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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.

       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.

       Presently (Linux  2.6.9):  in_fd,  must  correspond  to  a  file  which
       supports  mmap(2)-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  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.

       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 glibc 2.1.


       Not specified in POSIX.1-2001, or other standards.

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


       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.


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


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