How to loop select() to poll for data ad infinitum

Question

#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>

int main ()
{
char            name[20];
fd_set          input_set;
struct timeval  timeout;
int             ready_for_reading = 0;
int             read_bytes = 0;

/* Empty the FD Set */
FD_ZERO(&input_set );
/* Listen to the input descriptor */
FD_SET(0, &input_set);

/* Waiting for some seconds */
timeout.tv_sec = 10;    // 10 seconds
timeout.tv_usec = 0;    // 0 milliseconds

/* Invitation for the user to write something */
printf("Enter Username: (in 15 seconds)\n");
printf("Time start now!!!\n");

/* Listening for input stream for any activity */
ready_for_reading = select(1, &input_set, NULL, NULL, &timeout);
/* Here, first parameter is value of the socket descriptor + 1 (STDIN descriptor is 0, so  
 * 0 +1 = 1)  
 * in the set, second is our FD set for reading,
 * third is the FD set in which any write activity needs to updated, which is not required
 * in this case. Fourth is timeout
 */

if (ready_for_reading == -1) {
    /* Some error has occured in input */
    printf("Unable to read your input\n");
    return -1;
} else {
    if (ready_for_reading) {
        read_bytes = read(0, name, 19);
        printf("Read, %d bytes from input : %s \n", read_bytes, name);
    } else {
        printf(" 10 Seconds are over - no data input \n");
    }
}

return 0;

}

How to do the same, but not just once, but in infinite loop which breaks after encountering 'quit' string (for example). Every way I tried - failed. So if no data has been inputed after 10 seconds program just prints "10 secs are over - no data input" and then starts waiting again. Same after input - just begins again and behave the same every time in infinite loop.
Am little desperate already, please - help.
Thanks.

Put everything in a `while(!strcmp(name, "quit"))` loop or something? :-) — Eitan T, May 19 '12 at 13:38
"Here, first parameter is number of FDs in the set" **no**. It must be the highest FD number plus one. Please go recheck the man page. — Mat, May 19 '12 at 13:41
The example at the bottom of http://linux.die.net/man/2/select_tut is exactly what you want. — Adam Liss, May 19 '12 at 13:42
@EitanT try to do this with while(1) and U will understand my question ;) — azrahel, May 19 '12 at 13:57
@user471548 Did you try to enclose the FD_ZERO and FD_SET in the loop as well? — Eitan T, May 19 '12 at 14:00
@Mat yes i know, didnt notice, i copypasted the code from another topic. Fixed. — azrahel, May 19 '12 at 14:01
@EitanT no. and have no idea how to do this. Can You please provide a code snippet ? — azrahel, May 19 '12 at 14:05
@user471548, I've outlined the general idea for you in my answer. See if it works for you... — Eitan T, May 19 '12 at 14:45

Eitan T · Accepted Answer · 2012-05-19T14:43:59.037

I don't really see the problem here. Basically just put everything you want in the loop, and let it run. Did you try this?

int main ()
{
   /* Declarations and stuff */
   /* ... */

   /* The loop */
   int break_condition = 0;
   while (!break_condition)
   {
       /* Selection */
       FD_ZERO(&input_set );   /* Empty the FD Set */
       FD_SET(0, &input_set);  /* Listen to the input descriptor */
       ready_for_reading = select(1, &input_set, NULL, NULL, &timeout);

       /* Selection handling */
       if (ready_for_reading)
       {
          /* Do something clever with the input */
       }
       else
       {
          /* Handle the error */
       }

       /* Test the breaking condition */
       break_condition = some_calculation();
   }
   return 0;
}

Note that you have to have keep resetting the selection inside the loop so that it will respond again in the next iteration.

Thank You all very much. Solved at last ! :))) The problem was that I initialized selection arguments only once, and than use them multiple times, while I should initialize them each time I use select. Really thank You very much. !!!! Greetings — azrahel, May 19 '12 at 14:57

score 0 · Answer 2 · answered May 19 '12 at 14:19

The select() function can be told to block indefinitely by setting timeout to NULL. See select(2) man page:

timeout is an upper bound on the amount of time elapsed before select() returns. If both fields of the timeval stucture are zero, then select() returns immediately. (This is useful for polling.) If timeout is NULL (no timeout), select() can block indefinitely.

So what you want is:

...
ready_for_reading = select(1, &input_set, NULL, NULL, NULL);
...

How to loop select() to poll for data ad infinitum

2 Answers2

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