C safe and proper way to get user input [closed] - c

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Safe input is harder than I thought. I looked around and found this, but it is from 2011 and things might have changed since then.
I'm looking for something that can handle string input and single character input. For example:
Hello, what's your name?
My name is: _
Are you sure?
[Y] Yes, hello
[N] No, goodbye
_
Here is the way I'm doing it right now:
char input[16];
fgets(input, 16, stdin);
char input = getchar();
My problem has always been that the user may input arbitrarily long input or invalid input. How can I read input safely and ensure future inputs won't get borked?
Looking for solutions that work in C and across Linux / Windows.
Thank you for your time.

Answer 1

You are able to read infinite standard input safely. You can just use a continuously reallocating buffer.
#include <stdio.h>
#include <stdlib.h>
char* get_line(size_t* length) {
size_t len = 0;
size_t cap = 4;
char* buffer = malloc(cap);
if (buffer == NULL) {
fprintf(stderr, "Virtual memory exhausted\n");
return NULL;
}
char current;
while ((current = getchar()) != '\n') {
if (len + 1 > cap) {
cap *= 2;
buffer = realloc(buffer, cap);
}
buffer[len++] = current;
}
// One last time for NUL terminator
if (len + 1 > cap) {
cap *= 2;
buffer = realloc(buffer, cap);
}
buffer[len] = 0;
if (length) *length = len;
return buffer;
}
This function will read stdin char-by-char and add it to a string which is returned. Optionally you can ask for its length. Don't forget to free.
int main() {
char* line = get_line(NULL);
printf("You entered: %s\n", line);
free(line);
}
NOTE: This code is meant to be of little verbosity so some checks were omitted. In production code checking realloc and using int for getchar() to test for EOF is necessary.