Ubuntu Manpage: splice - splice data to/from a pipe

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:

• If fd_in refers to a pipe, then off_in must be NULL.

• If fd_in does not refer to a pipe and off_in is NULL, then bytes are read from fd_in
starting from the file offset, and the file offset is adjusted appropriately.

• If fd_in does not refer to a pipe and off_in is not NULL, then off_in must point to a
buffer which specifies the starting offset from which bytes will be read from fd_in; in
this case, the file offset of fd_in is not changed.

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

       EAGAIN SPLICE_F_NONBLOCK  was  specified  in flags or one of the file descriptors had been
              marked as nonblocking (O_NONBLOCK), and the operation would block.

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

       EINVAL The target filesystem doesn't support splicing.

       EINVAL The target file is opened in append mode.

       EINVAL Neither of the file descriptors refers to a pipe.

       EINVAL An offset was given for nonseekable device (e.g., a pipe).

       EINVAL fd_in and fd_out refer to the same pipe.

       ENOMEM Out of memory.

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

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.

       tee(2) "copies" the data 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).