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:


Unused for vmsplice(); see splice(2).


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.


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.



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.


splice(2), tee(2), pipe(7)


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