输出以下代码的原因：

Question

I have a few lines of code for which I could't understand the reason for this output..

int main()
{
int a=5;
float b=10.5,c=11.0;
printf("%d",b);
printf("\n%d",c);
printf("\n%f",a);
return 0; 
}

O/p in Visual C++ :- 0 ,0 ,0.000000

gcc compiler :- 0,0, 11.000000

Answer 1

When you call a variadic function like printf , float s undergo promotion to double . int s are passed as-is. printf therefore expects a double when you write %f , and an int when you write %d .

Not giving it a double , but an int , is therefore undefined behaviour . Similarly, passing a double when the function expects an int is also undefined.

As usual, undefined behaviour means "anything could happen". Never, ever rely on undefined behaviour.

Answer 2

You are playing with undefined or unspecified behavior. Not sure which one of them it is. On my Debian with gcc 4.7.2, I get -780714744, 4195886, 11.000000 on the output.