Conflicting types error in compiler - c

Question

My issue is that I keep getting the same type of error and I can not understand why. I'm fairly sure I declared and defined it before the main function.
Here is my code.
#include <stdio.h>
void functn (int x);
int functn(int x, int result){
result = (x-1)+2;
if (x <= 0){
return 0;
}
else{
return result;
}
}
int main (){
int x, y;
printf ("Enter the value of x: ");
scanf ("%d", &x);
y = f(x);
printf ("%d", y);
return 0;
}

Answer 1

You have a number of problems in functn. Primarily passing result. When you pass result to functn, functn receives a copy of result and the only way to get the modified value is to return the modified value. You can also pass a pointer to result and update *result in functn (similar to what you do now), that would eliminate the need to return a value, as any changes to result would be visible back in the calling function (main() here). Further, there is no need for global variables. Simply declare the variables local to main and pass as parameters, as required.
The following example declares functn (simply f below) to do both, take a pointer to result (e.g. &result) as a parameter, while also returning result, which allows you to either assign the return or not, but always have the updated value for result back in main, e.g.
#include <stdio.h>
int f (int x, int *result);
int main (void){
int x, y, result = 0;
printf ("Enter the value of x: ");
if (scanf ("%d", &x) != 1) {
fprintf (stderr, "error scanf, invalid conversion.\n");
return 1;
}
y = f (x, &result);
printf ("y = f(%d) => %d\n", x, y);
return 0;
}
int f (int x, int *result)
{
*result = (x-1)+2;
if (x <= 0)
return 0;
return *result;
}
Example Use/Output
$ ./bin/yfx
Enter the value of x: 5
y = f(5) => 6
Look things over and let me know if you have questions.

Answer 2

Return type of functn() different in declaration and definition. So, use
int functn(int x);
instead of
int functn(int x, int result)
Remove int result from function argument and declare inside function. like,
int functn(int x)
{
int result = 0;
result = (x-1)+2;
if (x <= 0){
return 0;
}
else{
return result;
}
}
Also, correct function call, like
y = functn(x);
instead of
y = f(x);