Using stdin with select() in C

I have the following program:

 #include <stdio.h>
 #define STDIN 0

 int main()
 {

    fd_set fds;
    int maxfd;
    // sd is a UDP socket

    maxfd = (sd > STDIN)?sd:STDIN;

    while(1){

        FD_ZERO(&fds);
        FD_SET(sd, &fds); 
        FD_SET(STDIN, &fds); 

        select(maxfd+1, &fds, NULL, NULL, NULL); 

        if (FD_ISSET(STDIN, &fds)){
              printf("\nUser input - stdin");
        }
        if (FD_ISSET(sd, &fds)){
              // socket code
        }
     }
 }

The problem I face is that once input is detected on STDIN, the message "User input - stdin" keeps on printing...why doesn't it print just once and on next while loop check which of the descriptors has input ?

Thanks.

The select function only tells you when there is input available. If you don't actually consume it, select will continue falling straight through.

Because you are not reading STDIN, so next time around the loop there is still something to read.

You need to read STDIN to prevent this.

来源：https://stackoverflow.com/questions/10219340/using-stdin-with-select-in-c

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