imdecode() without temporary file

Hi all, It seems from looking at the OpenCV source that the function cv::imdecode() creates a temporary file under some circumstances and decodes that file to create the decoded Mat object. Does anyone know under what conditions a temporary file is created? I see from code that the condition is !decoder->setSource(buf), but don't really know what that means. Is there a way to avoid temporary file creation if all I have is a buffer to decode? Thanks