为什么这个程序没有显示任何错误？

Question

in the code below .

• i have defined function prototype with no argument
• in definition as well as in function call i have used one parameter.

i would like to know why i am not getting any error ?

# include <stdio.h>
float circle();       /* no parameter*/
int main()
{
    float area;
    int radius =2;
    area=circle(radius);
    printf("%f \n",area);
    return 0;
}

float circle( r) /* with one parameter even no parameter type */
{
    float a;
    a=3.14*r*r;
    return (a);
}

Answer 1

The

float circle();

is not a function with zero parameters. It's a function with an unspecified number of parameters.

The

float circle( r) {

is a K&R-style definition in which the type of r defaults to int . See https://stackoverflow.com/a/18433812/367273

Answer 2

This is because compiler treat r as int by default when no parameter is defined for circle . Try to run your code after declaring function prototype as

float circle(void);  

and you will get error.

Answer 3

That's because function

float circle();

declaration doesn't declare function that takes no arguments.
It's implicitly declared as a function that takes undefined number of integer variables as arguments.
Just like

function();

is valid function declaration. Implicitly this function will be treated as function taking int as arguments and returning int .
If you want to declare function function taking no arguments or not returning any value, you do it with void keyword:

void funct(void);