sendfile() can be used to transmit data from a "file" descriptor to a "socket" descriptor in order to get data from machine A to machine B. Is it possible to get the data at the receiving end from the "socket" descriptor to a file with similar zero-copy semantics? I think sendfile() doesn't help here because sendfile() needs the source of data to be "page/buffer" cache. Is my understanding correct? Can splice() help in this situation?
You're correct about the limitation of sendfile for this. And yes, splice can help, but it's not trivial: splice requires that at least one of the source or target file descriptors be a pipe. So you can't directly splice from a socket to a plain file descriptor.
Conceptually, what you can do to make it work is:
- setup your inbound socket fd and your output file fd as you would normally
- create a pipe with
pipe(2) - in a loop:
- read from the socket to the write side of the pipe with
splice - write from the read side of the pipe to the file with
splicealso
- read from the socket to the write side of the pipe with
Repeat the last steps until all the data is read.
Zero-Copy in Linux with sendfile() and splice() has an implementation of this technique.
来源：https://stackoverflow.com/questions/8626263/understanding-sendfile-and-splice