PrintWindow works in standalone application and do

问题:

I have a code, which captures a given window by PrintWindow function. Specifically I do capture web-pages in a browser. This code is tested in a standalone Windows application, and it works ok. The same code is incorporated into NPAPI plugin and loaded into Google Chrome. There problems happen. If Chrome has only one tab, the code works OK. If Chrome has 2 tabs, the code returns black box of expected size. I tested the standalone application and the plugin at the same moment, on the same window handle (Chrome_RenderWidgetHostHWND). Application does ALWAYS work, the plugin does ALWAYS fail if multiple tabs are open in the browser.

I'd really appreciate, if someone give a clue how to fix this.

Thanks in advance.

NB. Other methods of capturing windows are inapplicable due to even greater shortcomings for my usecase.

回答1:

I bet you're running into sandboxing issues.

Your app is likely running at the "normal" integrity level, so it has no problem posting the WM_PRINT or WM_PRINTCLIENT message to the Chrome process. (PrintWindow uses these messages under the covers.)

When you're a plugin in Chrome, I believe your code runs as a "low" integrity process (which reduces the chances of things like shatter attacks). Thus it doesn't surprise me that sending the message to a window in another process fails.

Unfortunately, that doesn't explain why it works when Chrome has only one tab. Are you finding the same instance of Chrome_RenderWidgetHostHWND in both the app and the plugin in the case where you have multiple tabs open? In general, there's one of these windows per tab. How are you finding the window?