In C, calling a function from main [closed]

Answer 1

you defined it again.. Just remove the void from the funciton. include a header a forward declartion so it will recognize it..

void printSum (void); <-------------------

int main(void){

  printSum();         <-------------------------
  printf("Hi!\n");

  return 0;
}

void printSum (void)
{
  printf("Please give two integers\n");
  int x,y;
  scanf("%d %d", &x,&y);
  printf("%d + %d is %d\n",x,y,x+y);
}

Answer 2

void printSum(void);

Does not actually call the function, it merely declares that it exists. You need to do that so the compiler will know how to call it even though it hasn't been defined yet, so leave that line in. But to actually call the function, you need:

printSum();

Answer 3

The following:-

void printSum(void);

is not a function call. It is a function declaration.

printSum();

is a function call. You should also forward declare the function before main.

Answer 4

Because instead of actually calling the function, you are just declaring it. To call the function you just need the function name and parameter list, which in this case is empty:

printSum();

Answer 5

Your program doesn't call printSum, it just declares it. Change this line:

void printSum(void);

to

printSum();

If your functions are in the same order in your source file as you put them here, you'll also need to forward declare or move the implementation of printSum above main to be correct.

You should probably look into getting a beginner C book.

Answer 6

It looks like you're just starting out with C. Hopefully you have some experience with other languages, as C has a steep learning curve. Anyway, it's important to note a few things about C. The first is what void printSum(void); really means. Breaking it down:

void printSum(void);

This declares a return signature. In other words, what the function gives back to you. In C, the word void basically means "no variable". Remember that specifically. Why? Because C has another similar word, NULL. NULL means "no value." This is another way to look at it.

Some valid variables: int, float, bool, void Some valid values: 1, 'c', 2.0f, NULL

In reality, NULL is actually just the number 0. Literally. NULL == 0 will return true.

Moving on...

void printSum(void);

This defines the name of the item.

void printSum ( void );

The parentheses mean this is a function.

void printSum(void);

This represents a variable being passed into the system. So this could have been int, float, etc.

void printSum(void) ;

The semi-colon represents the end of a statement. This is a bit trickier of a concept to explain, but just think of it as finishing a sentence.

Now, the important bit here is the very first void. If you tell C what kind of thing a function returns, it assumes you are talking about the function, not actually calling it. Omitting the first void makes C attempt to run the function instead of define or declare it.

The difference between defining a function and declaring it is interesting and might be best saved for when you're a bit more accustomed to C.