man2/vmsplice.2

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.TH VMSPLICE 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
vmsplice \- splice user pages to/from a pipe
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <fcntl.h>
.B #include <sys/uio.h>
.PP
.BI "ssize_t vmsplice(int " fd ", const struct iovec *" iov ,
.BI "                 size_t " nr_segs ", unsigned int " flags );
.fi
.\" Return type was long before glibc 2.7
.SH DESCRIPTION
.\" Linus: vmsplice() system call to basically do a "write to
.\" the buffer", but using the reference counting and VM traversal
.\" to actually fill the buffer. This means that the user needs to
.\" be careful not to reuse the user-space buffer it spliced into
.\" the kernel-space one (contrast this to "write()", which copies
.\" the actual data, and you can thus reuse the buffer immediately
.\" after a successful write), but that is often easy to do.
If
.I fd
is opened for writing, the
.BR vmsplice ()
system call maps
.I nr_segs
ranges of user memory described by
.I iov
into a pipe.
If
.I fd
is opened for reading,
.\" Since Linux 2.6.23
.\" commit 6a14b90bb6bc7cd83e2a444bf457a2ea645cbfe7
the
.BR vmsplice ()
system call fills
.I nr_segs
ranges of user memory described by
.I iov
from a pipe.
The file descriptor
.I fd
must refer to a pipe.
.PP
The pointer
.I iov
points to an array of
.I iovec
structures as defined in
.IR <sys/uio.h> :
.PP
.in +4n
.EX
struct iovec {
    void  *iov_base;        /* Starting address */
    size_t iov_len;         /* Number of bytes */
};
.EE
.in
.PP
The
.I flags
argument is a bit mask that is composed by ORing together
zero or more of the following values:
.TP
.B SPLICE_F_MOVE
Unused for
.BR vmsplice ();
see
.BR splice (2).
.TP
.B SPLICE_F_NONBLOCK
.\" Not used for vmsplice
.\" May be in the future -- therefore EAGAIN
Do not block on I/O; see
.BR splice (2)
for further details.
.TP
.B SPLICE_F_MORE
Currently has no effect for
.BR vmsplice (),
but may be implemented in the future; see
.BR splice (2).
.TP
.B SPLICE_F_GIFT
The user pages are a gift to the kernel.
The application may not modify this memory ever,
.\" FIXME . Explain the following line in a little more detail:
otherwise the page cache and on-disk data may differ.
Gifting pages to the kernel means that a subsequent
.BR splice (2)
.B SPLICE_F_MOVE
can successfully move the pages;
if this flag is not specified, then a subsequent
.BR splice (2)
.B SPLICE_F_MOVE
must copy the pages.
Data must also be properly page aligned, both in memory and length.
.\" FIXME
.\" It looks like the page-alignment requirement went away with
.\" commit bd1a68b59c8e3bce45fb76632c64e1e063c3962d
.\"
.\" .... if we expect to later SPLICE_F_MOVE to the cache.
.SH RETURN VALUE
Upon successful completion,
.BR vmsplice ()
returns the number of bytes transferred to the pipe.
On error,
.BR vmsplice ()
returns \-1 and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EAGAIN
.B SPLICE_F_NONBLOCK
was specified in
.IR flags ,
and the operation would block.
.TP
.B EBADF
.I fd
either not valid, or doesn't refer to a pipe.
.TP
.B EINVAL
.I nr_segs
is greater than
.BR IOV_MAX ;
or memory not aligned if
.B SPLICE_F_GIFT
set.
.TP
.B ENOMEM
Out of memory.
.SH VERSIONS
The
.BR vmsplice ()
system call first appeared in Linux 2.6.17;
library support was added to glibc in version 2.5.
.SH CONFORMING TO
This system call is Linux-specific.
.SH NOTES
.BR 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
.B IOV_MAX
as defined in
.IR <limits.h> .
Currently,
.\" UIO_MAXIOV in kernel source
this limit is 1024.
.PP
.\" commit 6a14b90bb6bc7cd83e2a444bf457a2ea645cbfe7
.BR vmsplice ()
really supports true splicing only from user memory to a pipe.
In the opposite direction, it actually just copies the data to user space.
But this makes the interface nice and symmetric and enables people to build on
.BR vmsplice ()
with room for future improvement in performance.
.SH SEE ALSO
.BR splice (2),
.BR tee (2),
.BR pipe (7)