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In this non compliant code a struct of type A is allocated on the memory region of s.B. The problem is that A has a member of type unsigned long which on IA32 systems is required to be aligned to 4. However B is an array of unsigned chars inside S and is not aligned to 4. After the call to new, the pointer a is pointing to an unaligned memory address. Writing val into a->i can cause the program to terminate abnormally.

int num = 5;
if (num * sizeof(SomeClass) > SIZE_MAX) {
  /* handle error */
}
SomeClass *sc = static_cast<SomeClass*>(malloc(sizeof(SomeClass)*num));
if (sc == NULL) {
  /* handle error */
}
// initialize sc array
// ...
// destroy sc
free(sc);
struct A{
	unsigned long i;
};

struct S{
	unsigned char x;
	unsigned char B[sizeof(A)];
}s;


int main()
{
	A *a = new(&s.B[0]) A;
	unsigned long val = 0xaabbccdd;
	a->i = val;
	return (0);
}

Compliant Solution

The following is a compliant solution on a IA32 system using either GCC or Microsoft's Visual Studio. In this solution the macro ALIGN was added to ensure alignment when compiling with both Microsoft VS compiler and g++ . The ALIGN macro calls either _declspec(align(#)) or _attribute_((aligned(#)))) to enforce alignment on a particular variable. In this case it is used on B to make it aligned to 4, which is the alignment restriction for unsigned long.

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