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       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

              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


       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.


       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.


       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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