Can a char array be used with any data type?

Question

The malloc() function returns a pointer of type void*. It allocates memory in bytes according to the size_t value passed as argument t

Answer 1

No you cannot use an arbitrary byte array for an arbitrary type because of possible alignment problems. The standard says in 6.3.2.3 Conversions/Pointers (emphasize mine):

A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned for the pointed-to type, the behavior is undefined. Otherwise, when converted back again, the result shall compare equal to the original pointer.

As a char as the smallest alignment requirement, you cannot make sure that your char array will be correctly aligned for any other type. That is why malloc guarantees that a buffer obtained by malloc (even if it is a void *) has the largest possible alignement requirement to be able to accept any other type.

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I think that

union {
    char buf[128];
    long long i;
    void * p;
    long double f;
};

should have correct alignment for any type as it is compatible with largest basic types (as defined in 6.2.5 Types). I am pretty sure that it will work for all common implementations (gcc, clang, msvc, ...) but unfortunately I could not find any confirmation that the standard allows it. Essentially because of the strict aliasing rule as defined in 6.5 Expression §7:

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:

• a type compatible with the effective type of the object,
• a qualified version of a type compatible with the effective type of the object,
• a type that is the signed or unsigned type corresponding to the effective type of the object,
• a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,
• an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or
• a character type.

So IMHO there is not portable and standard conformant way to build a custom allocator not using malloc.