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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.

       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  sendfile()  changes  the file offset


       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  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 function 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)


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