1SPLICE(2) Linux Programmer's Manual SPLICE(2)
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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, off64_t *off_in, int fd_out,
13 off64_t *off_out, size_t len, unsigned int flags);
14
16 splice() moves data between two file descriptors without copying be‐
17 tween kernel address space and user address space. It transfers up to
18 len bytes of data from the file descriptor fd_in to the file descriptor
19 fd_out, where one of the file descriptors must refer to a pipe.
20 The following semantics apply for fd_in and off_in:
22 * If fd_in refers to a pipe, then off_in must be NULL.
24 * If fd_in does not refer to a pipe and off_in is NULL, then bytes are
26 read from fd_in starting from the file offset, and the file offset
27 is adjusted appropriately.
28 * If fd_in does not refer to a pipe and off_in is not NULL, then
30 off_in must point to a buffer which specifies the starting offset
31 from which bytes will be read from fd_in; in this case, the file
32 offset of fd_in is not changed.
33 Analogous statements apply for fd_out and off_out.
35 The flags argument is a bit mask that is composed by ORing together
37 zero or more of the following values:
38 SPLICE_F_MOVE
40 Attempt to move pages instead of copying. This is only a hint
41 to the kernel: pages may still be copied if the kernel cannot
42 move the pages from the pipe, or if the pipe buffers don't refer
43 to full pages. The initial implementation of this flag was
44 buggy: therefore starting in Linux 2.6.21 it is a no-op (but is
45 still permitted in a splice() call); in the future, a correct
46 implementation may be restored.
47 SPLICE_F_NONBLOCK
49 Do not block on I/O. This makes the splice pipe operations non‐
50 blocking, but splice() may nevertheless block because the file
51 descriptors that are spliced to/from may block (unless they have
52 the O_NONBLOCK flag set).
53 SPLICE_F_MORE
55 More data will be coming in a subsequent splice. This is a
56 helpful hint when the fd_out refers to a socket (see also the
57 description of MSG_MORE in send(2), and the description of
58 TCP_CORK in tcp(7)).
59 SPLICE_F_GIFT
61 Unused for splice(); see vmsplice(2).
62
64 Upon successful completion, splice() returns the number of bytes
65 spliced to or from the pipe.
66 A return value of 0 means end of input. If fd_in refers to a pipe,
68 then this means that there was no data to transfer, and it would not
69 make sense to block because there are no writers connected to the write
70 end of the pipe.
71 On error, splice() returns -1 and errno is set to indicate the error.
73
75 EAGAIN SPLICE_F_NONBLOCK was specified in flags or one of the file de‐
76 scriptors had been marked as nonblocking (O_NONBLOCK), and the
77 operation would block.
78 EBADF One or both file descriptors are not valid, or do not have
80 proper read-write mode.
81 EINVAL The target filesystem doesn't support splicing.
83 EINVAL The target file is opened in append mode.
85 EINVAL Neither of the file descriptors refers to a pipe.
87 EINVAL An offset was given for nonseekable device (e.g., a pipe).
89 EINVAL fd_in and fd_out refer to the same pipe.
91 ENOMEM Out of memory.
93 ESPIPE Either off_in or off_out was not NULL, but the corresponding
95 file descriptor refers to a pipe.
96
98 The splice() system call first appeared in Linux 2.6.17; library sup‐
99 port was added to glibc in version 2.5.
100
102 This system call is Linux-specific.
103
105 The three system calls splice(), vmsplice(2), and tee(2), provide user-
106 space programs with full control over an arbitrary kernel buffer, im‐
107 plemented within the kernel using the same type of buffer that is used
108 for a pipe. In overview, these system calls perform the following
109 tasks:
110 • splice() moves data from the buffer to an arbitrary file descriptor,
112 or vice versa, or from one buffer to another.
113 • tee(2) "copies" the data from one buffer to another.
115 • vmsplice(2) "copies" data from user space into the buffer.
117 Though we talk of copying, actual copies are generally avoided. The
119 kernel does this by implementing a pipe buffer as a set of reference-
120 counted pointers to pages of kernel memory. The kernel creates
121 "copies" of pages in a buffer by creating new pointers (for the output
122 buffer) referring to the pages, and increasing the reference counts for
123 the pages: only pointers are copied, not the pages of the buffer.
124 In Linux 2.6.30 and earlier, exactly one of fd_in and fd_out was re‐
126 quired to be a pipe. Since Linux 2.6.31, both arguments may refer to
127 pipes.
128
130 See tee(2).
131
133 copy_file_range(2), sendfile(2), tee(2), vmsplice(2), pipe(7)
134
136 This page is part of release 5.13 of the Linux man-pages project. A
137 description of the project, information about reporting bugs, and the
138 latest version of this page, can be found at
139 https://www.kernel.org/doc/man-pages/.
140Linux 2021-03-22 SPLICE(2)