1SPLICE(2)                  Linux Programmer's Manual                 SPLICE(2)
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NAME

6       splice - splice data to/from a pipe
7

SYNOPSIS

9       #define _GNU_SOURCE         /* See feature_test_macros(7) */
10       #include <fcntl.h>
11
12       ssize_t splice(int fd_in, loff_t *off_in, int fd_out,
13                      loff_t *off_out, size_t len, unsigned int flags);
14

DESCRIPTION

16       splice()  moves  data  between  two  file  descriptors  without copying
17       between kernel address space and user address space.  It  transfers  up
18       to  len  bytes  of  data  from  the  file  descriptor fd_in to the file
19       descriptor fd_out, where one of the file descriptors must  refer  to  a
20       pipe.
21
22       The following semantics apply for fd_in and off_in:
23
24       *  If fd_in refers to a pipe, then off_in must be NULL.
25
26       *  If fd_in does not refer to a pipe and off_in is NULL, then bytes are
27          read from fd_in starting from the file offset, and the  file  offset
28          is adjusted appropriately.
29
30       *  If  fd_in  does  not  refer  to  a pipe and off_in is not NULL, then
31          off_in must point to a buffer which specifies  the  starting  offset
32          from  which  bytes  will  be read from fd_in; in this case, the file
33          offset of fd_in is not changed.
34
35       Analogous statements apply for fd_out and off_out.
36
37       The flags argument is a bit mask that is  composed  by  ORing  together
38       zero or more of the following values:
39
40       SPLICE_F_MOVE
41              Attempt  to  move pages instead of copying.  This is only a hint
42              to the kernel: pages may still be copied if  the  kernel  cannot
43              move the pages from the pipe, or if the pipe buffers don't refer
44              to full pages.  The initial  implementation  of  this  flag  was
45              buggy:  therefore starting in Linux 2.6.21 it is a no-op (but is
46              still permitted in a splice() call); in the  future,  a  correct
47              implementation may be restored.
48
49       SPLICE_F_NONBLOCK
50              Do not block on I/O.  This makes the splice pipe operations non‐
51              blocking, but splice() may nevertheless block because  the  file
52              descriptors that are spliced to/from may block (unless they have
53              the O_NONBLOCK flag set).
54
55       SPLICE_F_MORE
56              More data will be coming in a  subsequent  splice.   This  is  a
57              helpful  hint  when  the fd_out refers to a socket (see also the
58              description of MSG_MORE  in  send(2),  and  the  description  of
59              TCP_CORK in tcp(7)).
60
61       SPLICE_F_GIFT
62              Unused for splice(); see vmsplice(2).
63

RETURN VALUE

65       Upon  successful  completion,  splice()  returns  the  number  of bytes
66       spliced to or from the pipe.
67
68       A return value of 0 means end of input.  If fd_in  refers  to  a  pipe,
69       then  this  means  that there was no data to transfer, and it would not
70       make sense to block because there are no writers connected to the write
71       end of the pipe.
72
73       On error, splice() returns -1 and errno is set to indicate the error.
74

ERRORS

76       EAGAIN SPLICE_F_NONBLOCK  was  specified  in  flags  or one of the file
77              descriptors had been marked as nonblocking (O_NONBLOCK), and the
78              operation would block.
79
80       EBADF  One  or  both  file  descriptors  are  not valid, or do not have
81              proper read-write mode.
82
83       EINVAL The target filesystem doesn't support splicing.
84
85       EINVAL The target file is opened in append mode.
86
87       EINVAL Neither of the file descriptors refers to a pipe.
88
89       EINVAL An offset was given for nonseekable device (e.g., a pipe).
90
91       EINVAL fd_in and fd_out refer to the same pipe.
92
93       ENOMEM Out of memory.
94
95       ESPIPE Either off_in or off_out was not  NULL,  but  the  corresponding
96              file descriptor refers to a pipe.
97

VERSIONS

99       The  splice()  system call first appeared in Linux 2.6.17; library sup‐
100       port was added to glibc in version 2.5.
101

CONFORMING TO

103       This system call is Linux-specific.
104

NOTES

106       The three system calls splice(), vmsplice(2), and tee(2), provide user-
107       space  programs  with  full  control  over  an arbitrary kernel buffer,
108       implemented within the kernel using the same type  of  buffer  that  is
109       used for a pipe.  In overview, these system calls perform the following
110       tasks:
111
112       splice()    moves data from the buffer to an arbitrary file descriptor,
113                   or vice versa, or from one buffer to another.
114
115       tee(2)      "copies" the data from one buffer to another.
116
117       vmsplice(2) "copies" data from user space into the buffer.
118
119       Though  we  talk  of copying, actual copies are generally avoided.  The
120       kernel does this by implementing a pipe buffer as a set  of  reference-
121       counted  pointers  to  pages  of  kernel  memory.   The  kernel creates
122       "copies" of pages in a buffer by creating new pointers (for the  output
123       buffer) referring to the pages, and increasing the reference counts for
124       the pages: only pointers are copied, not the pages of the buffer.
125
126       In Linux 2.6.30 and earlier,  exactly  one  of  fd_in  and  fd_out  was
127       required to be a pipe.  Since Linux 2.6.31, both arguments may refer to
128       pipes.
129

EXAMPLE

131       See tee(2).
132

SEE ALSO

134       copy_file_range(2), sendfile(2), tee(2), vmsplice(2), pipe(7)
135

COLOPHON

137       This page is part of release 5.04 of the Linux  man-pages  project.   A
138       description  of  the project, information about reporting bugs, and the
139       latest    version    of    this    page,    can     be     found     at
140       https://www.kernel.org/doc/man-pages/.
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144Linux                             2019-05-09                         SPLICE(2)
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