Ubuntu Manpage: splice - splice data to/from a pipe

NAME

       splice - splice data to/from a pipe

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

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.

STANDARDS

       Linux.

HISTORY

       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

       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.

       _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

       See tee(2).