c - need an alternative for fflush

Question

I am only allowed to use the standard c library so fpurge() is not an option form me.

int dim = 0;
int c = 0;

printf("Please enter a number");

while ( (( c = scanf("%d",&dim)) != 1 ) || (dim < 1) || (dim > UCHAR_MAX) )   
{
    if (c == EOF)
    {
        return 0;
    }
    fflush(stdin);
    printf("[ERR] Enter a number\n");
    printf("Please enter a number");
}

The program should read in a number big than one and if there is any “wrong” input like letters it should go and deliver an error message. It works with fflush on my windows pc put the program should run on a Linux system.

How can I replace fflush that the programm still works? Because when I do not use it I come in an infinite loop.

The number which is entered determines how often a certain line of code is used in the rest of the program that is why I need an array.

Why can't you use `fflush`? That's part of the standard lib. — Iguananaut, Nov 02 '17 at 10:44
[`fflush(stdin);` is undefined behavior, don't do it.](https://stackoverflow.com/a/38325926/2173917) — Sourav Ghosh, Nov 02 '17 at 10:45
@Iguananaut Yes, but since it has an undefined behaviour it does not work with Linux — Mark, Nov 02 '17 at 10:48
Don't use `scanf` but use `fgets` and parse the input yourself. Then you don't need to flush anything. `scanf` is not very useful for user input. — Jabberwocky, Nov 02 '17 at 10:49

Jabberwocky · Accepted Answer · 2017-11-02T14:18:43.587

You probably want something like this:

#include <stdio.h>
#include <stdlib.h>

int main()
{
  int dim = 0;

  for (;;)
  {
    printf("Please enter a number: ");
    char buffer[30];
    fgets(buffer, sizeof(buffer), stdin);

    char *endpointer;
    dim = strtol(buffer, &endpointer, 10);

    if (*endpointer != '\n' || (dim < 1) || (dim > UCHAR_MAX))
    {
      printf("[ERR] Enter a number\n");
      continue;
    }
    break;
  }

  printf("The number you entered is: %d\n", dim);
}

After the call to strtol, endptr points to the first non digit char entered. If only digits have been entered, endptr will point to the \n that terminates the line, otherwise if e.g. 12x has been entered, endptr will point to the 'x'.

You may improve this by writing a GetNumber function so you can just write

dim = GetNumber();

or

dim = GetNumber("Please enter a number: ", "[ERR] Enter a number\n", 1, UCHAR_MAX);

c - need an alternative for fflush

1 Answers1

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