NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       splice - splice data to/from a pipe

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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 file 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         top

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

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       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         top

       See tee(2).

SEE ALSO         top

       copy_file_range(2), sendfile(2), tee(2), vmsplice(2)

COLOPHON         top

       This page is part of release 4.08 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
       https://www.kernel.org/doc/man-pages/.

Linux                            2016-03-15                        SPLICE(2)