Splice (system call)

splice() is a Linux-specific system call that moves data between a file descriptor and a pipe without a round trip to user space. The related system call vmsplice() moves or copies data between a pipe and user space. Ideally, splice and vmsplice work by remapping pages and do not actually copy any data, which may improve I/O performance. As linear addresses do not necessarily correspond to contiguous physical addresses, this may not be possible in all cases and on all hardware combinations.

Workings

With splice(), one can move data from one file descriptor to another without incurring any copies from user space into kernel space, which is usually required to enforce system security and also to keep a simple interface for processes to read and write to files. splice() works by using the pipe buffer. A pipe buffer is an in-kernel memory buffer that is opaque to the userland process. A user process can splice the contents of a source file into this pipe buffer, then splice the pipe buffer into the destination file, all without moving any data through userland.

Origins

Linus Torvalds described splice() in a 2006 email, which was included in a KernelTrap article.

The Linux splice implementation borrows some ideas from an original proposal by Larry McVoy in 1998.