when _malloca can not allocate memory on heap

Question

When alloca can't allocate memory on heap it creates structured exception stackoverflow and program halt with Stackoverflow. Ok. But when _malloca can not allocate memory on heap it says nothing. I allocate great amount of memory and after that use it, but get access violation exception. Example

#include <stdio.h>
#include <malloc.h>
#include <conio.h>

void foo(size_t n) {
    int *arr = (int*) _malloca(n*sizeof(int));
    size_t i;

    for (i = 0; i < n; i++) {
        arr[i] = rand();
    }
    for (i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    _freea(arr);
}

void main() {
    foo(900000000000);
    _getch();
}

BUT when I use only part of allocated memory, I get no exceptions at all. Example

#include <stdio.h>
#include <malloc.h>
#include <conio.h>

void foo(size_t n) {
    int *arr = (int*) _malloca(n*sizeof(int));
    size_t i;

    for (i = 0; i < 100; i++) {
        arr[i] = rand();
    }
    for (i = 0; i < 100; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    _freea(arr);
}

void main() {
    foo(900000000000);
    _getch();
}

VSE2013 WinDesktop. Oki, you may say, try to catch exception

#include <stdio.h>
#include <malloc.h>
#include <conio.h>
#include <windows.h>

void foo(size_t n) {
    int *arr = NULL; 
    size_t i;

    __try {
        arr = (int*)_malloca(n*sizeof(int));
    } __except (GetExceptionCode() == STATUS_STACK_OVERFLOW) {
        int errcode;
        printf("_malloca failed!\n");
        _getch();
        errcode = _resetstkoflw();
        if (errcode) {
            printf("Could not reset the stack!");
            _getch();
            _exit(1);
        }
    }
    for (i = 0; i < 100; i++) {
        arr[i] = rand();
    }
    for (i = 0; i < 100; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    _freea(arr);
}

void main() {
    foo(900000000000);
    _getch();
}

but it continue to work. And if use all element of array then again get access violation.
Question: it is bug or a feature?

Answer 1

Yey! I was right that _malloca returns NULL when can not allocate memory using malloc on heap. The problem is in call, that is stupid(((

foo(900000000000);

is invalid, because it is bigger than size_t size on my computer. malloc.h use this function to check size is normal

__inline int _MallocaIsSizeInRange(size_t size)
{
    return size + _ALLOCA_S_MARKER_SIZE > size;
}

when I call

foo(INT_MAX);

it returns NULL, as memory can not be allocated.