write_buffer() - 将缓冲区写入文件或读入数据源到缓冲区
int write_buffer( string dest, int start, mixed source );
int write_buffer( buffer dest, int start, mixed source );
If 'dest' is a file, then 'source' must be an int (and will be written
to the file in network-byte-order), a buffer, or a string, and 'source'
will be written to the file 'dest' starting at byte # 'start'.
If 'dest' is a buffer, then 'source' will be written into the buffer
starting at byte # 'start' in the buffer. If 'source' is an int, it
will be written in network-byte-order.