Using stdin with select() in C

Question

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.

Solution 1

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

Solution 2

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.

Comments

[Jake]

If I have a running program, and I type in any text on console and press Enter, won't that be read by STDIN?

[Ed Heal]

@Jake - No. The console is just sending the input to thee program and you program is not reading it. The input is in a buffer until it is read by your code.