vmsplice - splice user pages to/from a pipe
#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);
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:
Do not block on I/O; see splice(2) for further details.
Currently has no effect for vmsplice(), but may be implemented in the future; see splice(2).
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.
SPLICE_F_NONBLOCK was specified in flags
, and the operation would block.
fd
either not valid, or doesn't refer to a pipe.
nr_segs
is greater than IOV_MAX; or memory not aligned if SPLICE_F_GIFT set.
Out of memory.
The vmsplice() system call first appeared in Linux 2.6.17; library support was added to glibc in version 2.5.
This system call is Linux-specific.
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 performance.
This page is part of release 5.10 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/.