returning pointer to a local variable

Question

What happens when a pointer to a local variable is returned by a function?

for example

int* foo()
{
    int local;
    int* ptr = &local;
    return ptr;
}

will compiler issue a warning or will it compile and produce unexpected results??

int* foo() { int local = 42; int* ptr = &local; return ptr; } int main () { return *(foo()); } — Eliot B., Dec 06 '16 at 08:54
This should be obvious: the lifetime of a local variable is the function invocation. — , Dec 06 '16 at 09:04
"why don't you try?" is actually a bad advice in this case. It is asked before and it is undefined behavior meaning that trying it can get different results. — stefaanv, Dec 06 '16 at 09:13

artm · Answer 1 · 2016-12-06T09:00:55.330

4

What happens when a pointer to a local variable is returned by a function?

Undefined behaviour. Anything can happen. The compiler will give you a warning.

This is g++ warning for that mistake:

g++ -Wall -std=c++11 -O3 test.cpp -o test
warning: function returns address of local variable [-Wreturn-local-addr]

edited Dec 06 '16 at 09:00

answered Dec 06 '16 at 08:55

artm

17,291
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GCC doesn't give any warning, even with -Wall -Wextra -Werror – Eliot B. Dec 06 '16 at 08:56
@WARhead Please read this on what is undefined behavior: http://stackoverflow.com/questions/2397984/undefined-unspecified-and-implementation-defined-behavior – alexeykuzmin0 Dec 06 '16 at 08:56
You should use g++ for .cpp file – artm Dec 06 '16 at 08:56
1

*should give a warning. static analyzer really really should give a warning. otherwise they really failed :) – jonas_toth Dec 06 '16 at 08:57
Yep, I didn't notice the c++ tag, sorry @artm – Eliot B. Dec 06 '16 at 08:57
@EliotB. there is a warning, see the `g++` flag – artm Dec 06 '16 at 09:01
1

Both GCC and Clang warn for `return &local;`, but not for `int* p = &local; return p;` cc @EliotB. – chris Dec 06 '16 at 09:03
@chris I read your comments more carefully now. That's strange `gcc` and `clang` doesn't give a warning; but `g++` does – artm Dec 06 '16 at 09:08

score 2 · Accepted Answer · edited May 23 '17 at 12:01

2

Similar kind of question has already been asked : Stack Overflow, local pointer

There are somethings in C which are left for compiler vendor to implement in the way they like. The behaviour of such things are not defined by the creators. Compiler vendors can implement such things as in the way they feel easy and faster. This falls in the same category. The behaviour is undefined and depends on the compiler you are using.

One more such thing is use of more than one pre-increment (pre-decrement) and/or post-increment (post-decrement). When the same code runs on different compilers, you'll may get different output.

edited May 23 '17 at 12:01

Community

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answered Dec 06 '16 at 09:02

AnkitAti

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For more information on UB, better refer to http://stackoverflow.com/questions/2397984/undefined-unspecified-and-implementation-defined-behavior where you can find all the info you need – stefaanv Dec 06 '16 at 09:39

returning pointer to a local variable

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