Why have prototypes...am I understanding this correctly?

[Robert Nash]

I am learning code from scratch, so bear with me.

Using the following code as an example:

---
int add(int,int);

int add(int i, int j)
{
return i+j;
}

int main()
{
printf("%d\n",add(3,3));
}
----

The above code works without the first line. Why have it there?

[weaknessforcats]

Not all of your source code will be in the same file.

If this code:

int add(int i, int j)
{
return i+j;
}

is in a separate file then how does the compiler know that this is correct:

int main()
{
printf("%d\n",add(3,3));
}

?

The answer is that you tell the compiler that there is a function named addthat takes two int arguments and returns an int. You do this using a prototype:

int add(int, int);

The compiler takes your word for that
and marks "int add(int,int)" as an unresolved external reference. The linker will locate the function and fix up the addresses so that the call works.

Note that your function prototypes are exactly the first line of the function. The function body between the {} is replaced with a semi-colon:


Alos, try moving the code to the end of the file with main() and then compile with and without your prototype. See what happens.

[Robert Nash]

Thanks for this. I'm using a free online source http://www.cprogramming.com/

It's great, but sometimes it's not enough. There is a lot you have to take for granted.

[donbock]

C made its appearance sometime around 1972. Function prototypes were codified in the ANSI C Standard in 1989. Function prototypes were added to the language because they would eliminate a certain class of widespread and pernicious bugs.

I want to emphasize that weaknessforcats described a very real and pervasive problem -- not some academic and abstract possibility.