Stream in PHP socket communication_ Understanding of select method

About php stream_select Research understanding of methods

function stream_select (array &$read, array &$write, array &$except, $tv_sec, $tv_usec = null) {
    }

stream_select The condition returned by the function is :

1. $read A readable event occurs on any connection in the array
2. $write A writable event occurs on any connection in the array
3. $except Any connection in the array has out of band data (OOB)
4. The current process received a system signal (signal), Such as :pcntl_alarm Generated alarm signal , That is to say, the signal can be interrupted stream_select Call to , Make it return immediately
5. stream_select The waiting time for blocking is longer than （$tv_sec＋$tv_usec） The total time of , If $tv_sec=&null when , Then unlimited blocking up to the top 4 Any of the return conditions has occurred

Return value

• The return value is the number of connections whose status has changed in all the monitored connections , For example, when there is 3 Writeable events occurred on connections , Cause method to return , The return value is 3(integer type )
• If it is interrupted by the system signal, the return value is false And output a warning ( Interrupted by signal ).
• If nothing happens after waiting until the timeout , Then return to ０

About out of band data

• The out of band data length only supports 1 Characters , For emergency data , For example, pause such functions , An emergency character is sent to the opposite end first , Further emergency treatment

stream_select How to determine whether the connection is readable or writable

• every last socket Each connection has a read buffer (readBuffer) And a write buffer (writeBuffer). Each buffer has a low water mark . Receive buffer low watermark （ Used to judge readable ） And send buffer low watermark （ Used to judge whether it is writable ）stream_select The bottom layer traverses all to be monitored socket Connect , And determine the readability and Writeability of each connection in the following way ：
  
  1. Can be read :　 When readBuffer The amount of data in ” Equal to ” When receiving the low water mark value . By default, the low water mark value is received =1; There is another case where a readable event occurs , When the opposite end closes the connection , You can use !is_resource($fd) || feof($fd)==true Judge that the connection has been disconnected
  2. Can write :　 When writeBuffer Free space remaining in ” Equal to ” When sending the low watermark value ,　 Send low watermark value by default =2048