Recover: While loop repeats but does'nt add variable

Question

My code:

//create space save info from card
unsigned char buffer[512];
int jpgCount = 0;
char title[7];
do
{
    if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff)
    {
        sprintf(title, "00%i.jpg", jpgCount);
        FILE* img = fopen(title,"a");
        printf("This is the start of a jpg file! This is JPG number: %i \n",jpgCount);
        fclose(img);
        jpgCount++;
    }
}
while (fread(buffer, 512, 1, file) == 1);

fclose(file);

return 0;

}

The outcome is not what I expected though.The program correctly identifies the jpgs 50 times, printing out"This is the start of a jpg file! This is JPG number: 0 " 50 times! Why doesn't the variable add up? Also, only one file is opened. What am I missing that's messing this while loop up?

Appreciate your help!

Answer 1

Welcome to the wonderful, wacky, wtf world of memory leak! This allocation char title[7]; is causing the problem. This sprintf(title, "00%i.jpg", jpgCount); is stomping on jpgCount's memory. How so? Let's take i = 0. sprintf will create title as 001.jpg. Which is 7 chars. But wait, there's more! Where's it going to put the null terminator (and it will add a null terminator)? It will put the null terminator in memory it doesn't own, which in this case belongs to jpgCount.

Take it for a spin in gdb. Inspect jpgCount after sprintf(title, "00%i.jpg", jpgCount); executes to get a visual.

Once you fix the allocation, you will still have a problem because of the format string "00%i.jpg". The format string "%03i.jpg" would work. format strings are like their own language, I have found this article useful. Or try "printf leading 0" in your favorite search engine.

Only one file is opened because you are using "a"ppend mode. More info here.