Determining the size of a JPEG (JFIF) image

Maybe something like this

int GetJpgSize(unsigned char *pData, DWORD FileSizeLow, unsigned short *pWidth, unsigned short *pHeight)
{
  unsigned int i = 0;


  if ((pData[i] == 0xFF) && (pData[i + 1] == 0xD8) && (pData[i + 2] == 0xFF) && (pData[i + 3] == 0xE0)) {
    i += 4;

    // Check for valid JPEG header (null terminated JFIF)
    if ((pData[i + 2] == 'J') && (pData[i + 3] == 'F') && (pData[i + 4] == 'I') && (pData[i + 5] == 'F')
        && (pData[i + 6] == 0x00)) {

      //Retrieve the block length of the first block since the first block will not contain the size of file
      unsigned short block_length = pData[i] * 256 + pData[i + 1];

      while (i < FileSizeLow) {
        //Increase the file index to get to the next block
        i += block_length; 

        if (i >= FileSizeLow) {
          //Check to protect against segmentation faults
          return -1;
        }

        if (pData[i] != 0xFF) {
          return -2;
        } 

        if (pData[i + 1] == 0xC0) {
          //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]
          *pHeight = pData[i + 5] * 256 + pData[i + 6];
          *pWidth = pData[i + 7] * 256 + pData[i + 8];

          return 0;
        }
        else {
          i += 2; //Skip the block marker

          //Go to the next block
          block_length = pData[i] * 256 + pData[i + 1];
        }
      }

      //If this point is reached then no size was found
      return -3;
    }
    else {
      return -4;
    } //Not a valid JFIF string
  }
  else {
    return -5;
  } //Not a valid SOI header

  return -6;
}  // GetJpgSize

The compressed data will not include SOI or EOI bytes, so you are safe there. But the comment, application data, or other headers might. Fortunately, you can identify and skip these sections as the length is given.

The JPEG specification tells you what you need:
http://www.w3.org/Graphics/JPEG/itu-t81.pdf

Look at Table B.1, on page 32. The symbols that have an * do not have a length field following it (RST, SOI, EOI, TEM). The others do.

You will need to skip over the various fields, but it is not too bad.

How to go through:

1. Start reading SOI (FFD8). This is the start. It should be the first thing in the stream.

   • Then, progress through the file, finding more markers and skipping over the headers:

   • SOI marker (FFD8): Corrupted image. You should have found an EOI already!

   • TEM (FF01): standalone marker, keep going.

   • RST (FFD0 through FFD7): standalone marker, keep going. You could validate that the restart markers count up from FFD0 through FFD7 and repeat, but that is not necessary for measuring the length.

   • EOI marker (FFD9): You're done!

   • Any marker that is not RST, SOI, EOI, TEM (FF01 through FFFE, minus the exceptions above): After the marker, read the next 2 bytes, this is the 16-bit big-endian length of that frame header (not including the 2-byte marker, but including the length field). Skip the given amount (typically length minus 2, since you already got those bytes).

   • If you get an end-of-file before EOI, then you've got a corrupted image.

   • Once you've got an EOI, you've gotten through the JPEG and should have the length. You can start again by reading another SOI if you expect more than one JPEG in your stream.