The purpose of union with only one member inside the struct

Question

I saw such union inside struct definition in pure c code in Linux kernel sources struct cma_multicast (it's not the only one place. Seems that it is some common practice):

struct cma_multicast {
    struct rdma_id_private *id_priv;
    union {
        struct ib_sa_multicast *ib;
    } multicast;
    struct list_head    list;
    void            *context;
    struct sockaddr_storage addr;
    struct kref     mcref;
};

But I can't figure out what is the purpose of union with only one member inside the struct? Why can't we just type struct ib_sa_multicast *ib; ?

I read this post but it has no usage explanation and has C++ specificity only.

UPD:
Posted example from Linux kernel instead of proprietary code.

Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackoverflow.com/rooms/219214/discussion-on-question-by-budoattack-the-purpose-of-union-with-only-one-member-i). — Machavity, Aug 04 '20 at 18:08

score 2 · Answer 1 · answered Aug 03 '20 at 15:54

2

This says in this case multicast can have only one polymorphic dispatcher.

It is object oriented programming made in C. The union is kept only for uniformity of naming.

answered Aug 03 '20 at 15:54

alinsoar

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The purpose of union with only one member inside the struct

1 Answers1