Send data from a file to a socket
#include <io.h>
int64 io_sendfile(int64 sock,int64 fd,uint64 off,uint64 n);
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.