[cairo] Need help in understanding some cairo code

[John Allerdyce]

I come across the following cairo code, I appreciate if someone can help me
understand them:

1. Why pass tmpBuf.get()  to cairo_image_surface_create_for_data()?
2. What is the purpose of cairo_set_source_surface() and later with
cairo_paint_with_alpha() with the parameter mCairo?
3. What does it save to the file? the data in tmpBuf? or mCairo?

   // Create a temporary surface to hold the full-size image in cairo
    // image format.
    nsAutoArrayPtr<PRUint8> tmpBuf(new PRUint8[
aSurfaceSize.width*aSurfaceSize.height*4]);

 cairo_surface_t *tmpSurf =
        cairo_image_surface_create_for_data(tmpBuf.get(),
                                            CAIRO_FORMAT_ARGB32,
aSurfaceSize.width, aSurfaceSize.height,
                                            aSurfaceSize.width*4);

    cairo_set_source_surface(mCairo, tmpSurf, 0, 0);
    cairo_paint_with_alpha(mCairo, mGlobalAlpha);

    cairo_surface_write_to_png(tmpSurf, "my.png");

Thank you.
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