Select function

usage ：int select(int maxfdp,fd_set *readfds,fd_set *writefds,fd_set *errorfds,struct timeval *timeout);

effect ： Used to monitor the file descriptors we need to monitor （ File descriptors in the read or write file set ） Change of state . And can tell us through the returned value .

Parameter interpretation ：

int maxfdp： The range of all file descriptors in the collection , Add... To the maximum of all file descriptors 1.

fd_set *readfds： Read file set to monitor .

fd_set *writefds ： Set of write files to monitor .

fd_set *errorfds： Used to monitor abnormal data .

struct timeval* timeout：select Timeout for , It can make select In three states ：

• First of all , If the NULL Pass in as a formal parameter , That is to say, the time structure is not introduced , Namely take select Put it in a blocked state , Be sure to wait until a file descriptor in the set of monitored file descriptors changes ;
• second , If the time value is set to 0 second 0 millisecond , It becomes a pure non blocking function , Whether or not the file descriptor changes , All immediately return to continue execution , File no change return 0, Change returns a positive value ;
• Third ,timeout The value is greater than 0, This is the waiting timeout , namely select stay timeout During the blocking time , When an event arrives within the timeout period, it returns , Otherwise, after the timeout, it must return anyway .

struct timeval timeout; timeout.tv_sec = 0; // second  timeout.tv_usec = dwTimeout * 1000; // Microsecond  1 millisecond  = 1000 Microsecond 

Return value Introduction ：

>0： The monitored file descriptor has changed , The return bit is still 1 Of fd Total of .
-1： error 
0 ： Overtime 

give an example ：

REF

Linux Operations on file descriptors in （FD_ZERO、FD_SET、FD_CLR、FD_ISSET）_houxiaoni01 The blog of -CSDN Blog _ File descriptor fd

select The function and fd_set Introduce - cs_wu - Blog Garden

Select Analysis of function realization principle _liitlefrogyyh The column -CSDN Blog