splice(2)

NAME
splice - splice data to/from a pipe

SYNOPSIS
#define _GNU_SOURCE
#include <fcntl.h>

long splice(int fd_in, off_t *off_in, int fd_out,
            off_t *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 descriptors must refer to a pipe.

If in_fd refers to a pipe, then in_off must be NULL. If in_fd does not refer to a pipe and in_off is NULL, then bytes are read from in_fd starting from the current file offset, and the current file offset is adjusted appropriately. If in_fd does not refer to a pipe and in_off is not NULL, then in_off must point to a buffer which specifies the starting offset from which bytes will be read from in_fd; in this case, the current file offset of in_fd is not changed. Analogous statements apply for out_fd and out_off.

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.

SPLICE_F_NONBLOCK
    Do not block on I/O. This makes the splice pipe operations non-blocking, 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 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 referred to by fd_in.

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

ERRORS

EBADF
    One or both file descriptors are not valid, or do not have proper read-write mode.

EINVAL
    Target file system doesn't support splicing; neither of the descriptors refers to a pipe; or offset given for non-seekable device.

ENOMEM
    Out of memory.

ESPIPE
    Either off_in or off_out was not NULL, but the corresponding file descriptor refers to a pipe.

HISTORY
The splice(2) system call first appeared in Linux-2.6.17.

NOTES
The three system calls splice(2), vmsplice(2), and tee(2)), provide userspace 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.

tee()
    "copies" the data from one buffer to another.

vmsplice()
    "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.

EXAMPLE
See tee(2).

CONFORMING TO
This system call is Linux specific.

SEE ALSO
sendfile(2), splice(2), tee(2)