Question

I am having trouble with some code in C accessing the contents of this chain of pointers:

I have these structs:

typedef struct {
    unsigned int hash;
    char string[10]; 
    void *memory;
} thing;

typedef struct {
    int num; 
    thing *thing;
} node;


typedef struct {
    int size;
    thing* things;
    node* nodes;
} t_system;

Ok. Now I initialize everything like this:

thing* things = NULL;
things = calloc(10, sizeof(thing));

node* nodes = NULL;
nodes = calloc(10, sizeof(node));

t_system* theSystem = NULL;
theSystem = calloc(1, sizeof(t_system));

    theSystem->things = things;
    theSystem->nodes = nodes;

And now, I want to set this:

theSystem->nodes[2].thing = &theSystem->things[1];

After that line, if I debug and set a breakpoint theSystem nodes points to 0x0

Where am I going wrong?

---

if (theSystem->nodes[2].thing == NULL) {
    theSystem->nodes[2].thing = &theSystem->things[1]; //this is executed
}
if (theSystem->nodes[2].thing == NULL) {
    //this is not executed...
}

---

I can do this:

theSystem->nodes[2].thing->hash = 123;

And debugging shows the correct value for hash, and thing, but not for nodes. it points to 0x0.

Answer 1

You initialize nodes to NULL on this line:

node* nodes = NULL; 

Then you assign nodes to theSystem->nodes on this line:

theSystem->nodes = nodes; 

Since you never change the value of nodes in between, theSystem->nodes will also be NULL.

Are you sure the following line is correct:

tree = calloc(10, sizeof(node));  

Shouldn’t you assign this to nodes instead?