code:

VMSPLICE(2)                Linux Programmer's Manual               VMSPLICE(2)



NAME
       vmsplice - splice user pages into a pipe

SYNOPSIS
       #define _GNU_SOURCE
       #include <fcntl.h>
       #include <sys/uio.h>

       long vmsplice(int fd, const struct iovec *iov,
                     unsigned long nr_segs, unsigned int flags);

DESCRIPTION
       The vmsplice() system call maps nr_segs ranges of user memory described
       by iov into 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 appli-
                          cation  may  not  modify  this  memory ever, or 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
       EBADF  fd either not valid, or doesn't refer to a pipe.

       EINVAL nr_segs is 0 or greater than IOV_MAX; or memory not  aligned  if
              SPLICE_F_GIFT set.

       ENOMEM Out of memory.

VERSIONS
       The vmsplice(2) system call first appeared in Linux 2.6.17.

CONFORMING TO
       This system call is Linux specific.

NOTES
       vmsplice()  follows the other vectorized read/write type functions when
       it comes to limitations on number of segments being  passed  in.   This
       limit  is  IOV_MAX as defined in <limits.h>.  At the time of this writ-
       ing, that limit is 1024.

SEE ALSO
       splice(2), tee(2), feature_test_macros(7)



Linux                             2006-04-28                       VMSPLICE(2)

Quote:

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ботай. http://lwn.net/SubscriberLink/268783/6928eb6921800e25/

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code:
static inline size_t iov_length(const struct iovec *iov, unsigned long nr_segs)
{
        unsigned long seg;
        size_t ret = 0;

        for (seg = 0; seg < nr_segs; seg++)
                ret += iov[seg].iov_len;
        return ret;
}