debian man page for io_sendfile

io_sendfile(3)						     Library Functions Manual						    io_sendfile(3)

NAME
       io_sendfile - send data from a file to a socket

SYNTAX
       #include <io.h>

       int64 io_sendfile(int64 sock,int64 fd,uint64 off,uint64 n);

DESCRIPTION
       io_sendfile  sends  data from a file to a socket.  This function tries to do this in a way that allows high quality operating systems to do
       zero-copy TCP (serving the data without copying the file contents to user space or inside kernel space).  This function	will  use  special
       operating  system  "sendfile" primitives where available and use memory mapped I/O otherwise.  It should always be faster than using read()
       and write() to copy the data yourself.

       io_sendfile will return the number of bytes sent.  If an error occurred, it will return -1 for EAGAIN, or -3 otherwise.

SEE ALSO
       io_wait(3), io_waituntil(3)

																    io_sendfile(3)