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I have created this sensational program: #include <stdio.h> int* func (void) { int local=5; return &local; } int main (void) { printf("%d\n", *func()); } This prints 5 even thoug...
#1: Initial revision
by
Lundin
·
2020-08-25T06:34:53Z (about 4 years ago)
What is undefined behavior and how does it work?
I have created this sensational program:
#include <stdio.h>
int* func (void)
{
int local=5;
return &local;
}
int main (void)
{
printf("%d\n", *func());
}
This prints `5` even though I'm returning a pointer to a local variable. It did not produce an error!
And then I made another sensational program:
int main (void)
{
int arr[5] = {1,2,3,4,5};
for(int i=0; i<10; i++)
{
printf("%d ", arr[i]);
}
}
It did not produce an error! But it prints 1 2 3 4 5 and then some other stuff.
I was told that these cases where examples of "undefined behavior" and that we aren't allowed to write code like that.
But it works! The compiler did not protest, the OS didn't give me a "segmentation fault" and nothing bad happened. Isn't some sort of error supposed to happen when we do things like these?
What is this "undefined behavior" and who is responsible for taking care of it?