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



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 descrip‐
       tor  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 nonblock‐
                          ing, 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
       EAGAIN SPLICE_F_NONBLOCK was specified in flags, and the operation would block.

       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 fol‐
       lowing 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  mem‐
       ory.   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).

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

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.



Linux                                       2014-07-08                                  SPLICE(2)