I am not able to flush stdin.

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

How to flush the stdin??

Flushing input streams is invoking Undefined Behavior. Don't try it.

You can only flush output streams.

Answer 2

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);

Answer 3

You are overriding the last element of the input in arg with '\0'. That line should be arg[i]='\0'; instead (after error and boundary checking you are missing.)

Other's already commented of the flushing part.

Answer 4

From the comp.lang.c FAQ, see:

• How can I flush pending input so that a user's typeahead isn't read at the next prompt? Will fflush(stdin) work?
• If fflush won't work, what can I use to flush input?