Undefined behaviour when function return type is not int

March 23, 2022

Consider snippet 1:

#include <stdio.h>

int main() {
    float val;
    val = fun(2, 4);
    printf("%f", val);
    
    return 0;
}
float fun(int x, int y){
    return x+y;
}

Code snippet 2:

#include <stdio.h>

int main() {
    int val;
    val = fun(2, 4);
    printf("%d", val);
    
    return 0;
}
int fun(int x, int y){
    return x+y;
}

In case of snippet 1, it gives me error, as follows:

gcc /tmp/1cUl25Ux4O.c -lm
/tmp/1cUl25Ux4O.c: In function 'main':
/tmp/1cUl25Ux4O.c:6:11: warning: implicit declaration of function 'fun' [-Wimplicit-function-declaration]
    6 |     val = fun(2, 4);
      |           ^~~
/tmp/1cUl25Ux4O.c: At top level:
/tmp/1cUl25Ux4O.c:11:7: error: conflicting types for 'fun'
   11 | float fun(int x, int y){
      |       ^~~
/tmp/1cUl25Ux4O.c:6:11: note: previous implicit declaration of 'fun' was here
    6 |     val = fun(2, 4);
      |           ^~~

But in case of second snippet, i’m getting correct output. (i.e 6).
In both the cases I’ve omitted function declaration and just the definition part is present, and that too after main().
But, if incase of snippet 1, if i change the order and define fun(int, int) before main(), i’m getting the desired output.(i.e 6.0000)
I’m not able to figure it out.

Why is this happening?
Is it due to the order in which functions are pushed into stack?
If that is the reason, then why code in snippet 2 is working?
Please provide me either with references/ explanation. Thanks!

Update:
In the error log for snippet 1, it says that there are conflicting types for fun. But there’s just one fun(int, int) present in the code. What is creating conflict?

>Solution :

First snippet:

int main() {
    float val;
    val = fun(2, 4);
    printf("%f", val);
    
    return 0;
}
float fun(int x, int y){
    return x+y;
}

When you write val = fun(2, 4); in main the compiler has not yet encountered float fun(int x, int y). Therefore it assumes that fun is declared like this: int fun(); (function returning an int and taking any number of parameters of any type.

Then the compiler encounters float fun(int x, int y), but as explained in the preceeding paragraph, the compiler assumes now that fun returns an int, hence the conflict.

If you put float fun(int x, int y) before main, the compiler encounters float fun(int x, int y) and now knows that fun returns a float and takes exactly two int arguments.

Implicit declarations are a thing of the past, and they shouldn’t be used nowadays ever.