Please explain How prog works

Question

#include<stdio.h>
int f();

int main()
{

    f(1);
    f(1,2);
    f(1,2,3);
}

f(int i,int j,int k)
{

    printf("%d %d %d",i,j,k);

}

it is running fine(without any error) ...can u plz explain how it executes ? how f(1) and f(1,2) links to f(int,int,int) ?

Answer 1

You must have a different definition of "error" to me :-) What gets printed the first two times you call your f function? I get

1 -1216175936 134513787
1  2          134513787
1  2          3

for my three function calls.

What you're seeing is a holdover from the very early days of C when people played footloose and fancy-free with their function calls.

All that is happening is that you are calling a function f and it's printing out three values from the stack (yes, even when you only give it one or two). What happens when you don't provide enough is that your program will most likely just use what was there anyway, usually leading to data issues when reading and catastrophic failure when writing.

This is perfectly compilable, though very unwise, C. And I mean that in a very real, "undefined behaviour", sense of the word (referring specifically to C99: "If the expression that denotes the called function has a type that does not include a prototype, ... if the number of arguments does not equal the number of parameters, the behaviour is undefined").

You should really provide fully formed function prototypes such as:

void f(int,int,int);

to ensure your compiler picks up this problem, and use ellipses (...) in variable parameter functions.

---

As an aside, what usually happens under the covers is that the calling function starts with a stack like:

12345678
11111111

and pushes (for example) two values onto a stack, so that it ends up like:

12345678
11111111
2
1

When the called function uses the first three values on the stack (since that's what it wants), it finds that it has 1, 2 and 11111111.

It does what it has to do then returns and the calling function clears those two values off the stack (this is called a caller-makes-good strategy). Woe betide anyone who tries this with a callee-makes-good strategy :-) although that's pretty unusual in C since it makes variable argument functions like printf a little hard to do.