select works only once

Question

   /* Wait up to 1 min */
   tv.tv_sec = 60;
   tv.tv_usec = 0;

   FD_ZERO(&readfd);
   FD_ZERO(&writefd);
   for(i=0;i<3;i++)
   {
     FD_SET(my_rdfd[i], &readfd);
     FD_SET(my_wrfd[i], &writefd);
   }

   for(int i=0;i<10;i++)
   {
     retval = select((NRFDS)+1, &readfd, &writefd, (fd_set*)0, &tv);//NRFDS is a macro

     if (retval == -1)
       printf("select() error");
     else if (retval)
     {
       printf("data found\n");
       //do something with the data recieved
     }
     else
       printf("Timeout.\n");
   }

The thing is that for the first time select() inside the for loop works as expected, fine, for subsequent loops, it doesn't work. What am I doing wrong?

Thank you for your help.

Answer 1

Try putting this within the for loop:

for(i=0;i<10;i++)
{
  tv.tv_sec = 60;
   tv.tv_usec = 0;

   FD_ZERO(&readfd);
   FD_ZERO(&writefd);
   for(j=0;j<3;j++)
   {
     FD_SET(my_rdfd[j], &readfd);
     FD_SET(my_wrfd[j], &writefd);//Note that you should use a different inside forloop
   }

   //select statement here

}

When select() returns, it changes the sets to show which file descriptors have become ready for read/write/exception. All other flags would have been cleared.

It's important that you re-enable the file descriptors that were cleared prior to starting another select, otherwise, you will no longer be waiting on those file descriptors.