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

Provided by: manpages-dev_3.54-1ubuntu1_all

NAME

       splice - splice data to/from a pipe

SYNOPSIS

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

       ssize_t splice(int fd_in, loff_t *off_in, int fd_out,
                      loff_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 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 current file offset,
and the current 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 current 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 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  filesystem  doesn't support splicing; target file is opened in append mode;
              neither of the descriptors refers to  a  pipe;  or  offset  given  for  nonseekable
              device.

       ENOMEM Out of memory.

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

VERSIONS

       The splice() system call first appeared in Linux 2.6.17;  library  support  was  added  to
       glibc in version 2.5.

CONFORMING TO

       This system call is Linux-specific.

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.

EXAMPLE

       See tee(2).

COLOPHON

       This  page  is  part of release 3.54 of the Linux man-pages project.  A description of the
       project,    and    information    about    reporting    bugs,    can    be    found     at
       http://www.kernel.org/doc/man-pages/.