Understanding sendfile() and splice()
linux, sendfile, splice
Solution
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 `splice` also
Repeat the last steps until all the data is read.
Zero-Copy in Linux with sendfile() and splice() has an implementation of this technique.
Problem
`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?