C Compiler ignores scanf

Question

I have problem with this code:

printf("Select your math: \n'+'addition \n'-'subtraction \n'*'multiplication \n'/' division \n");
char do_math;
scanf("%c", &do_math);
printf("Type 1 st number: ");

Problem is, that program doesn't wait until I type "do_math" but it displays "Type 1 st number: " right after first printf. Any ideas?

The compiler don't ignore `scanf`. You could look at the generated assembly code (e.g. with `gcc -Wall -fverbose-asm -O -S`). You'll find that `scanf` is still referenced there. You are just using `scanf` *wrongly* — Basile Starynkevitch, Nov 04 '14 at 21:01

score 3 · Answer 1 · answered Nov 04 '14 at 20:57

The only way your compiler will miscompile that is if you deliberately defined scanf() as a do-nothing macro. Don't blame your compiler!

There is probably a previous scanf() call in the program that left a carriage return in the input buffer. You can confirm that hypothesis by printing the value of do_math.

Try scanf(" %c", &do_math); (with a space before %c) to discard such whitespace.

(It's also a good idea to pay attention to the return value from scanf().)

Instead of a "thank you" comment, could you please mark the answer as accepted? — 200_success, Nov 04 '14 at 21:05

C Compiler ignores scanf

1 Answers1