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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>
ssize_t vmsplice(int fd, const struct iovec *iov,
size_t 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 vm‐
splice() 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 er‐
ror.
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 support 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 user space. But this
makes the interface nice and symmetric and enables people to build on vmsplice() with room for future improvement in performance.
SEE ALSO
splice(2), tee(2), pipe(7)
Linux 2021-03-22 VMSPLICE(2)