The Linux "copy problem"

Posted May 30, 2019 13:54 UTC (Thu) by ecree (guest, #95790)
In reply to: The Linux "copy problem" by desbma
Parent article: The Linux "copy problem"

> > one of the file descriptors must refer to a pipe
Yes, so you make two splice calls:

int p[2];
pipe(p);
splice(fd_in, NULL, p[1], NULL, len, flags);
splice(p[0], NULL, fd_out, NULL, len, flags);

I haven't actually tried this, but in theory it should enable the kernel to do a zero-copy copy where the underlying files support that. The pipe is really no more than a way to associate a userspace handle with a kernel buffer; see https://yarchive.net/comp/linux/splice.html for details.

The Linux "copy problem"

Posted May 30, 2019 14:00 UTC (Thu) by desbma (guest, #118820) [Link]

> I haven't actually tried this, but in theory it should enable the kernel to do a zero-copy copy where the underlying files support that.

This is exactly what sendfile does, with a single system call, instead of 3 for your example.