请，您能帮我这个递归函数吗？

Question

#include <stdio.h> 
int  fun (int x)
 {
      if (x<1)
        return(1);
      else
        printf("%d %d \n", x, fun(x-1));
 }
int main()
 { int x,y;
   x = 5;
   y = fun(x);
   printf("\n x = %d f(x) = %d \n", x, y);
   return 0;
  }

This program contains a recursive function that count some numbers. There is something in the output I cannot understand. There is a screenshot of the output at the following link:

https://onedrive.live.com/redir?resid=BE4862D617298D2C!886&authkey=!AA03bF8dQ5W4S9Y&v=3&ithint=photo%2cpng

Why the right column (red circuled) is as shown? I thought that this column will be all ones instead of that.

Answer 1

Because the function fun didn't have a return value when x >= 1.

And the 5 is the return value of printf("%d %d \\n", x, fun(x-1)); because it has output 5 characters.

Answer 2

You are not returning anything if x>=1 , causing undefined behaviour when the execution reaches the end of the function. That means the value could be anything, you got 5 by chance.

6.9.1. p12:

If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

The return value is used in your example.

int fun(int x)
{
    if (x<1)
        return(1);
    else
        printf("%d %d \n", x, fun(x-1));

    return x ;//just return something
}

You probably want to return something relevant to what you function does.