About

splice(2) moves data between two file descriptors without copying between kernel address space and user address space. It transfers up to nbytes bytes of data from the file descriptor in to the file descriptor out.

zero-copy

Normally when you copy data from one data stream to another, the data to be copied is first stored in a buffer in userspace and is then copied back to the target data stream from the user space which introduces a certain overhead.

zero-copy allows us to operate on data without the use of copying data to userspace. It essentialy transfers the data by remapping pages and not actually performing the copying of data, resulting in improved performance.

Illustrated below is a simple example of copying data from one file to another using the splice(2) system call. For the complete documentation see API Documentation.

# copy data from one file to another using splice

from splice import splice

to_read = open("read.txt")
to_write = open("write.txt", "w+")

splice(to_read.fileno(), to_write.fileno())

This copying of the data twice (once into the userland buffer, and once out from that userland buffer) imposes some performance and resource penalties. splice(2) syscall avoids these penalties by avoiding any use of userland buffers; it also results in a single system call (and thus only one context switch), rather than the series of read(2) / write(2) system calls (each system call requiring a context switch) used internally for the data copying.

API Documentation

sendfile module provides a single function: sendfile().

• splice.splice(out, in, offset, nbytes, flags)

  Copy nbytes bytes from file descriptor in (a regular file) to file descriptor out (a regular file) starting at offset. Return the number of bytes just being sent. When the end of file is reached return 0. If offset is not specified, the bytes are read from the current position of in and the position of in is updated. If nbytes is not specified, the whole of in is copied over to out.

  Required arguments

  • in: file descriptor of the file from which data is to be read.

  • out: file descriptor of the file to which data is to be transferred.

  Positional optional arguments

  • offset: offset from where the input file is read from.

  • nbytes: number of bytes to be copied in total, default value

  • flags: a bit mask which can be composed by ORing together the following.

    • splice.SPLICE_F_MOVE

    • splice.SPLICE_F_NONBLOCK

    • splice.SPLICE_F_MORE

    • splice.SPLICE_F_GIFT

  More information on what each of the flag means can be found on the splice(2) man page here.

Usage

>>> from splice import splice

# init file objects
>>> to_read = open("read.txt") # file to read from
>>> to_write = open("write.txt", "w+") # file to write to

>>> len(to_read.read())
50

# copying whole file
>>> splice(to_read.fileno(), to_write.fileno())
50  # bytes copied

# copying file starting from an offset
>>> splice(to_read.fileno(), to_write.fileno(), offset=10)
40

# copying certain amount of bytes
>>> splice(to_read.fileno(), to_write.fileno(), nbytes=20)
20

# copying certain amount of bytes beginning from an offset
>>> splice(to_read.fileno(), to_write.fileno(), offset=10, nbytes=20)
20

# specifying flags
>>> import splice
>>> splice(to_read.fileno(), to_write.fileno(), flags=splice.SPLICE_F_MORE)
50

Why would I use this?

splice(2) is supposed to be better in terms of performance when compared to traditional read/write methods since it avoids overhead of copying the data to user address space and instead, does the transfer by remapping pages in kernel address space.

Supported platforms

The splice(2) system call is (GNU)Linux-specific.

Support

Feel free to add improvements, report issues or contact me about anything related to the project.

LICENSE

GNU GPL