int main(void) {

  char input_buffer_array[100];

  while(1) {
    user_menu( input_buffer_array);

    update_input ( input_buffer_array); // this function will always be delayed
  }

}

void update_input( char input_buffer_array[100]) {

  int i;
  for( i = 0; i < 100; i++) {
    input_buffer_array[i] = ...
    ...
    ...
  }

}

void user_menu( char input_buffer_array[100]) {

  int choice;
  fflush(stdin);
  scanf("%d," &choice);

  ...
  ...
  printf("\n%s\n", input_buffer_array);
  ...

}

itsme@itsme:~/C$ cat select.c
#include <stdio.h>
#include <sys/select.h>
#include <time.h>

void do_keyboard(void)
{
  char buf[128];

  fgets(buf, sizeof(buf), stdin);
  printf("You typed: %s", buf);
}

void do_time(void)
{
  time_t now = time(NULL);

  printf("Current time is: %s", ctime(&now));
}

int main(void)
{
  struct timeval tv;
  fd_set inpset;

  tv.tv_sec = 5;
  tv.tv_usec = 0;

  while(1)
  {
    FD_ZERO(&inpset);
    FD_SET(fileno(stdin), &inpset);
    select(fileno(stdin)+1, &inpset, NULL, NULL, &tv);

    if(FD_ISSET(fileno(stdin), &inpset))
      do_keyboard();
    else
    {
      do_time();
      tv.tv_sec = 5;
      tv.tv_usec = 0;
    }
  }

  return 0;
}