Passing parameters to function in different file (gcc) - c

Question

I wrote an add function for int in Add.c. Then, I passed real numbers to this add function in Main.c. The codes are shown as follows.
If I have declaration of add function before calling it, the answer is correct as 30. However, if the declaration does not exist, the answer is incorrect. Why?
Add.c
int add(int x, int y) { return x + y;}
Main.c
#include<stdio.h>
int add(int, int); // If this does not exist, the answer is not 30.
int main(void) {
printf("%d\n", add(10.5, 20.5));
}

Answer 1

Without the declaration, which is more specifically a prototype (a declaration with specified parameter types), you're relying on an implicit prototype-less declaration of the function (which is now an obsolescent feature of C).
The implicit prototype for function f is int f()—a function returning int and taking an unspecified number of parameters.
The way functions without prototypes are called is the arguments are promoted (lesser than int integer types to int and floats to doubles) and passed to the function in an implementation-defined way. If the arguments don't match the definition, the behavior is undefined (your case).
You could fix this (while still relying on the implicit declaration) by providing explicit casts:
#include<stdio.h>
//nothing (probably will get a compiler warning) or
int add(); /*=params unspecified*/
int main(void) {
printf("%d\n", add((int)10.5, (int)20.5));
}
thereby removing the UB (undefined behavior), or you can include the prototype as you have, which will make the compiler convert passed-in floats to match the expected types (int). In other words, with the prototype, the compiler will insert these casts for you, seeing that a conversion from float to int in (argument) assignment.
The preferred way to solve this is with a prototype included via a header which is also included in the implementation file (Add.c) so as to allow to compiler to verify the call is consistent with the definition.

Answer 2

To make this work, first of all, you have to include the "add.c" file into the "main.c". Coz main.c doesn't know about "add" function in add.c. (Also you should not make '.c' file to add a function or something. Instead use '.h')
Now the solution should look like this...
add.h
int add(int x, int y){
return x + y;
}
main.c
#include<stdio.h>
#include "add.h"
int main(void){
printf("%d\n", add(10, 20));
return 0;
}
Make sure to keep both the files in same directory.

Answer 3

Before using add in Main.c, it needs to know the declaration for this function. Without using function prototype in this case the behavior of the program would be undefined. You can either add a prototype before main as you did or you can create a separate Add.h file and put the prototype there.
In second case you need to add Add.h to both .c files.
Add.h
#ifndef ADD_H
#define ADD_H
int add(int x, int y;
#endif
Add.c
#include "Add.h"
int add(int x, int y) { return x + y;}
Main.c
#include<stdio.h>
#inclide "Add.h"
int main(void) {
printf("%d\n", add(10, 20));
}