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           #include <sys/sendfile.h>
           ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);


           sendfile()  copies  data  between  one  file  descriptor  and  another.
           Because this copying is done within  the  kernel,  sendfile()  is  more
           efficient  than  the  combination  of read(2) and write(2), which would
           require transferring data to and from user space.
           in_fd should be a file descriptor opened for reading and out_fd  should
           be a descriptor opened for writing.
           If  offset  is  not NULL, then it points to a variable holding the file
           offset from which sendfile() will start reading data from in_fd.   When
           sendfile() returns, this variable will be set to the offset of the byte
           following the last byte that was read.  If offset  is  not  NULL,  then
           sendfile()  does not modify the current file offset of in_fd; otherwise
           the current file offset is adjusted to reflect the number of bytes read
           from in_fd.
           If  offset  is  NULL, then data will be read from in_fd starting at the
           current file offset, and the file offset will be updated by the call.
           count is the number of bytes to copy between the file descriptors.
           The  in_fd  argument  must  correspond  to  a   file   which   supports
           mmap(2)-like operations (i.e., it cannot be a socket).
           In  Linux  kernels before 2.6.33, out_fd must refer to a socket.  Since
           Linux 2.6.33 it can be any file.  If it is a regular file,  then  send-
           file() changes the file offset appropriately.


           If  the  transfer was successful, the number of bytes written to out_fd
           is returned.  On error, -1 is returned, and errno is set appropriately.


           EAGAIN Nonblocking I/O has been selected using O_NONBLOCK and the write
                  would block.
           EBADF  The input file was not opened for reading or the output file was
                  not opened for writing.
           EFAULT Bad address.
           EINVAL Descriptor  is not valid or locked, or an mmap(2)-like operation
                  is not available for in_fd.
           EIO    Unspecified error while reading from in_fd.
           need  to  send some header data in front of the file contents, you will
           find it useful to employ the TCP_CORK option, described in  tcp(7),  to
           minimize the number of packets and to tune performance.
           In  Linux  2.4  and earlier, out_fd could also refer to a regular file,
           and sendfile() changed the current offset of that file.
           The original Linux sendfile() system call was not  designed  to  handle
           large file offsets.  Consequently, Linux 2.4 added sendfile64(), with a
           wider type for the offset argument.  The glibc sendfile() wrapper func-
           tion transparently deals with the kernel differences.
           Applications  may  wish  to  fall  back to read(2)/write(2) in the case
           where sendfile() fails with EINVAL or ENOSYS.
           The Linux-specific splice(2) call supports  transferring  data  between
           arbitrary files (e.g., a pair of sockets).


           mmap(2), open(2), socket(2), splice(2)

    Linux 2011-09-14 SENDFILE(2)


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