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 5.10 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)