how difference between including the header file and source file in main file

Hi,

i have two files. 1.c and 2.h like below


2.h file

Code:

---

int fun()
{
        printf ("hai how are you\n");
        return 0;
}

---

1.c file

Code:

---

#include<stdio.h>
#include"2.h"
int i;
int j;
int main()
{
        int fun();
        printf ("%d\n",i);
        fun();
        printf ("%d\n",i);
}

---

if run the above program by using #cc 1.c , i should get the output as

"0
hai how are you
0".

if i rename the 2.h to 2.c and reflect the same change in include file on 1.c , then also i get the same output.


actualy what is the correct procedure?

in which condition i include .h file and in which condition i need to include .c file?

i would appreciate any help.

Header files should never have any executable code in them. They just define the interface to your .c files. A .h file should basically contain function prototypes and extern'ed variable declarations, and struct, enum and type definitions that are needed in other .c files. You should only #include a .h file, never a .c file. The .c file will be compiled with the other .c files to make the final executable. Fixing your example:

1.c

Code:

---

#include <stdio.h>
#include "2.h"
int main(void)
{
    int i = 0;  // globals are bad
    printf("%d\n", i);
    fun();
    printf("%d\n", i);
    return 0;
}

---

2.h

Code:

---

#ifndef include_guard_2_h
#ifndef include_guard_2_h
void fun(void);
#ifndef include_guard_2_h

---

2.c

Code:

---

void fun(void)
{
    printf("hai how are you\n");
}

---

The bold code is things you need to fix:

1. Global Variables Are Bad
   
2. You said main would return an int, so make it do so
3. You should always use include guards with your header files: Include guard - Wikipedia, the free encyclopedia
   
4. There's no need for fun() to return int, since it has no significance (it's always 0), and you don't use the return value.
5. Be explicit. If you don't want your function to take parameters, then put a void in the param list.

Quote:

---

Originally Posted by anduril462

You said main would return an int, so make it do so

---

Note that anduril462's suggestion is unnecessary if you are compiling with respect to C99, but generally it is indeed better to be explicit than implicit.

The include guard example should be:

Code:

---

#ifndef include_guard_2_h
#define include_guard_2_h

void fun(void);

#endif

---

As a common convention, macro names should be fully capitalised, so your actual include guard names should likewise be fully capitalised.

Quote:

---

Originally Posted by laserlight

The include guard example should be:

---

Doh! Got a little overzealous with the copy-paste. Thanks.

Hi All,

Thanks for your explanations.

Hi ,

I have one more small doubt,

i have ran the below program

//extern.c

Code:

---

int i;

---

//main.c

Code:

---

#include<stdio.h>
int mian()
{
        printf("%d\n",i);
        return 0;
}

---

I am getting below error if i compile the program with even i including the all files as input to compiler

#cc main.c extern.c

# cc main.c extern.c
main.c: In function `main':
main.c:4: error: `i' undeclared (first use in this function)
main.c:4: error: (Each undeclared identifier is reported only once
main.c:4: error: for each function it appears in.)


why i am getting error even if i am including all the defined files.?




Note:
1.if i have function fun() in 4th line of main.c and defined in extern.c. compilation is success(please see below code) with #cc main.c extern.c. why its working for functions and its not working for integers?

//extern.c

Code:

---

void fun()
{
      printf("hai");
}

---

//main.c

Code:

---

#include<stdio.h>
int main()
{
      fun();
      return 0;
}

---

#cc main.c extern.c

Quote:

---

Originally Posted by nkrao123@gmail.

I am getting below error if i compile the program with even i including the all files as input to compiler

---

Focus on main.c. It is obvious that i was not declared anywhere. Maybe you object: i is defined in extern.c! Yes, but when the compiler compiles main.c, it does not consider extern.c. Therefore, you must declare i in main.c. You don't want to define i again, so you declare i extern, e.g.,

Code:

---

#include<stdio.h>

extern int i;

int mian()
{
    printf("%d\n", i);
    return 0;
}

---

Hi Laserlight,

From your earlier reply's, i understand that the usage of "extern". i can understand that main.c will compile about if we use "extern int i" in main file.

my doubt was fun() also not defined in main.c and even we did not included extern.c in main file. but its working.

please give comment on why its working for functions and its not working for variables?

why compiler did not considering extern.c for variable? and why it is considering extern.c for functions?

I hope Laze doesn't mind me taking a shot at this...

I'll bet your last try where fun() from the second file is not included ended in an error saying it could not find the function.
The code did not compile, leaving the earlier version where you did include fun() into the main file in the folder.

Try this... go into your project's folders and delete all object (.o or .obj) and executable (.exe) files and try that again.

Then when you compile keep a close eye on your compiler's messages ... you ignore them at your own peril.

Hi,

I am doing the below steps

1. I am created new folder and created below files and executed.
1. created extern.c file
//extern.c

Code:

---

#include<stdio.h>
void fun()
{
        printf("hai\n");
}

---

2.created main.c file
//main.c

Code:

---

#include<stdio.h>
int main()
{
    fun();
    return 0;
}

---

3.compile the above program with #cc main.c extern.c
4.The program compiling with out errors and creates binary file
4.after running the binary file , we will get the output hai
5.Please see the below execution stpes

Code:

---

$ vi extern.c
$ cat extern.c
#include<stdio.h>
void fun()
{
        printf("hai\n");
}
$ vi main.c
$ cat main.c
#include<stdio.h>
int main()
{
        fun();
        return 0;
}
$ cc main.c extern.c  -Wall
main.c: In function ‘main’:
main.c:4: warning: implicit declaration of function ‘fun’
$ ls
a.out                extern.c        main.c
$ ./a.out
hai
$

---

At least it showing output as wt i expected by warning messages at compilation time.

my question is

why it it compiling for functions with waring messages. why it is not compiling(showing error messages) for variables with out using "extern int i"? is it limitaion?. For variables i should include that "extern.c" file?

thanks in advance.

Quote:

---

Originally Posted by nkrao123@gmail.

3.compile the above program with #cc main.c extern.c

---

I am not sure which compiler you are using, but increase your warning level and you should find the compiler complaining about what CommonTater has been talking about. For example, if cc is gcc, you can pass the -Wall flag.

Basically, what you are observing here is the use of an implicit declaration of foo: since no declaration for foo has been seen by the compiler in the translation unit up to that point, foo is assumed to be a function with no parameters, that returns an int. It so happens that this is really the case, but you should not rely on that.

Quote:

---

Originally Posted by nkrao123@gmail.

Code:

---

$ cc main.c extern.c  -Wall
main.c: In function ‘main’:
main.c:4: warning: implicit declaration of function ‘fun’
$ ls
a.out                extern.c        main.c
$ ./a.out
hai
$

---

---

As Lase has pointed out... the compiler is taking it's best guess trying to finish it's job, and in this case you got lucky.

This is probably the worst kind of "undefined behaviour", because there's a very high probability the compiler will guess wrong and will either bail or produce severely flawed code.

IOW... your experiment worked by virture of pure dumb luck.