Malloc & Init - MACRO

Question

Is it possible to rewrite the following code, so that it would be ISO C compliant ? The following macros are doing a malloc & init for a given type and value.

The current code works with the gcc compilers (uses a gcc extension), but it's not standard. If I use -pedantic, I receive warnings.

#ifdef __GNUC__

#define NM_CVPTR(type, value) \
    ({ \
        type * var = NULL; \
        var = nm_malloc(sizeof(*var)); \
        *var = value; \
        (const void*) var; \
    }) \

#define NM_VPTR(type, value) \
    ({ \
        type * var = NULL; \
        var = nm_malloc(sizeof(*var)); \
        *var = value; \
        (void*) var; \
    }) \

#define NM_PTR(type, value) \
    ({ \
        type * var = NULL; \
        var = nm_malloc(sizeof(*var)); \
        *var = value; \
        (type *) var; \
    }) \

#endif

Answer 1

You can use memcpy to assign a value and then have the pointer returned. The following uses two different versions depending on whether or not your initial value is a primitive type (integer,float, pointers...) or if it is a struct. The value version uses a compound literal (type){ (value) }, so it is only valid in C99.

#include <stdlib.h>
#include <string.h>

static inline
void* memcpy_safe(void* target, void const* source, size_t n) {
  if (target) memcpy(target, source, n);
  return target;
}

#define NM_PTR_RVALUE(type, rvalue)                                     \
  ((type*)memcpy_safe(malloc(sizeof(type)), &(type){ (rvalue) }, sizeof(type)))

#define NM_PTR_LVALUE(type, lvalue)                                     \
  ((type*)memcpy_safe(malloc(sizeof(type)), &(lvalue), sizeof(type)))

typedef struct {
  int a;
} hoi;

hoi H7 = {.a = 7};

int main() {
  int* a = NM_PTR_RVALUE(int, 7);
  hoi* b = NM_PTR_LVALUE(hoi, H7);
}

(Added a NULL check here that uses an inline function, although this wasn't requested originally.)

BTW, in C++ in contrast to C the assignment operator = returns an lvalue, so for C++ you could probably play games with that.