I am not able to flush stdin. How can I flush stdin in C?

Question

How to flush the stdin??

Why is it not working in the following code snippet?

#include <string.h>
#include <stdio.h>
#include <malloc.h>
#include <fcntl.h>

int main() {
    int i = 0, j = 0, sat;
    char arg[256];
    char *argq;
    argq = malloc(sizeof(char) * 10);
    
    printf("Input the line\n");
    i = read(0, arg, sizeof(char) * 9);
    arg[i - 1] = '\0';
    fflush(stdin);
    
    i = read(0, argq, sizeof(char) * 5);
    argq[i - 1] = '\0';

    puts(arg);
    puts(argq);
    
    return 0;
}

Now if I give the input as 11 characters, only 9 should be read but the remaining two characters in the stdin are not flushed and read again in the argq. Why?

Input: 123 456 789

Output:

123 456
89

Why am I getting this 89 as the output?

Answer 1

I believe fflush is only used with output streams.

You might try fpurge or __fpurge on Linux. Note that fpurge is nonstandard and not portable. It may not be available to you.

From a Linux fpurge man page: Usually it is a mistake to want to discard input buffers.

The most portable solution for flushing stdin would probably be something along the lines of the following:

int c;
while ((c = getchar()) != '\n' && c != EOF);