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cstructunionsc11offsetof

How to portably allocate space for a particular member of a union embedded in a struct

Consider the following type in C11, where MyType1 and MyType2 are previously declared types:

typedef struct {
  int tag;
  union {
    MyType1 type1;
    MyType2 type2;
  }
} MyStruct;

I'd like to allocate enough memory using malloc to hold the tag attribute and type1. Can this be done in a portable way? I guess, sizeof(tag) + sizeof(type1) may not work due to alignment issues.

Can I calculate the offset of type1 from the beginning of the structure in a portable way?

Solution

  • You can use offsetof(), and since that will include both the size of tag and any padding, it's enough to then add the size of type1:

    void *mys = malloc(offsetof(MyStruct, type1) + sizeof (MyType1));