splice

NAME

splice - splice data to/from a pipe

LIBRARY

Standard C library (libc, -lc)

SYNOPSIS

#define _GNU_SOURCE /* See feature_test_macros(7) */
#define _FILE_OFFSET_BITS 64
#include <fcntl.h>

ssize_t splice(int fd_in, off_t *_Nullable off_in,
int fd_out, off_t *_Nullable off_out,
size_t len, unsigned int flags);

DESCRIPTION

splice() moves data between two file descriptors without copying between kernel address space and user address space. It transfers up to len bytes of data from the file descriptor fd_in to the file descriptor fd_out, where one of the file descriptors must refer to a pipe.

The following semantics apply for fd_in and off_in:

Analogous statements apply for fd_out and off_out.

The flags argument is a bit mask that is composed by ORing together zero or more of the following values:
SPLICE_F_MOVE

Attempt to move pages instead of copying. This is only a hint to the kernel: pages may still be copied if the kernel cannot move the pages from the pipe, or if the pipe buffers don’t refer to full pages. The initial implementation of this flag was buggy: therefore starting in Linux 2.6.21 it is a no-op (but is still permitted in a splice() call); in the future, a correct implementation may be restored.

SPLICE_F_NONBLOCK

Do not block on I/O. This makes the splice pipe operations nonblocking, but splice() may nevertheless block because the file descriptors that are spliced to/from may block (unless they have the O_NONBLOCK flag set).

SPLICE_F_MORE

More data will be coming in a subsequent splice. This is a helpful hint when the fd_out refers to a socket (see also the description of MSG_MORE in send(2), and the description of TCP_CORK in tcp(7)).

SPLICE_F_GIFT

Unused for splice(); see vmsplice(2).

RETURN VALUE

Upon successful completion, splice() returns the number of bytes spliced to or from the pipe.

A return value of 0 means end of input. If fd_in refers to a pipe, then this means that there was no data to transfer, and it would not make sense to block because there are no writers connected to the write end of the pipe.

On error, splice() returns -1 and errno is set to indicate the error.

ERRORS

STANDARDS

Linux.

HISTORY

Linux 2.6.17, glibc 2.5.

In Linux 2.6.30 and earlier, exactly one of fd_in and fd_out was required to be a pipe. Since Linux 2.6.31, both arguments may refer to pipes.

NOTES

The three system calls splice(), vmsplice(2), and tee(2), provide user-space programs with full control over an arbitrary kernel buffer, implemented within the kernel using the same type of buffer that is used for a pipe. In overview, these system calls perform the following tasks:
splice()

moves data from the buffer to an arbitrary file descriptor, or vice versa, or from one buffer to another.

vmsplice(2)

"copies" data from user space into the buffer.

Though we talk of copying, actual copies are generally avoided. The kernel does this by implementing a pipe buffer as a set of reference-counted pointers to pages of kernel memory. The kernel creates "copies" of pages in a buffer by creating new pointers (for the output buffer) referring to the pages, and increasing the reference counts for the pages: only pointers are copied, not the pages of the buffer.

_FILE_OFFSET_BITS should be defined to be 64 in code that uses non-null off_in or off_out or that takes the address of splice, if the code is intended to be portable to traditional 32-bit x86 and ARM platforms where off_t’s width defaults to 32 bits.

EXAMPLES

See tee(2).

SEE ALSO

copy_file_range(2), sendfile(2), tee(2), vmsplice(2), pipe(7)