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

       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

       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)

       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the
       project,  and  information  about  reporting  bugs,  can  be  found   at   http://www.ker-

Linux					    2010-02-15				      SENDFILE(2)
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