Pointer to struct with pointer to array of pointers to structs. Dynamic memory in C

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.

You are making something up here. The line theSystem->nodes[2].thing = theSystem->things[1] will not even compile. Left-hand side is a pointer. Right-hand side is a struct. Please, don't make up code. Post the real code or at least something sufficiently close to real code. — AnT stands with Russia
– AnT stands with Russia, Commented Jul 17, 2012 at 6:46
That is exactly why im here. if (theSystem->nodes[2].thing == NULL) is false, but debugging says otherwise. — Nicolas S
– Nicolas S, Commented Jul 17, 2012 at 6:50
No, no, no. Again, the code you posted will not compile. It cannot be run or debugged. The line I referred to is not compilable by any existing compiler, unless you are using something totally unknown. What compiler are you using? — AnT stands with Russia
– AnT stands with Russia, Commented Jul 17, 2012 at 6:52
Well, I don't know what kind of major malfunction in GCC allows one to compile theSystem->nodes[2].thing = theSystem->things[1] line, but in any case that assignment makes no sense whatsoever. You cannot assign a struct to a pointer. The code is simply meaningless. What do you expect this assignment should do? — AnT stands with Russia
– AnT stands with Russia, Commented Jul 17, 2012 at 6:58

ecatmur · Accepted Answer · 2012-07-17 06:39:13Z

2

You wrote

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

You should have written

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

answered Jul 17, 2012 at 6:39

ecatmur

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1 Comment

Nicolas S Over a year ago

I have update the code to reflect this fix, please take a look.

Patrick · Accepted Answer · 2012-07-17 06:40:07Z

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?

answered Jul 17, 2012 at 6:40

Patrick

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1 Comment

Nicolas S Over a year ago

Yes that was wrong, but just in this question. The actual code is correct, i have updated the main question.

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