warning: implicit declaration of function is invalid in C99?

Question

This is a header file

#include <stdio.h>

int m = 18;
int x = 4;

int singles (n) {
    if (n == 1)
         return 0;
    return doubles(n-1);
} 

int doubles (n) {
    if (n == 1)
        return 0;

    return triples(n-1);
}

int triples (n) {
    if (n == 1)
        return m;

    return (singles(n-1) + doubles (n-1) + triples (n-1))*(m-1);
}

and this is the main file

#include <stdio.h>
#include "test.h"

int main () {
    printf("%d",singles (x));
}

So this is pretty complicated for me at-least.The idea is that in the main function i will call singles(x) where x =4 so its more like singles (4),it will call doubles (3),that will call triples (2),that will call all of singles(1) which will return 0,doubles (1) that returns 0 and triples (1) that will return m.

So the error i am getting is

./test.h:13:12: warning: implicit declaration of function 'doubles' is invalid
      in C99 [-Wimplicit-function-declaration]
    return doubles(n-1);
       ^
./test.h:20:12: warning: implicit declaration of function 'triples' is invalid
      in C99 [-Wimplicit-function-declaration]
    return triples(n-1);
       ^
2 warnings generated.

I tried to create a header file .h with the first script and then made a second .c script that i try to compile that won't work.I tried importing the header to try to avoid this error but it doesn't seem to work. Thanks a lot

Highly doubt this is related to C, C++, *and* C#. Please only tag the languages that are really involved (and in this case seems to be only C). — crashmstr, Oct 13 '15 at 19:05
Why do you have function definitions (content) in a header file? Usually only the function signatures go in a header file. — Thomas Matthews, Oct 13 '15 at 19:06
I recommend you move your variable declarations from header file to source file. — Thomas Matthews, Oct 13 '15 at 19:08
Possible duplicate of [Warning: Implicit declaration of function is invalid in C99](http://stackoverflow.com/questions/15850042/warning-implicit-declaration-of-function-is-invalid-in-c99) — AShelly, Oct 13 '15 at 19:09
The linked question shows the causes and solutions for this class of error. — AShelly, Oct 13 '15 at 19:09
@AShelly Ive looked the at the link,it is what prompted me to make a header.when writing the function before main didn't work. — Charana, Oct 13 '15 at 19:12
c#, c and c++ are definitely not the same thing. You better remove unrelated tags — 463035818_is_not_an_ai, Oct 13 '15 at 19:14
You don't need `#include ` in the header file because none of the function declarations use identifiers from that header file. The `main` does, so move the `#include ` to your main file. — Thomas Matthews, Oct 13 '15 at 19:15

score 13 · Accepted Answer · answered Oct 13 '15 at 19:13

Inside of singles, you're using doubles before it's defined. Similarly in doubles, you're using triples before it's defined. That's why you're getting the implicit declaration errors.

Also, you're not defining the type of the n parameter to any of these functions.

You need to specify function prototypes, which declare the function without defining it:

int singles(int n);
int doubles(int n);
int triples(int n);

Also, you shouldn't define functions in a header file. If you include this header in multiple .c files and then link them together, you'll get an error because you'll have multiple definitions of those functions.

Take all of the function definitions and put them in test.c. Then in test.h, put only the prototypes above. Then you can compile everything as follows:

gcc -c test.c
gcc -c main.c
gcc -o main main.o test.o

Or in a single line:

gcc -o main test.c main.c

score 4 · Answer 2 · edited May 12 '17 at 19:31

4

The term "implicit declaration" in an error message is usually generated when the compiler sees ~~the implementation of~~ a call to a function before the declaration (prototype).

For example, you should have:
header file

int singles(int x);
int doubles(int x);
int triples(int x);

In your source file:

#include "header_file.h"

Also, in your function definitions (implementations) you need to specify the type of the argument:

int singles (/* data type */ parameter_name)
{
 //...
}

Edit 1: Changed implementation to function call per @John Bollinger.

edited May 12 '17 at 19:31

Sparkzz

367
5
14

answered Oct 13 '15 at 19:13

Thomas Matthews

56,849
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thanks for the input,didn't solve my question tho.thanks anyways – Charana Oct 13 '15 at 19:18
1

You misspoke: an "implicit definition" occurs when the compilers sees a function *call* (not implementation) before any declaration of the called function. – John Bollinger Oct 13 '15 at 19:27

warning: implicit declaration of function is invalid in C99?

2 Answers2

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