SPLICE(2)                 (2020-06-09)                  SPLICE(2)

          splice - splice data to/from a pipe

          #define _GNU_SOURCE         /* See feature_test_macros(7) */
          #include <fcntl.h>

          ssize_t splice(int fd_in, loff_t *off_in, int fd_out
                         loff_t *off_out, size_t len, unsigned int flags

          splice() moves data between two file descriptors without
          copying between kernel address space and user address space.
          It transfers up to len bytes of data from the file descrip-
          tor fd_in to the file descriptor fd_out, where one of the
          file descriptors must refer to a pipe.

          The following semantics apply for fd_in and off_in:

          *  If fd_in refers to a pipe, then off_in must be NULL.

          *  If fd_in does not refer to a pipe and off_in is NULL,
             then bytes are read from fd_in starting from the file
             offset, and the file offset is adjusted appropriately.

          *  If fd_in does not refer to a pipe and off_in is not NULL,
             then off_in must point to a buffer which specifies the
             starting offset from which bytes will be read from fd_in;
             in this case, the file offset of fd_in is not changed.

          Analogous statements apply for fd_out and off_out.

          The flags argument is a bit mask that is composed by ORing
          together zero or more of the following values:

               Attempt to move pages instead of copying.  This is only
               a hint to the kernel: pages may still be copied if the
               kernel cannot move the pages from the pipe, or if the
               pipe buffers don't refer to full pages.  The initial
               implementation of this flag was buggy: therefore start-
               ing in Linux 2.6.21 it is a no-op (but is still permit-
               ted in a splice() call); in the future, a correct
               implementation may be restored.

               Do not block on I/O.  This makes the splice pipe opera-
               tions nonblocking, but splice() may nevertheless block
               because the file descriptors that are spliced to/from
               may block (unless they have the O_NONBLOCK flag set).

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               More data will be coming in a subsequent splice.  This
               is a helpful hint when the fd_out refers to a socket
               (see also the description of MSG_MORE in send(2), and
               the description of TCP_CORK in tcp(7)).

               Unused for splice(); see vmsplice(2).

          Upon successful completion, splice() returns the number of
          bytes spliced to or from the pipe.

          A return value of 0 means end of input.  If fd_in refers to
          a pipe, then this means that there was no data to transfer,
          and it would not make sense to block because there are no
          writers connected to the write end of the pipe.

          On error, splice() returns -1 and errno is set to indicate
          the error.

               SPLICE_F_NONBLOCK was specified in flags or one of the
               file descriptors had been marked as nonblocking
               (O_NONBLOCK), and the operation would block.

               One or both file descriptors are not valid, or do not
               have proper read-write mode.

               The target filesystem doesn't support splicing.

               The target file is opened in append mode.

               Neither of the file descriptors refers to a pipe.

               An offset was given for nonseekable device (e.g., a

               fd_in and fd_out refer to the same pipe.

               Out of memory.

               Either off_in or off_out was not NULL, but the

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               corresponding file descriptor refers to a pipe.

          The splice() 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.

          The three system calls splice(), vmsplice(2), and tee(2),
          provide user-space programs with full control over an arbi-
          trary kernel buffer, implemented within the kernel using the
          same type of buffer that is used for a pipe.  In overview,
          these system calls perform the following tasks:

          +o splice() moves data from the buffer to an arbitrary file
            descriptor, or vice versa, or from one buffer to another.

          +o tee(2) "copies" the data from one buffer to another.

          +o vmsplice(2) "copies" data from user space into the buffer.

          Though we talk of copying, actual copies are generally
          avoided.  The kernel does this by implementing a pipe buffer
          as a set of reference-counted pointers to pages of kernel
          memory.  The kernel creates "copies" of pages in a buffer by
          creating new pointers (for the output buffer) referring to
          the pages, and increasing the reference counts for the
          pages: only pointers are copied, not the pages of the

          In Linux 2.6.30 and earlier, exactly one of fd_in and fd_out
          was required to be a pipe.  Since Linux 2.6.31, both argu-
          ments may refer to pipes.

          See tee(2).

          copy_file_range(2), sendfile(2), tee(2), vmsplice(2),

          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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