C vs C++ placing structs in unsigned char buffer

Question

Does C have anything similar to C++ where one can place structs in an unsigned char buffer as is done in C++ as shown in the standard sec. 6.7.2

template<typename ...T>
struct AlignedUnion {
  alignas(T...) unsigned char data[max(sizeof(T)...)];
};
int f() {
  AlignedUnion<int, char> au;
  int *p = new (au.data) int;           // OK, au.data provides storage
  char *c = new (au.data) char();       // OK, ends lifetime of *p
  char *d = new (au.data + 1) char();
  return *c + *d;                       // OK
}

In C I can certainly memcpy a struct of things(or int as shown above) into an unsigned char buffer, but then using a pointer to this struct one runs into strict aliasing violations; the buffer has different declared type.

So suppose one would want to replicate the second line in f the C++ above in C. One would do something like this

#include<string.h>
#include<stdio.h>
struct Buffer {
  unsigned char data[sizeof(int)];
};

int main()
{ 
  struct Buffer b;
  int n = 5;
  int* p = memcpy(&b.data,&n,sizeof(int));
  printf("%d",*p);  // aliasing violation here as unsigned char is accessed as int
  return 0;
}

Unions are often suggested i.e. union Buffer {int i;unsigned char b[sizeof(int)]}; but this is not quite as nice if the aim of the buffer is to act as storage (i.e. placing different sized types in there, by advancing a pointer into the buffer to the free part + potenially some more for proper alignment).

Answer 1

Have you tried using a union?

#include <string.h>
#include <stdio.h>

union Buffer {
    int int_;
    double double_;
    long double long_double_;
    unsigned char data[1];
};

int main() {
    union Buffer b;
    int n = 5;
    int *p = memcpy(&b.data, &n, sizeof(int));
    printf("%d", *p);  // aliasing violation here as unsigned char is accessed as int
    return 0;
}

The Buffer aligns data member according the type with the greatest alignment requirement.