chris360 chris360 - 1 year ago 124
C Question

Flushing standard input buffer in C

This code is a simplification from a larger project I'm working on and it sums up the problem in a simple example. I am obtaining input from the user, their name, and then clearing the buffer from any input that did not fit in the C-string. The problem is that After entering the name, the user has to push enter twice for the program to respond, and because I am using getchar() to flush the buffer there is just a clear misunderstanding in the logic of the loop I created. How can I keep the user from entering Enter twice, in otherword what am I missing? Thanks!

#include "stdafx.h"
#include <stdio.h>
#include <string.h>

#define BUFFSIZE 10

int main(void)

unsigned char name[BUFFSIZE];

printf("ENTER YOUR NAME: ");
fgets(name, BUFFSIZE, stdin);
name[strcspn(name, "\n")] = '\0';

//flush the input buffer
int flush;
while (flush = getchar() != '\n' && flush != EOF);

printf("Your name is: %s!\n ", name);
printf("Press enter to continue...");

return 0;

Kaz Kaz
Answer Source

The problem in your program is that you don't distinguish the two cases: a) the user's input fit into the buffer versus b) the input didn't fit. What distinguishes these cases is the presence of the newline character in the buffer. That information is destroyed when you overwrite that character:

fgets(name, BUFFSIZE, stdin);
name[strcspn(name, "\n")] = '\0';

What we need here is something like:

size_t nlcspn = strcspn(name, "\n");
bool incomplete = name[nlcspn] == 0;
name[nlcspn] = 0;

Now we have an incomplete flag to test. Only when this flag informs that the input didn't contain a newline, then we can go ahead and complete the "get line" operation of fgets with a little loop that scans until a newline is received. (In that case, some error recovery might also be a good idea, like informing the user that the input had been too long, and creating an opportunity to rectify that).

Another thing to note is that fgets returns a value which should be checked. If it returns a null pointer, it means that the stream ended before any input was consumed. The problem is that in that case, fgets doesn't put anything into the array. The array retains its previous value, which may be previously read input, or an indeterminate value ("garbage") due to uninitialized contents.

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