loop is running Infinitely - c

Question

I have written the below small code with several conditions on number entered by the user
#include<stdio.h>
int main()
{
int userInput;
while(1)
{
printf("Press '1':To Print\n");
scanf("%d",&userInput);
if(userInput==1)
{
printf("ABCD\n");
}
else
{
printf("WARNING:Pressed invalid key");
}
}
}
When I press any number then it works fine, but when I press up arrow/any character key and press enter the loop runs infinite times.
Why is this happening and is there any way to prevent it?

Answer 1

You're not checking whether scanf failed. May something like this help?
#include <stdio.h>
#include <ctype.h>
#include <stdbool.h>
void printHelp(bool afterInvalid) {
if(afterInvalid)
fprintf(stderr, "\nInvalid input, try again!\n");
printf("Press 1 to print\nPress 3 to break\nInput: ");
}
int main() {
while(1) {
int userInput;
printHelp(false);
if(scanf("%d", &userInput) == 0) {
char ch;
bool reachedNewline = false;
do {
if(reachedNewline) {
printHelp(true);
reachedNewline = false;
}
ch = getchar();
if(ch == '\n')
reachedNewline = true;
} while(!isdigit(ch));
ungetc(ch, stdin);
continue;
}
if(userInput == 1)
printf("ABCD\n");
else if(userInput == 3) {
printf("Breaking!\n");
break;
} else
printHelp(true);
}
return 0;
}

Answer 2

this makes it an infinite loop that keep asking to print..
#include<stdio.h>
int main()
{
int userInput;
while(1!=EOF)
{
printf("Press '1':To Print\n");
scanf("%d",&userInput);
if(userInput==1)
{
printf("ABCD\n");
}
else
{
printf("WARNING:Pressed invalid key");
}
}
}

Answer 3

Check the return value from scanf("%d"... and consume the non-numeric input as needed.
#include<stdio.h>
int main(void) { // Add void
int ch = 0;
while (ch != EOF) {
int userInput;
printf("Press '1':To Print\n");
// Expect 0, 1 or EOF to be returned
int fields_scanned = scanf("%d",&userInput);
if (fields_scanned == 1) {
printf("ABCD %d\n", userInput);
}
else if (fields_scanned == 0) {
printf("WARNING:Pressed invalid key\n");
}
// Consume remaining characters in the line
while ((ch = fgetc(stdin)) != '\n' && ch != EOF) {
;
}
}
return 0; // Not required, yet good practice.
}
All in all, recommend using fgets()/other code instead of scanf()
#define INT_MAX_SIZE (sizeof (int) * CHAR_BITS /3 + 3)
fputs(Prompt, stdout);
char buf[INT_MAX_SIZE*2 +1]; // I like 2x expected size, no need to be stingy here.
if (fgets(buf, sizeof buf, stdin) == NULL) break; // No more input or error
// Use sscanf() or strtol(), etc
if (sscanf(buf, "%d", &userInput) == 1) {
; // Success
} else {
; // Failure
}