1SPLICE(2) Linux Programmer's Manual SPLICE(2)
2
6 splice - splice data to/from a pipe
7
9 #define _GNU_SOURCE /* See feature_test_macros(7) */
10 #include <fcntl.h>
11 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
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 descriptors must refer to a pipe.
20 If fd_in refers to a pipe, then off_in must be NULL. If fd_in does not
22 refer to a pipe and off_in is NULL, then bytes are read from fd_in
23 starting from the current file offset, and the current file offset is
24 adjusted appropriately. If fd_in does not refer to a pipe and off_in
25 is not NULL, then off_in must point to a buffer which specifies the
26 starting offset from which bytes will be read from fd_in; in this case,
27 the current file offset of fd_in is not changed. Analogous statements
28 apply for fd_out and off_out.
29 The flags argument is a bit mask that is composed by ORing together
31 zero or more of the following values:
32 SPLICE_F_MOVE Attempt to move pages instead of copying. This is
34 only a hint to the kernel: pages may still be copied
35 if the kernel cannot move the pages from the pipe,
36 or if the pipe buffers don't refer to full pages.
37 The initial implementation of this flag was buggy:
38 therefore starting in Linux 2.6.21 it is a no-op
39 (but is still permitted in a splice() call); in the
40 future, a correct implementation may be restored.
41 SPLICE_F_NONBLOCK Do not block on I/O. This makes the splice pipe
43 operations nonblocking, but splice() may neverthe‐
44 less block because the file descriptors that are
45 spliced to/from may block (unless they have the
46 O_NONBLOCK flag set).
47 SPLICE_F_MORE More data will be coming in a subsequent splice.
49 This is a helpful hint when the fd_out refers to a
50 socket (see also the description of MSG_MORE in
51 send(2), and the description of TCP_CORK in tcp(7))
52 SPLICE_F_GIFT Unused for splice(); see vmsplice(2).
54
56 Upon successful completion, splice() returns the number of bytes
57 spliced to or from the pipe. A return value of 0 means that there was
58 no data to transfer, and it would not make sense to block, because
59 there are no writers connected to the write end of the pipe referred to
60 by fd_in.
61 On error, splice() returns -1 and errno is set to indicate the error.
63
65 EBADF One or both file descriptors are not valid, or do not have
66 proper read-write mode.
67 EINVAL Target file system doesn't support splicing; target file is
69 opened in append mode; neither of the descriptors refers to a
70 pipe; or offset given for nonseekable device.
71 ENOMEM Out of memory.
73 ESPIPE Either off_in or off_out was not NULL, but the corresponding
75 file descriptor refers to a pipe.
76
78 The splice() system call first appeared in Linux 2.6.17; library sup‐
79 port was added to glibc in version 2.5.
80
82 This system call is Linux-specific.
83
85 The three system calls splice(), vmsplice(2), and tee(2), provide user-
86 space programs with full control over an arbitrary kernel buffer,
87 implemented within the kernel using the same type of buffer that is
88 used for a pipe. In overview, these system calls perform the following
89 tasks:
90 splice() moves data from the buffer to an arbitrary file descriptor,
92 or vice versa, or from one buffer to another.
93 tee(2) "copies" the data from one buffer to another.
95 vmsplice(2) "copies" data from user space into the buffer.
97 Though we talk of copying, actual copies are generally avoided. The
99 kernel does this by implementing a pipe buffer as a set of reference-
100 counted pointers to pages of kernel memory. The kernel creates
101 "copies" of pages in a buffer by creating new pointers (for the output
102 buffer) referring to the pages, and increasing the reference counts for
103 the pages: only pointers are copied, not the pages of the buffer.
104
106 See tee(2).
107
109 sendfile(2), tee(2), vmsplice(2)
110
112 This page is part of release 3.53 of the Linux man-pages project. A
113 description of the project, and information about reporting bugs, can
114 be found at http://www.kernel.org/doc/man-pages/.
115Linux 2012-05-04 SPLICE(2)