Code::Blocks - Process Returned -1073741819 (0xc0000005) - c++

Question

I don't understand what this line of text is referring to "Process Returned -1073741819 (0xc0000005".
CMD Execution
It is coincident with the following warning in my log:
Log Warning
"Warning: no return statement in function returning non-void [-Wreturn-type]"
This is the code:
#include<iostream>
using namespace std;
float milesToKm(float distance)
{
return distance*1.60934;
}
float milesToKm_(float& distance)
{
distance = distance*1.60934;
}
int main()
{
float distance;
cout << "Enter the distance in miles \n";
cin >> distance;
cout << "\nDistance in km is : " << distance*1.60934 ;
cout << "\nDistance in km is : " << milesToKm(distance) ;
milesToKm_(distance);
cout << "\nDistance in km is : " << distance;
return 0;
}
Can someone explain what this means to me? Do so the same way you'd explain it to a lobotomized Australian Dung Beetle
I copied this code into Code::Blocks, I was not expecting a warning on line 12. I am also not entirely sure if the prior "Process Returned" text is related to it.

Answer 1

Process returned 0xc0000005 means it has crashed with an Access Violation.
Why would this code crash?
The warning ":12:1: warning: no return statement in function returning non-void [-Wreturn-type]" sounds like gcc, so assuming that.
Let's look at the disassembly: https://godbolt.org/z/1x7vadoj7
milesToKm_(float&):
push rbp
mov rbp, rsp
//middle stuff removed
movss DWORD PTR [rax], xmm0
ud2
Now ud2 is an interesting instruction. This is an explicit undefined instruction that, if reached, will produce an illegal instruction exception and crash the process.
That's right, gcc made sure that if you fall off the end of the function without returning the float value you promised, your program will crash.
If you enable optimizations with -O2, something even more worrying happens:
milesToKm_(float&):
cvtss2sd xmm0, DWORD PTR [rdi]
mulsd xmm0, QWORD PTR .LC0[rip]
cvtsd2ss xmm0, xmm0
movss DWORD PTR [rdi], xmm0
.LC1:
.string "Enter the distance in miles \n"
.LC2:
.string "\nDistance in km is : "
The control just falls off the end of the function (as there is no RET instruction) and the CPU starts executing some strings or whatever happens to be in the memory above.
The "no return statement" warning may be somewhat survivable in C mode, in C++ this is almost always an error. I would recommend to add -Werror=return-type to your compile options for C++ to treat it as the error it is.
So how do you make it not crash? If you're not interested in returning a value, declare the function void:
void milesToKm_(float& distance)
{
distance = distance*1.60934;
}