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

This can be done using the comma operator, but in standard C you won’t be able to declare a variable as part of an expression, so you will have to pass the name of var‘s replacement to the macro:

// C - comma operator but not able to declare the storage during the
// expression.
#define NM_PTR(type, var, value) \
    (var = nm_malloc(sizeof(*var)), \
    *var = value, \
    (type * var))