the overwrite lies in the file name creation. When the code builds the file name, it stores it in jpgName. The file name is 7 chars long, 3 digits, a dot, and 3 letters. But, that's not the end of it. This string still requires a string terminator, \0, making a length needed of 8.

    char jpgName[7];

So, when the code creates a filename, it writes 8 characters, including the \0. That end of string marker gets written to the first byte of whatever memory follows jpgName, which happens to be memory[0], thus corrupting the signature after it has been detected, but before it is written to the output file.

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