Is it possible to `splice()` from a socket to a buffer with "zero-copy"?

Question

Is it possible to splice() from a socket to a buffer with "zero-copy"? If yes, how?

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The example code I found for splice() has two steps (splice example code):

1. splice from socket to pipe
2. splice from pipe to file descriptor

Is there always going to be an intermediary step of using a pipe (i.e. you can't do a zero-copy operation into a buffer in user space)?

Answer 1

@yeyo has answer this question in the question's comments IMO but did not provide a timely answer. So I will summarize:

You can not splice() from a socket to a buffer with “zero-copy”.

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The reason you can't use splice() is because splice() "requires either the source or destination to be a pipe". In other words when the source is a socket then the destination must be a pipe because of the API's restrictions ('either the source or destination or both must be a pipe').

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That answers the yes or no question but not the "why".

I still had questions about the "why" or specifically:

• "Why does splice() require the source or destination to be a pipe?"
• and
• "Why can't I splice directly from a socket to user memory?"

Linus Torvalds answers those questions directly in the link and his answer seems clear and concise to me (if not with a little studying of what he writes): http://yarchive.net/comp/linux/splice.html