Expected behavior for using pointer beyond malloc allocated memory

Question

Just wondering, because I can't figure out a way to test this. Imagine the scenario whereby I have 10 bytes of memory

I malloc varA with 5 bytes
Assign a string with 7 characters (which use up 8 bytes)
I malloc varB with 5 bytes

Will the program run into an error? Or just end up with gibberish memory?

Does the behaviour varies from a c/c++ and a cuda program?

This question is basically "What will my program do if there's a bug in it?". The answer is basically "Probably not what you want or expect. That's why we avoid bugs." — David Schwartz
– David Schwartz, Commented Mar 16, 2014 at 9:33
You don't do this! This is not even a possibility. When you have a piece of paper and you write on it, when it's full you don't continue on the desk... or your mom will punish you. Same here! — CodeAngry
– CodeAngry, Commented Mar 16, 2014 at 9:34
Leaks are when you loose grip of pointers you allocated and did not free. That memory ends up in limbo. Overflows are when you overstep your boundaries writing. Both are deadly: first is a slower death, second not so much. — CodeAngry
– CodeAngry, Commented Mar 16, 2014 at 9:36
Haha, I like your comments. I pondered upon this because I was running someone else's source (on cuda) and it gave me serious headache because it worked while such a thing happened. But as you already know, when I continuously run the program... problems.... — springcold
– springcold, Commented Mar 16, 2014 at 9:38
@springcold Go tell that guy that pointers are not just a starting point of writing. They have a SIZE and you don't overstep it. So it has both a beginning and an ending. — CodeAngry
– CodeAngry, Commented Mar 16, 2014 at 9:41

Some programmer dude · Accepted Answer · 2014-03-16 09:31:00Z

7

That's not a memory leak, it's a buffer overflow. And those leads to undefined behavior, which will most likely give you weird problems (or even crashes) during run-time.

Unless you mean point 2 literally, like in

char *str = malloc(5);
str = "foobar";

Then you do have a memory leak, and not a buffer overflow.

answered Mar 16, 2014 at 9:31

Some programmer dude

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3 Comments

user2485710 Over a year ago

I think that is safe to say that this is undefined behaviour, I don't know if it's equally true the fact that this will cause buffer overflow.

springcold Over a year ago

May I ask, will the behaviour likely be the same in Cuda as well?

πάντα ῥεῖ Over a year ago

@springcold The behavior is the same with any C-library.

Balu · Accepted Answer · 2014-03-16 09:36:43Z

0

It is an undefined behavior to write beyond allocated memory.

answered Mar 16, 2014 at 9:36

Balu

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