NAME

       vmsplice - splice user pages to/from a pipe

SYNOPSIS

       #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);

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 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:

       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.

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 userspace.  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)

COLOPHON

       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/.

Linux                                              2019-03-06                                        VMSPLICE(2)