Thread: flushing stdin

In the faq, there is a section saying that it is unsafe to fflush(stdin) (it doesn't work correctly anyway). I'm having a problem with a function and an infinate loop with switch statement where the stream has not been cleared and outputs unwanted printf data instead of pausing to read scanf. Here's a snippet:
This is the call to the function in question from main:

Code:

 case '4': getroom3: {
                                                  printf("Check occupant into what room? 1 - 10: ");
                                                  scanf("%d", &roomnum);
                                                  if (roomnum < 1 || roomnum > 10) {
                                                          printf("1 - 10 only!");
                                                          goto getroom3;
                                                  }
                                          }
                                          if ((addguest(roomnum)) < 0)
                                                  printf("The room is occupied\n");
                                          else
                                                  printf("The guest was added\n");
                                          break;

This is just the call in a switch statement.
Here is the definition to addguest:

Code:

#include "hotel.h"
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>

int fd;
char buf[NLINES];

void fillbuf();

int
addguest(int roomnum)
{
        off_t offset;
        ssize_t nread;
        int i;

        offset = (roomnum - 1) * NLINES;

        if (lseek(fd, offset, SEEK_SET) < 0)
                perror("problem with lseek in addguest: line 24\n");

        if ( (nread = read(fd, buf, sizeof(buf))) < 0)
                perror("problem with read in addguest: line 27\n");

        for (i = 0; i < nread; ++i) {
                if (isalpha(buf[i]))
                        return -1;
        }

        fillbuf();

        if (lseek(fd, offset, SEEK_SET) < 0)
                perror("problem with lseek in addguest: line 24\n");
        if (write(fd, buf, sizeof(buf)) < 0)
                perror("problem with write in addguest function: line 36\n");

    return 1;
}

void
fillbuf()
{
    char *lname, *fname;
    int i;

    lname = malloc(22);
    fname = malloc(18);

    memset(lname, ' ', sizeof(lname));
    memset(fname, ' ', sizeof(fname));
    memset(buf, ' ', sizeof(buf));

    lastname: {
        fflush(stdin);           /* [red]had this as fflus(NULL) but didn't work[/red] */
        printf("Enter last name. Do not exceed 19 chars: ");
        fgets(lname, (sizeof(lname) - 3), stdin);
    }

    /* check for newline */

    for (i = 0; i < sizeof(lname); ++i) {
        if (isalpha(lname[i]))
            continue;
        else if (lname[i] == '\0')
            break;
        else if (lname[i] == '\n') {
            lname[i] = '\0';
            break;
        } else {
            printf("There was an error in your input, try again:\n");
            goto lastname;
        }
    }
   memcpy(buf, lname, strlen(lname));

    /* tac on ", " to lname */

    buf[strlen(buf)] = ',';
    buf[strlen(buf) + 1] = ' ';
    buf[strlen(buf) + 2] = '\0';


    firstname: {
        fflush(stdin);           /*[red]again tried fflush(NULL) per man page on fflush[/red]*/
        printf("Enter first name. Do not exceed 17 chars: ");
        fgets(fname, (sizeof(fname) - 1), stdin);
    }

    /* check for newline again */

    for (i = 0; i < sizeof(fname); ++i) {
        if (isalpha(fname[i]))
            continue;
        else if (lname[i] == '\0')
            break;
        else if (lname[i] == '\n') {
            lname[i] = '\0';
            break;
        } else {
            printf("There was an error in your input, try again:\n");
    /*      goto firstname; */
        }
    }

    strncat(buf, fname, strlen(fname));

    return;
}

Here's a trial run:
1: Print a list of the rooms:
2: Retrieve an occupier:
3: Free a room
4: Add a guest:
5: Find a free room
Enter a number 1 - 5: 4
Check occupant into what room? 1 - 10: 1
Enter last name. Do not exceed 19 chars: There was an error in your input, try again:
There was an error in your input, try again:
There was an error in your input, try again:
There was an error in your input, try again:
Enter first name. Do not exceed 17 chars: There was an error in your input, try again:
There was an error in your input, try again:
There was an error in your input, try again:
There was an error in your input, try again:
The guest was added
1: Print a list of the rooms:
2: Retrieve an occupier:
3: Free a room
4: Add a guest:
5: Find a free room
Enter a number 1 - 5:
----------------------------
Maybe writing the data to /dev/null would help? How would I do that if that would be a solution?