vmsplice

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



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 5.08 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)