splice

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 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.

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.

       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.

EXAMPLES
       See tee(2).

SEE ALSO
       copy_file_range(2), sendfile(2), tee(2), vmsplice(2), pipe(7)

COLOPHON
       This page is part of release 5.08 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                             2020-06-09                         SPLICE(2)