SPLICE(2)                                           Linux Programmer's Manual                                          SPLICE(2)



NAME
       splice - splice data to/from a pipe

SYNOPSIS
       #define _GNU_SOURCE
       #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 descrip-
       tors 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.   Analo-
       gous 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 file system 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.

CONFORMING TO
       This system call is Linux-specific.

NOTES
       The  three system calls splice(), 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(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  creat-
       ing  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).

SEE ALSO
       sendfile(2), tee(2), vmsplice(2), feature_test_macros(7)

COLOPHON
       This page is part of release 3.25 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/.



Linux                                                      2009-09-15                                                  SPLICE(2)