splice(2) — Linux manual page


splice(2)                  System Calls Manual                 splice(2)

NAME         top

       splice - splice data to/from a pipe

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #define _FILE_OFFSET_BITS 64
       #include <fcntl.h>

       ssize_t splice(int fd_in, off_t *_Nullable off_in,
                      int fd_out, off_t *_Nullable 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:

              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.

              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

              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)).

              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

ERRORS         top

       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.

STANDARDS         top


HISTORY         top

       Linux 2.6.17, glibc 2.5.

       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.

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:

              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.

              "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.

       _FILE_OFFSET_BITS should be defined to be 64 in code that uses
       non-null off_in or off_out or that takes the address of splice,
       if the code is intended to be portable to traditional 32-bit x86
       and ARM platforms where off_t's width defaults to 32 bits.

EXAMPLES         top

       See tee(2).

SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                        splice(2)

Pages that refer to this page: pv(1)copy_file_range(2)io_uring_enter2(2)io_uring_enter(2)pipe(2)sendfile(2)syscalls(2)tee(2)vmsplice(2)io_uring_prep_splice(3)io_uring_prep_tee(3)tracefs_cpu_read_size(3)pipe(7)