Struct inside union inside struct in c

Question

hello lets say i have this code

typedef struct entry {
    union {
        struct A {
            char  *c;
        } *A;
        struct B {
            char *c;
        } *B;
    } value;
} *TableEntry;

i m doing a malloc for entry and now i want to copy a string to c from struct A . do i have to allocate memory for struct A and then for c or the first malloc for table entry allocates for all of them ? thank you in advance

Its not C++. You can argue that C is a subset of C++, and you would be wrong. — littleadv, May 04 '14 at 21:21
@littleadv: It is valid C++, so give a **good** reason to remove the tag. — Deduplicator, May 04 '14 at 21:22
@Deduplicator as I said - you can argue that and you would be wrong. One good reason: the OP said its C. — littleadv, May 04 '14 at 21:23
Ok, reacted to strongly on seeing those cannot be C++ assertions. The title says the OP is only interested in C. That's the only C specific part of the question and all answers though. — Deduplicator, May 04 '14 at 21:29

score 3 · Answer 1 · answered May 04 '14 at 21:17

3

you have to allocate memory for both of them

answered May 04 '14 at 21:17

user3520514

88
7

score 2 · Answer 2 · answered May 04 '14 at 21:18

2

When you allocate the TableEntry - you allocate the memory for the whole union. Pointers in it are allocated, but what they point to - are not. So you should assign values that you allocate to c members of the struct and A/B members of the union.

Note that the A and B share the same space.

answered May 04 '14 at 21:18

littleadv

20,100
2
36
50

so we have a malloc for the whole struct and then only for c is that right ? – user3187860 May 04 '14 at 21:19
@user3187860 yes. And you only allocate one struct, either A or B - you cannot have both. – littleadv May 04 '14 at 21:20

score 0 · Answer 3 · answered May 22 '14 at 04:30

To clarify, three allocs are needed, e.g.:

TableEntry *t = malloc(sizeof *t);
t->A = malloc(sizeof *t->A);
t->A->c = malloc(50);

This design is questionable though, as there is no way of telling which is currently active out of A or B. You will have to have another index or something which keeps track of whether this entry is an A or a B.

Struct inside union inside struct in c

3 Answers3