printf 在不为格式字符串 C 传递变量的情况下以某种方式工作

Question

So I'm currently learning C and had to write a program that took in a character and checked if it was a vowel or consonant. While doing this I compiled but forgot the pass the char into printf and yet it still somehow worked and even displayed the char. Any clue how this could have happened or why?

#include <stdio.h>
#include <ctype.h>

int main() {
    char chr;
    int ascii;

    printf("Enter a character: ");
    scanf(" %c", &chr);

    ascii = (int) toupper(chr);

    if (ascii == 65 || ascii == 69 || ascii == 73 || ascii == 79 || ascii == 85) {
        printf("%c is a VOWEL.\n");
    } else if (ascii >= 65 && ascii <= 90) {
        printf("%c is a CONSONANT.\n");
    } else {
        printf("%c is not a letter.\n");
    }

    return 0;
}

Answer 1

When a function is called, arguments are passed to it. These arguments are passed according to some specification that says where they are passed, commonly in processor registers or on the stack.

In the simplest implementation, the called function takes the values for its parameters from the locations where you are supposed to pass arguments. When you do not put a desired value in the right place, the function uses whatever value is there. If the program happened to use the same register for some variable, say ascii , that would be used for passing the argument, then it is possible that printf happens to get the desired value and print it.

However, that is just the simplest behavior. Modern compilers optimize programs considerably and may transform function calls into code that would be equivalent if your program had defined behavior. When you have undefined behavior, this optimization can result in your program misbehaving in surprising ways. It may print incorrect values, terminate with an error, or execute code other than what you expected.

To avoid some of these problems, compile with most warnings enabled ( -Wall in GCC and at least -Wmost in Clang) and elevated to error messages ( -Werror with GCC and Clang).