vmsplice(2)

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Options · 8

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.
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.

NAME

vmsplice - splice user pages to/from a pipe

SYNOPSIS

bash

#define _GNU_SOURCE\n         /* See feature_test_macros(7) */
\n#include <fcntl.h>

bash

ssize_t vmsplice(int \nfd\n, const struct iovec *\niov\n,\n
\n                 size_t \nnr_segs\n, unsigned int \nflags\n);

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 described in iovec(3type).

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.

STANDARDS

Linux.

HISTORY

Linux 2.6.17, glibc 2.5.

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 user space. But this makes the interface nice and symmetric and enables people to build on vmsplice() with room for future improvement in performance.