pset4 recover segfault: no such file/directory

Question

My code seems to have trouble making the .jpg files in which to place the blocks of data.

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

uint8_t buffer[512];
uint8_t jpeg1[4] = {0xff, 0xd8, 0xff, 0xe0};
uint8_t jpeg2[4] = {0xff, 0xd8, 0xff, 0xe1};

int main(int argc, char* argv[])
{
    //open memory card file - fread the raw data

    int picNum = 0;
    char title[8];
    FILE* img;
    FILE* inptr = fopen("card.raw", "r");
    if(inptr == NULL)
    {   
        fclose(inptr);
        fprintf(stderr, "Wasn't able to open the cardfile.\n");
        return 1;
    }

    do
    {
        fread(&buffer, 1, 512, inptr);    
    }
    while((buffer[0] != jpeg1[0]) && (buffer[1] != jpeg1[1]) && (buffer[2] != jpeg1[2]) && ((buffer[3] != jpeg1[3]) || (buffer[3] != jpeg2[3])) && (fread(buffer, 1, 512, inptr) == 1));
    while(fread(&buffer, 512, 1, inptr) == 1)
    {

        if((buffer[0] == jpeg1[0]) && (buffer[1] == jpeg1[1]) && (buffer[2] == jpeg1[2]) && ((buffer[3] == jpeg1[3]) || (buffer[3] == jpeg2[3])))
        {
            if(img != NULL)
            {
                fclose(img);
                picNum++;
            }
            sprintf(title, "%3d.jpg", picNum);
            img = fopen(title, "a"); 

        }
        fwrite(&buffer, 1, 512, img);
        fread(&buffer, 1, 512, inptr);
    }
    fclose(img);
    return 0;
}

Get this from GDB:

41          if((buffer[0] == jpeg1[0]) && (buffer[1] == jpeg1[1]) && (buffer[2] == jpeg1[2]) && ((buffer[3] == jpeg1[3]) || (buffer[3] == jpeg2[3])))
(gdb) 
52          fwrite(&buffer, 1, 512, img);
(gdb) 

Program received signal SIGSEGV, Segmentation fault.
__GI__IO_fwrite (buf=0x804a044 <buffer>, size=1, count=512, fp=0x1)
    at iofwrite.c:41
41  iofwrite.c: No such file or directory.

Why isn't the jpg being made? Help on fixing this segfault please!

Answer 1

For your do while loop, the very very last block read is the start block of the jpg. However, in the following while loop, you immediately call fread again and thus buffer now points to the 2nd block of the jpg. You then check if this block has the jpg signature, which it doesn't. Thus, the if loop isn't executed and img isn't opened. This creates the error when your try to write to img