Thread: main.c: In function 'main': main.c:12:28: warning: implicit declaration of function '

First, I have created two header files: add.h and subtract.h
Next, created two source files: add.c and subtract.c.

Finally, created main program in main.c:

gcc compiler

Code:

//add.h

#ifndef ADD_H
#define ADD_H

int add(int x, int y);

#endif

//subtract.h

#ifndef SUBTRACT_H
#define SUBTRACT_H

int subtract(int x, int y);

#endif


//add.c
#include "add.h"

int add(int x, int y) {
    return x + y;
}

//subtract.c
#include "subtract.h"

int subtract(int x, int y) {
    return x - y;
}

//main.c 

#include <stdio.h>
#include "add.h"
#include "subtract.h"

int main() {
    int x = 10, y = 5;
    
    printf("x + y = %d\n", add(x, y));
    printf("x - y = %d\n", subtract(x, y));
    
    return 0;
}

Here's the compile command:
gcc -o main add.c subtract.c main.c

Code:

main.c: In function 'main':
main.c:12:28: warning: implicit declaration of function 'subtract' [-Wimplicit-function-declaration]
     printf("x - y = %d\n", subtract(x, y));
                            ^~~~~~~~
C:\Users\33\AppData\Local\Temp\ccfJeU2u.o:subtract.c:(.text+0x0): multiple definition of `add'
C:\Users\33\AppData\Local\Temp\ccGHzJAD.o:add.c:(.text+0x0): first defined here
C:\Users\33\AppData\Local\Temp\ccqKrrPu.o:main.c:(.text+0x0): multiple definition of `add'
C:\Users\33\AppData\Local\Temp\ccGHzJAD.o:add.c:(.text+0x0): first defined here
collect2.exe: error: ld returned 1 exit status

Works for me.

Code:

$ for i in * ; do echo $i ; echo ========== ; cat $i ; done
add.c
==========
#include "add.h"
 
int add(int x, int y) {
    return x + y;
}
add.h
==========
#ifndef ADD_H
#define ADD_H
 
int add(int x, int y);
 
#endif
main.c
==========
#include <stdio.h>
#include "add.h"
#include "subtract.h"
 
int main() {
    int x = 10, y = 5;
     
    printf("x + y = %d\n", add(x, y));
    printf("x - y = %d\n", subtract(x, y));
     
    return 0;
}
subtract.c
==========
#include "subtract.h"
 
int subtract(int x, int y) {
    return x - y;
}
subtract.h
==========
#ifndef SUBTRACT_H
#define SUBTRACT_H
 
int subtract(int x, int y);
 
#endif
$ gcc -o main add.c subtract.c main.c
$ ./main 
x + y = 15
x - y = 5

> implicit declaration of function 'subtract' [-Wimplicit-function-declaration]
You typically see this when you miss including a header file, or you have a spelling mistake somewhere.

> C:\Users\33\AppData\Local\Temp\ccfJeU2u.o:subtract .c:(.text+0x0): multiple definition of `add'
You get multiple definition errors by having multiple copies of the same thing.
One way to get this is by doing #include on the source code, not the header file.

Originally Posted by Salem

One way to get this is by doing #include on the source code, not the header file.

Now it is working for me also. maybe i did something wrong.

I don't understand if there are many source files in a project, then where should the header files be included?

As an example I have three source files and two header files if I include both the header file in main source file the program gives correct output.


If I include the header files with their related source file. program give the warning i but gives the output

Take for example subtract.h

You include subtract.h in subtract.c, so that the compiler can check that the prototype in the .h file is consistent with the definition in the .c file.
If you edited one without changing the other, you stand a chance of getting an error message.

Everywhere you want to call subtract would need to include the subtract.h header file, so that the compiler knows how to call it.

If a source file doesn't need to call subtract, then there is no need for it to include the subtract.h file.