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       sendfile - transfer data between file descriptors


       #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.

       offset  is  a  pointer 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.  sendfile() does not  modify  the  current
       file offset of in_fd.

       count is the number of bytes to copy between the file descriptors.

       Presently  (Linux  2.6.9):  in_fd,  must  correspond  to  a  file which
       supports mmap()-like operations (i.e., it  cannot  be  a  socket);  and
       out_fd must refer to a socket.

       Applications  may  wish  to  fall  back to read(2)/write(2) in the case
       where sendfile() fails with EINVAL or ENOSYS.


       If you plan to use sendfile() for sending files to a  TCP  socket,  but
       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 refer to a regular file, and
       sendfile() changed the current offset of that file.


       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 Non-blocking 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()-like  operation
              is not available for in_fd.

       EIO    Unspecified error while reading from in_fd.

       ENOMEM Insufficient memory to read from in_fd.


       sendfile()   is   a  new  feature  in  Linux  2.2.   The  include  file
       <sys/sendfile.h> is present since glibc2.1.

       Other  Unixes  implement  sendfile()  with  different   semantics   and
       prototypes. It should not be used in portable programs.


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