How to get the width/height of jpeg file without using library?

Question

Firstly I want to say I tried many times to find the answer by using google search, and I found many results but I did not understand, because I don\'t know the idea of read

Answer 1

int  GetJpegDimensions(
    char            *pImage,
    size_t          nSize,
    unsigned32      *u32Width,
    unsigned32      *u32Height,
    char            *szErrMsg)
{
    int             nIndex;
    int             nStartOfFrame;
    int             nError = NO_ERROR;
    bool            markerFound = false;
    unsigned char   ucWord0;
    unsigned char   ucWord1;

    // verify START OF IMAGE marker = FF D8
    nIndex = 0;
    ucWord0 = pImage[nIndex];
    ucWord1 = pImage[nIndex+1];

    // marker FF D8  starts a valid JPEG
    if ((ucWord0 == 0xFF)  && (ucWord1 == 0xD8))
    {
        // search for START OF FRAME 0  marker  FF C0
        for (nIndex = 2;
            (nIndex < nSize-2) && (markerFound == false);
             nIndex += 2)
        {
            ucWord0 = pImage[nIndex];
            ucWord1 = pImage[nIndex+1];
            if (ucWord0 == 0xFF)
            {
                if (ucWord1 == 0xC0)
                {
                    markerFound = true;
                    nStartOfFrame = nIndex;
                }
            }
            if (ucWord1 == 0xFF)
            {
                ucWord0 = pImage[nIndex+2];
                if (ucWord0 == 0xC0)
                {
                    markerFound = true;
                    nStartOfFrame = nIndex+1;
                }
            }
        } // while

        if (markerFound)
        {
            nError  = NO_ERROR;
            ucWord0 = pImage[nStartOfFrame+5];
            ucWord1 = pImage[nStartOfFrame+6];
            *u32Height = ucWord1 + (ucWord0 << 8);

            ucWord0 = pImage[nStartOfFrame+7];
            ucWord1 = pImage[nStartOfFrame+8];
            *u32Width =  ucWord1 + (ucWord0 << 8);
        }
        else
        {
            // start of frame 0 not found
            nError = -2;
            sprintf(szErrMsg,
              "Not a valid JPEG image. START OF FRAME 0 marker FFC0 not found");
        }
    }
    else   // START OF IMAGE marker not found
    {
        nError = -1;
        sprintf(szErrMsg,
          "Not a valid JPEG image. START OF IMAGE marker FFD8 not found");
    }
    return nError;
}