Setting struct variables - C - c

Question

I have some code in the following kind of layout, I believe that the topExample/botExample aren't being set properly when I call addTopBotExample. I think this is due to the top bot variables being on the functions stack and so being cleared when the function ends? I have a feeling that perhaps I need to malloc the memory first, but am not sure how I would go about doing this are even if its the right approach.
typedef struct Example Example;
struct Example {
/* normal variables ...*/
Example *topExample;
Example *botExample;
};
....
void addTopBotExample(Example **example, int someVariable) {
Example top = createTopExample(int someVariable); //(createTopExample returns a
//type Example based on some input)
Example bot = createBotExample(int someVariable);
(*example)->topExample = ⊤
(*example)->botExample = ⊥
return;
}

Answer 1

If createTopExample isn't allocating memory, this is going to cause problems the moment it's called more than once. Rewrite createTopExample and createBotExample to use malloc and return an Example*. Something like this:
Example* createTopExample(stuff)
{
Example *example = malloc(sizeof(Example));
// ... stuff you do
return example;
}
Then your addTopBotExample would look like this:
void addTopBotExample(Example **example, int someVariable) {
if ((*example)->topExample)
free((*example)->topExample)
if ((*example)->botExample)
free((*example)->botExample)
(*example)->topExample = createTopExample(int someVariable);
(*example)->botExample = createBotExample(int someVariable);
return;
}
Note that this addTopBotExample will free the allocated memory before calling malloc again but before your program ends, you need to call free on any lingering Examples that used this addTopBotExample function:
free(exampleInstanceThatWasPassedIntoAddTopBotExampleAtSomePoint.topExample);
free(exampleInstanceThatWasPassedIntoAddTopBotExampleAtSomePoint.botExample);

Answer 2

You have already everything together. Allocate the Example in createTopExample or createTopExample
Example *createTopExample(int someVariable)
{
Example *x = malloc(sizeof(Example));
/* initialize x */
return x;
}
and in addTopBotExample
void addTopBotExample(Example *example, int someVariable) {
Example *top = createTopExample(int someVariable); //(createTopExample returns a
//type Example based on some input)
Example *bot = createBotExample(int someVariable);
example->topExample = top;
example->botExample = bot;
return;
}

Answer 3

Ooooo, this is bad. The expression "Example top" in the addTopBotExample() function allocated that object on the stack. It'll be trashed after exiting from the function. (Same for "Example bot" on the following line.) Something like this will work better:
void addTopBotExample(Example **example, int someVariable) {
Example *top = createTopExample(someVariable); // NOTE THE *
Example *bot = createBotExample(someVariable); // NOTE THE *
(*example)->topExample = top; // NOT &top !!
(*example)->botExample = bot; // NOT &bot !!
return;
}
And you'll want to write createTopExample and createBotExample so they return pointers:
#include <stdlib.h> // For malloc!
Example *createTopExample(stuff) // Note *. It's returning a pointer.
{
Example *example = malloc(sizeof(Example)); // Allocate on the HEAP. Lives after this function call.
// Fill in the fields of example.
example->field1 = 25; // Note the "->": you're dereferencing a pointer.
example->title = "Example title";
return example;
}