VMSPLICE(2)               (2019-03-06)                VMSPLICE(2)

          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 applica-
               tion 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.


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     VMSPLICE(2)               (2019-03-06)                VMSPLICE(2)

          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 oper-
               ation 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 seg-
          ments 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 mem-
          ory 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)

          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/.

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