.\" This manpage is Copyright (C) 2006 Jens Axboe .\" and Copyright (C) 2006 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .TH VMSPLICE 2 2017-09-15 "Linux" "Linux Programmer's Manual" .SH NAME vmsplice \- splice user pages into a pipe .SH SYNOPSIS .nf .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .B #include .B #include .PP .BI "ssize_t vmsplice(int " fd ", const struct iovec *" iov , .BI " unsigned long " 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. The .BR vmsplice () system call maps .I nr_segs ranges of user memory described by .I iov into 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 : .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 . Currently, .\" UIO_MAXIOV in kernel source this limit is 1024. .SH SEE ALSO .BR splice (2), .BR tee (2), .BR pipe (7) .SH COLOPHON This page is part of release 4.16 of the Linux .I 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/.