I keep getting a compiler error whenever I try to use functions

Question

I'm learning C, and we have been recently been taught functions. User defined functions strike me as fascinating, and so I decided to write a simple program to try out how a function works. Here's my code:

#include"stdio.h"

int main()
{
    func();
}
func()
{
    printf("This program uses a function.");
}

I pressed "compile" and I got a compliler error. The exact message it gave me is:

fpermissive]
func()
^
Return code is not 0

The diagnostics do not imply a mistake in the code that I wrote, so I'm unable to find out where I made a mistake. Can someone help me fix this program, and also successfully learn how to use user defined functions?

Edit:

I had forgotten to mention the return type of the function. I did that now, and here's my modified code:

#include"stdio.h"

int main()
{
    func();
    return 0;
}
int func()
{
    printf("This program uses a function.");
    return 0;
}

I pressed compile again, and got a different error message. This one says:

was not declared in this scope
func();
^
Return code is not 0

Can someone help me?

What is the return type of your function `func()`? You need to mention that such as `void func()` or `int func()` — Aditi Rawat, Nov 05 '17 at 07:04
You are using an incomplete function prototype. Traditionally, this meant it returns an `int` and its arguments are unspecified, but you really shouldn't do that today. If you don't want to return a value, then declare it void. Similarly, if you don't want to pass any arguments, declare a void argument list. I.e. `void func(void)`. Finally, add a forward declaration for the function so that the compiler knows how to call it. All of this should have been covered in your tutorial. — Tom Karzes, Nov 05 '17 at 07:06
@TomKarzes Wonderful, thanks. You explained everything that I needed to know. Can you convert that comment into an answer so I can accept it as one? — Pritt Balagopal, Nov 05 '17 at 07:15

Costantino Grana · Accepted Answer · 2017-11-05T12:46:35.900

4

You need to study the syntax better. Your function doesn't return anything, so you need to put a void in front of the name. You also need to put a void between parenthesis of your functions. And move the function definition before the main, or, as other answers mention, add a function declaration before the use of the variable.

A function declaration is composed by:

the optional keyword extern
the first line of the function, i.e. the type returned, the name, parameters between parenthesis
a semicolon

In your case: extern void func(void);, or simply void func(void);.

edited Nov 05 '17 at 12:46

answered Nov 05 '17 at 07:08

Costantino Grana

3,132
1
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35

*"And move the function definition before the main."* Ahh I had been thinking about that for quite a while. My CS teacher put the function definition after the main(), so I had thought we could do it after as well. Anyway, thanks for patching this up. – Pritt Balagopal Nov 05 '17 at 07:11
Or add a forward declaration, which is a good idea in general. You don't need to move the definition. – Tom Karzes Nov 05 '17 at 07:11
My personal preference for students is to start by saying that what you use has to be defined before its use. After some practice than introduce declarations. – Costantino Grana Nov 05 '17 at 07:16

Aditi Rawat · Answer 2 · 2017-11-05T07:19:48.310

2

You need to take care of two things: (a) Declaring func() before main() , (b) mentioning the return type of func().

#include<stdio.h>

void func(void); //forward declaration

int main()
{
    func();
}
void func(void) //specifying return type as **void**
{
    printf("This program uses a function.");
}

edited Nov 05 '17 at 07:19

answered Nov 05 '17 at 07:07

Aditi Rawat

784
1
12
15

You forgot to declare a void argument list. Bad practice. These are not variadic functions. – Tom Karzes Nov 05 '17 at 07:10
Also, it is better to use , not "stdio.h" – Tom Karzes Nov 05 '17 at 07:10
@TomKarzes Can you explain why is preferred over "stdio.h"? I'm on my Android phone using CPPDroid, and reaching the "<" and ">" symbols is harder than quotations. – Pritt Balagopal Nov 05 '17 at 07:14
The argument lists are still wrong. They are improperly declared. – Tom Karzes Nov 05 '17 at 07:14
1

Using `<>` in an include causes the preprocessor to search only the include path, and not the source directory. So you use `<>` when including external header files. – Tom Karzes Nov 05 '17 at 07:15
https://stackoverflow.com/questions/3845415/what-is-the-difference-between-stdio-h-and-stdio-h – Aditi Rawat Nov 05 '17 at 07:15
Making progress, but forward decl and your function decl need to match (use `void` for both arg lists). – Tom Karzes Nov 05 '17 at 07:17

score 1 · Answer 3 · answered Nov 05 '17 at 08:16

The error occurred because the compiler didn't encounter any function named

func();

when it entered the main function. You need to, atleast, declare a function if not writing its body.

So, you have to do declare a function before main if you wan to write its code after main()

score 0 · Answer 4 · answered Nov 05 '17 at 07:05

0

You are missing return type of func(). Please look at syntax of way of defining functions.

answered Nov 05 '17 at 07:05

Prem

51
4

score 0 · Answer 5 · answered Nov 05 '17 at 07:07

0

#include"stdio.h"

void func();  // Forward declare func() here.

int main()
{
    func();
}

void func() // Ensure func() has a return type 'void'
{
    printf("This program uses a function.");
}

answered Nov 05 '17 at 07:07

Stewart

4,356
2
27
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You forgot to declare a `void` argument list. Bad practice. These are not variadic functions. – Tom Karzes Nov 05 '17 at 07:08
Also, it is better to use ``, not `"stdio.h"` – Tom Karzes Nov 05 '17 at 07:09
1

@TomKarzes Those coding practice improvements do not help the OP answer the original question. They are extra noise for this case. – Stewart Nov 05 '17 at 07:11
The `stdio` problem perhaps, but not the function prototype. That is OP's entire problem, so a correct prototype should be given in any answer. – Tom Karzes Nov 05 '17 at 07:13

I keep getting a compiler error whenever I try to use functions

5 Answers5