Problems with pointers allocation when dynamically build a binary tree in C

Question

studying C for few months, I encounter some difficulties with the use of pointers when dynamically building a binary tree:

Given my code below:

typedef struct TNoeud
{
    int data;
    struct TNoeud *pFilsGauche;
    struct TNoeud *pFilsDroit;
} TNoeud;


void insereData(int data, TNoeud **pRacine)
{
    TNoeud *noeud=malloc(sizeof(TNoeud));

    noeud->data=data;
    noeud->pFilsDroit=NULL;
    noeud->pFilsGauche=NULL;

    while((*pRacine)!=NULL)
    {
        if(data<(*pRacine)->data)
        {
            pRacine=&(**pRacine).pFilsGauche;
        }
        else
        {
            pRacine=&(**pRacine).pFilsDroit;
        }
    }
    if(pRacine==NULL)
    {
        *pRacine=noeud;
    }
    free(noeud);
}

And in the main:

int main(int argc, const char * argv[]) {

    TNoeud *pRacine=malloc(sizeof(TNoeud));

    pRacine->data=0;
    pRacine->pFilsGauche=NULL;
    pRacine->pFilsDroit=NULL;

    pRacine=&noeudRacine;

    insereData(4, &pRacine);

    return 0;
}

I read the following topic https://stackoverflow.com/a/28637104/7866010 for the BAD_ACCESS, but in my case, the pointer is not at NULL, as pRacine is assigned at 0.

I read the following topic https://stackoverflow.com/a/15154553/7866010 , but it didn't help.

I also tried the declaration variant

(*pRacine)->data

found in this topic https://stackoverflow.com/a/346739/7866010 without any difference.

So my questions are :

[SOLVED with TNoeud noeud as pointer instead of local variable. I also changed pRacine in the main the same way] Why do the pointer
```
*pRacine == NULL
```
when I pass a pointer to an assigned value as parameter of
```
insereData(4, &pRacine) ?
```
[SOLVED the same way] Why does the debugger give me random values to pointers
```
[1] = 0x00007fff5fbff700)
```
and datas
```
(int) data = 1606416544)
```
I didn't willingly assigned ?
[SOLVED: by deleting the if(pRacine==NULL) condition and replacing it by just (*pRacine)=noeud;] Now no more errors, but the result of
```
insereData(4, &pRacine);
```
doesn't impact pRacine : it should be
```
pRacine->pFilsDroit->data==4
```
but here it remains at NULL. I don't understand why, as it's not a local variable anymore.

Thanks all for your answers!

`TNoeud noeud;` is local auto variable. Its lifespan is through the scope(only inside function). — BLUEPIXY, Apr 16 '17 at 20:36
Your variable "TNoeud noeud" disappears as soon as the function it is defined in exits. Any pointer that used to point to it now points to a black hole. Any attempt to examine such pointer leads to a violation of the laws of physics. This is all explained im any C book. Get one. — n. m. could be an AI, Apr 16 '17 at 20:37

score 0 · Accepted Answer · answered Apr 16 '17 at 20:45

0

Some suggestions(not law).
First in order to not confuse yourself about pointers, just work with them as arrays. it really works and won't confuse you.
For instance given pointer int* ptr, for accessing the first element go as ptr[0].

And the problem is here

if(pRacine==NULL)
    {
        *pRacine=&noeud;
    }

as the noeud is not in dynamic memory, it gets halted.
You simply need to define the noeud as a pointer of the struct by malloc. But for memory sake, please keep an eye for free it once it's not needed.

answered Apr 16 '17 at 20:45

Ok, I edited it in my question to `TNoeud *noeud=malloc(sizeof(TNoeud));` and `free(noeud);`. – dxp Apr 17 '17 at 09:26
@dxp very awesome you could fix the problem yourself, perfect. Just one note again, please have an eye over `malloc` all the time. becasue you need to `free` the allocated memory yourself once it's not required. – Apr 17 '17 at 12:53

Problems with pointers allocation when dynamically build a binary tree in C

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