VMSPLICE(2) Linux Programmer's Manual VMSPLICE(2)
NAME
vmsplice - splice user pages to/from a pipe
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
#include <sys/uio.h>
ssize_t vmsplice(int fd, const struct iovec *iov,
unsigned long nr_segs, unsigned int flags);
DESCRIPTION
If fd is opened for writing, the vmsplice() system call maps nr_segs
ranges of user memory described by iov into a pipe. If fd is opened
for reading, the vmsplice() system call fills nr_segs ranges of user
memory described by iov from a pipe. The file descriptor fd must refer
to a pipe.
The pointer iov points to an array of iovec structures as defined in
<sys/uio.h>:
struct iovec {
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes */
};
The flags argument is a bit mask that is composed by ORing together
zero or more of the following values:
SPLICE_F_MOVE
Unused for vmsplice(); see splice(2).
SPLICE_F_NONBLOCK
Do not block on I/O; see splice(2) for further details.
SPLICE_F_MORE
Currently has no effect for vmsplice(), but may be implemented
in the future; see splice(2).
SPLICE_F_GIFT
The user pages are a gift to the kernel. The application may
not modify this memory ever, otherwise the page cache and on-
disk data may differ. Gifting pages to the kernel means that a
subsequent splice(2) SPLICE_F_MOVE can successfully move the
pages; if this flag is not specified, then a subsequent
splice(2) SPLICE_F_MOVE must copy the pages. Data must also be
properly page aligned, both in memory and length.
RETURN VALUE
Upon successful completion, vmsplice() returns the number of bytes
transferred to the pipe. On error, vmsplice() returns -1 and errno is
set to indicate the error.
ERRORS
EAGAIN SPLICE_F_NONBLOCK was specified in flags, and the operation
would block.
EBADF fd either not valid, or doesn't refer to a pipe.
EINVAL nr_segs is greater than IOV_MAX; or memory not aligned if
SPLICE_F_GIFT set.
ENOMEM Out of memory.
VERSIONS
The vmsplice() system call first appeared in Linux 2.6.17; library sup-
port was added to glibc in version 2.5.
CONFORMING TO
This system call is Linux-specific.
NOTES
vmsplice() follows the other vectorized read/write type functions when
it comes to limitations on the number of segments being passed in.
This limit is IOV_MAX as defined in <limits.h>. Currently, this limit
is 1024.
vmsplice() really supports true splicing only from user memory to a
pipe. In the opposite direction, it actually just copies the data to
userspace. But this makes the interface nice and symmetric and enables
people to build on vmsplice() with room for future improvement in per-
formance.
SEE ALSO
splice(2), tee(2), pipe(7)
COLOPHON
This page is part of release 5.07 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2019-03-06 VMSPLICE(2)