Question

I wrote this function to given filename(a jpeg file) shall print its size in pixels, w and h. According to tutorial that I’m reading,

//0xFFC0 is the “Start of frame” marker which contains the file size
//The structure of the 0xFFC0 block is quite simple [0xFFC0][ushort
length][uchar precision][ushort x][ushort y]

So, I wrote this struct

#pragma pack(1)
struct imagesize {
  unsigned short len; /* 2-bytes */
  unsigned char c;    /* 1-byte */
  unsigned short x;   /* 2-bytes */
  unsigned short y;   /* 2-bytes */
}; //sizeof(struct imagesize) == 7
#pragma pack()

and then:

#define SOF 0xC0 /* start of frame */

    void jpeg_test(const char *filename)
    {
      FILE *fh;
      unsigned char buf[4];
      unsigned char b;

      fh = fopen(filename, "rb");
      if(fh == NULL) 
        fprintf(stderr, "cannot open '%s' file\n", filename);

      while(!feof(fh)) {
        b = fgetc(fh);

        if(b == SOF) {

          struct imagesize img;
    #if 1
          ungetc(b, fh);
          fread(&img, 1, sizeof(struct imagesize), fh);
    #else
          fread(buf, 1, sizeof(buf), fh);
          int w = (buf[0] << 8) + buf[1];
          int h = (buf[2] << 8) + buf[3];
          img.x = w;
          img.y = h;
    #endif

          printf("%dx%d\n",
             img.x,
             img.y);

          break;
        }
      }

      fclose(fh);
    }

But I’m getting 520x537 instead of 700x537, that’s the real size.

Can someone point and explain where I’m wrong?

Answer 1

A JPEG file consists of a number of sections. Each section starts with 0xff, followed by 1-byte section identifier, followed by number of data bytes in the section (in 2 bytes), followed by the data bytes. The sequence 0xffc0, or any other 0xff-- two-byte sequence, inside the data byte sequence, has no significance and does not mark a start of a section.

As an exception, the very first section does not contain any data or length.

You have to read each section header in turn, parse the length, then skip corresponding number of bytes before starting to read next section. You cannot just search for 0xffc0, let alone just 0xc0, without regard to the section structure.

Source.