CODEWITHSUNDEEP
cstruct
Robert Simmons Jr.
Robert Simmons Jr.

Reputation: 1182

Confused about updating a struct member in C

It has been a LONG time (25y) since I have done C and so I forget some things so please forgive the question.

Given that I have the following declarations: 

typedef struct item {
  int field;
} Item;
typedef struct data {
  Item b;
} Data;

I have been trying to update the struct when its passed to a function and this doesn't work at all. 

static void foo(Data *data) {
  data->b.field = 3; // doesn't work, the struct remains unchanged.
}
static void test() {
  Data v = {.b = {.field = 2}};
  foo(&v);
}

However, if I alter the declaration slightly, use malloc to allocate it it works.

typedef struct data {
  Item *b;
};
static void foo(struct data *data) {
  data->b->field = 3; // works.
}
static void test() {
  Data v = (struct data*) malloc(sizeof(Data));
  Item i = (struct item*) malloc(sizeof(Item));
  foo(v);
  free(i);
  free(v);
}

Can someone inform me why this is? Is it not possible to have struct members that are updatable as members? How could I make the first example work?

Thanks in advance.

Upvotes: 0

Views: 170

Answers (1)

Stephan Lechner
Stephan Lechner

Reputation: 35154

Your first approach actually works (and I would have been surprised if it did not):

struct item {
    int field;
};

struct data {
    struct item b;
};

static void foo(struct data *data) {
    data->b.field = 3;
}
static void test() {
    struct data v = {.b = {.field = 2}};
    printf("v.b.field before calling foo: %d\n", v.b.field);
    foo(&v);
    printf("v.b.field afterwards: %d\n", v.b.field);
}

int main() {
    test();
}

Output:

v.b.field before calling foo: 2
v.b.field afterwards: 3

Probably your setting is a different one that than you've shown in the code. Mysterious things (i.e. undefined behaviour) often happens if you access an object after it's lifetime has ended. malloc often prevents such issues as it keeps an object alive until it is explicitly freed.

But in your case, there should not be any difference.

BTW: the typedef does not make sense, as you do not define an alias for the struct-type just declared. So 

struct item {
    int field;
};

is sufficient.

Upvotes: 6

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