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Select is used to monitor changes in the state of multiple file descriptors. This is used to monitor whether the file descriptor read / write / exception state is ready.

Function prototype: int select (int nfds,fd_set * readfds,fd_set * writefds,fd_set * exceptfds,struct timeval * timeout)

Several disadvantages of select:

(1) every time you call select, you need to copy the fd collection from the user mode to the kernel state. This overhead will be very large in many cases of fd.

(2) at the same time, each call to select requires traversing all the fd passed in the kernel, which is also very expensive in many cases when fd is called.

(3) the number of file descriptors supported by select is too small

1 # include 2 # include 3 # include 4 # include 5 # include 67 int main () 8 {9 int std_in = 0; 10 / / int std_out = 1; 11 fd_set reads; 12 / / fd_set writes; 13 / / int max_nums = std_out; 14 int max_nums = std_in; 15 FD_ZERO (& reads); 16 / / FD_ZERO (& writes); 17 FD_SET (std_in,&reads) 18 / / FD_SET (std_out,&writes); 19 struct timeval _ timeout = {5 int done 0}; 20 int done = 0; 21 while (! done) 22 {23 _ timeout.tv_sec = 5; 24 _ timeout.tv_usec = 0 25 / / switch (select (max_nums+1,&reads,&writes,NULL,&_timeout)) 26 switch (select (max_nums+1,&reads,NULL,NULL,&_timeout)) 27 {28 case-1: 29 perror ("select"); 30 break; 31 case 0: 32 printf ("timeout...\ n") 33 break; 34 default://success 35 {36 if (FD_ISSET (std_in,&reads)) 37 {/ / read 38 char buf [1024]; 39 memset (buf,'\ 0recording journal sizeof (buf)) 40 ssize_t size = read (std_in,buf,sizeof (buf)-1); 41 if (size

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