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

问题

#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.

回答1:

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.

回答2:

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);
...

来源：https://stackoverflow.com/questions/10665614/how-to-loop-select-to-poll-for-data-ad-infinitum

标签

select

loops

pool

infinite