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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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splice(2) System Calls Manual splice(2)
splice - splice data to/from a pipe
Standard C library (libc, -lc)
#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 size, unsigned int flags);
splice() moves data between two file descriptors without copying
between kernel address space and user address space. It transfers
up to size 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, and the offset pointed
to by off_in is adjusted appropriately instead.
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).
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.
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.
Linux.
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.
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.
See tee(2) for another example.
#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#include <err.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int
main(void)
{
int fd;
int pfd[2];
off_t off;
const char s[12] = "Hello, world";
fd = open("out", O_WRONLY | O_CREAT | O_EXCL, 0666);
if (fd == -1)
err(EXIT_FAILURE, "open");
if (pipe(pfd) == -1)
err(EXIT_FAILURE, "pipe");
if (write(pfd[1], s, sizeof(s)) != sizeof(s))
err(EXIT_FAILURE, "write");
if (close(pfd[1]) == -1)
err(EXIT_FAILURE, "close");
off = 10;
if (splice(pfd[0], NULL, fd, &off, sizeof(s), 0) != sizeof(s))
err(EXIT_FAILURE, "splice");
if (close(pfd[0]) == -1)
err(EXIT_FAILURE, "close");
printf("New offset is %jd\n", (intmax_t) off);
if (close(fd) == -1)
err(EXIT_FAILURE, "close");
exit(EXIT_SUCCESS);
}
copy_file_range(2), sendfile(2), tee(2), vmsplice(2), pipe(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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