Dynamically created string allocated on heap or stack - C

Question

Context

I was experimenting with getting C strings in C++ without allocating memory on the heap and came across this in testing:

#include <stddef.h>
#include <stdlib.h>

char* get_empty_c_string(size_t length) {
    char buffer[length];
    char *string = buffer;

    for (size_t i = 0; i ^ length; i++) *(string + i) = '\0';

    return string;
}

int main(void) {
    char *string = get_empty_c_string(20u); // Allocated on heap?
                                            // or stack?
    return 0;
}

Question

Is the C string returned allocated on heap or stack?

As far as I know:

Heap allocation occurs with the calloc, malloc & realloc C standard functions or new & new[] C++ keywords.
Stack allocation in most other cases.

Variable length array is valid only in C, not C++ as mentioned above. — openingnow
– openingnow, Commented Dec 13, 2019 at 2:47
And even if we are assuming that variable-length array such as this are valid in C++ or that the code is C, you are returning a pointer to stack-allocated memory that will be invalid as soon as the function returns. string is not usable in main. — walnut
– walnut, Commented Dec 13, 2019 at 2:48
I ran this code on GCC with all warning flags set using -Wall and this was not mentioned... — Lapys
– Lapys, Commented Dec 13, 2019 at 2:49
@Lapys GCC supports VLAs as compiler extension by default. In order for GCC to behave standard-conform, you need to add the -pedantic-errors flag. — walnut
– walnut, Commented Dec 13, 2019 at 2:50

dbush · Accepted Answer · 2019-12-13 02:50:14Z

1

The array buffer is a variable length array (VLA), meaning its size is determined at runtime. As a variable local to a function is resides on the stack. The pointer string then points to that array, and that pointer is returned. And because the returned pointer points to a local stack variable which goes out of scope, attempting to use that pointer will invoke undefined behavior.

Also, note that VLAs are a C only feature.

answered Dec 13, 2019 at 2:50

dbush

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Comments

walnut · Accepted Answer · 2019-12-13 03:00:17Z

1

There is no way in standard C++ to obtain runtime-sized memory of automatic storage duration (which usually maps to stack memory).

Therefore a proper string of any length cannot be obtained on the stack. You can only allocate a buffer with a maximal size and use strings up to that length in the program. (Something similar is usually done by std::string as so-called short string optimization.)

Furthermore, you cannot return pointers or references to variables with automatic storage duration from a function. When the function returns the variables are destroyed and the pointer/reference becomes invalid. You can only ever use the stack-allocation until the function returns. You can however return the variable by-value.

answered Dec 13, 2019 at 3:00

walnut

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1 Comment

Lapys Over a year ago

<i>Short string optimization</i>? Sounds like another interesting thing to learn

jhill515 · Accepted Answer · 2019-12-13 02:49:12Z

0

As @PaulMcKenzie points out, your implementation of get_empty_c_string() would fail to compile: In essence, arrays as temporary/instance variables of a function need to have a static size defined for them prior to compile time. This is because that volume of memory is pushed onto the stack at function invocation.

I can see that you're trying to have dynamic memory allocation as part of the function itself, which is why you need such heap-allocators.

answered Dec 13, 2019 at 2:49

jhill515

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1 Comment

Lapys Over a year ago

Hmm, understood. Without the -pedantic-errors flags though, the code compiles freely; As @walnut says, GCC supports VLAs as compiler extensions by default apparently

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