flushing stdin

This is a discussion on flushing stdin within the Linux Programming forums, part of the Platform Specific Boards category; In the faq, there is a section saying that it is unsafe to fflush(stdin) (it doesn't work correctly anyway). I'm ...

  1. #1
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    Apr 2006
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    31

    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?

  2. #2
    and the hat of wrongness Salem's Avatar
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    > memset(lname, ' ', sizeof(lname));
    sizeof() on a pointer does NOT tell you how much memory you're pointing at.

    > fgets(fname, (sizeof(fname) - 1), stdin);
    Ditto.

    And in several other places as well.


    > fflush(stdin);
    If you use fgets() for EVERY bit of input, then the need to flush input pretty much goes away.

    char buff[BUFSIZ];
    fgets( buff, BUFSIZ, stdin );
    Then validate buff in whatever way you choose, and copy the data to it's final destination.

    > /* tac on ", " to lname */
    Use strcat then!

    > buf[strlen(buf)] = ',';
    Oops, I just overwrote the \0, I really hope the next character is a \0 as well otherwise the following strlen() is lost.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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