Funcion prototype before its called.

[Humble]

There is always a thought that, the functon protoype has to be there before the function is called. Otherwise, function have to be defined before its getting called.. but, it is not *that* true.. That said, if the function is called without declaring it , the compiler assume it returns int .. If the function actually returns “int” , it can be later to the call statement..

Lets watch this:

[hchiramm@node]$ cat func.c 
#include <stdio.h>

int *func1 (int);

int main () {
	int j=100;
	printf ("Main: ");
	func1(j);
	return 0;
}

int *func1( int k)
{
	int *p = &k;
	printf ("%d is passed to me", k);
	return p;

}

[hchiramm@node]$ 


If you compile this program everything is fine. 

cc -Wall -g    func.c   -o func
[hchiramm@humbles-lap c-blog]$ ./func 
Main: 100 is passed to me


How-ever :

[root@node ]cat func1.c

#include <stdio.h>

/* int *func1 (int); */

int main () {
	int j=100;
	printf ("Main: ");
	func1(j);
	return 0;
}

int func1( int k)
{
	printf ("%d is passed to me", k);
	return k;

}

[hchiramm@node]$ 
cc -Wall -g    func1.c   -o func1
func1.c: In function ‘main’:
func1.c:8:2: warning: implicit declaration of function ‘func1’ [-Wimplicit-function-declaration]
  func1(j);
  ^
[hchiramm@node]$$ ./func1
Main: 100 is passed to me


You have a "Warning" about the "implicit declaration", but you are not blocked..

Now lets change the same program to return a different data type than int in the called function as in first program.

#include <stdio.h>

/*int *func1 (int); */

int main () {
	int j=100;
	printf ("Main: ");
	func1(j);
	return 0;
}

int *func1( int k)
{
	int *p = &k;
	printf ("%d is passed to me", k);
	return p;

cc -Wall -g    func1.c   -o func1
func1.c: In function ‘main’:
func1.c:8:2: warning: implicit declaration of function ‘func1’ [-Wimplicit-function-declaration]
  func1(j);
  ^
func1.c: At top level:
func1.c:13:6: error: conflicting types for ‘func1’
 int *func1( int k)
      ^
func1.c:8:2: note: previous implicit declaration of ‘func1’ was here
  func1(j);
  ^
func1.c: In function ‘func1’:
func1.c:16:2: warning: return makes pointer from integer without a cast [enabled by default]
  return k;
  ^
make: *** [func1] Error 1


Thats it..

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