声明结构时，总是将其设置为NULL？

Question

I'm declaring a pointer to a struct. When I'm printing its value it's not set to NULL and I get random numbers. Is this normal? You just need to look at the printf on line 3. I don't even use this pointer. I declared it just to see what happens because I was getting wrong values. I don't have any other pointers with the same name, and this function is being called correctly.

child * find_closer(int cid,child * root){
child *closer,*right,*left,*min,*not;
if(not!=NULL)
printf("    %d   ",not);
if(root!=NULL){
    if(root->cid < cid)
        closer=root;
    else
        min=root;
    if(root->lc!=NULL){
        if(closer==NULL || (root->lc->cid  <  cid  && closer->cid < root->lc->cid))
            closer=root->lc;
        if(min==NULL || min->cid > root->lc->cid)
            min=root->lc;
    }
    if(root->rc!=NULL){
        if(closer==NULL || (root->rc->cid  <  cid  && closer->cid < root->rc->cid))
            closer=root->rc;
        if(min==NULL || min->cid > root->rc->cid)
            min=root->rc;
    }
    if(closer==NULL){
        closer=min;
    }
    right=find_closer(cid,root->rc);
    left=find_closer(cid,root->lc); 
    if(closer->cid < cid){ 
        if(right!=NULL &&right->cid < cid && right->cid > closer->cid)
            closer=right;
        if(left!=NULL && left->cid < cid && left->cid > closer->cid)
            closer=left;
    }
    if(closer->cid<cid){
        return closer;
    }else{
        if(right!=NULL &&closer->cid > right->cid)
            closer=right;
        if( left!=NULL && closer->cid > left->cid)
            closer=left;
        return closer;
    }
}else
return NULL;

}

Answer 1

The only declarations guaranteed to be zeroed out in C are global and static declarations. If you declare a variable in a function, it will be uninitialised.

Answer 2

No, local pointers to struct, like the ones in your code sample, are not initialized to NULL on declaration.

Anything uninitialized can be treated as having "garbage" or "junk" contents. So in the case of your uninitialized child pointer, it contains a random memory address (could be NULL , could be 0xDEADBEEF , could be anything).

Uninitialized pointers are dangerous to dereference. Best case you could segfault. Worst case, you accidentally access memory that belongs to your process, possibly modifying some local variable, which can result in some nasty bugs to track down.

So yes, it is normal to see seemingly random contents in uninitialized variables. Always initialize pointers before you dereference them. Contrast this to a language like java which has auto initialization features.

Answer 3

Accessing uninitialized variables in Undefined Behavior .

When you declare the pointer to struct , they are not initialized to NULL .

---

NOTE

Static and global variables are initialized automatically to 0 .

Answer 4

If the pointer to your struct is global or a static variable, then it would be initialized to NULL. The local (automatic) variable defined inside a function cannot be guaranteed to NULL initialized. Thus it is always a good practice to explicitly initialize such variables.

Answer 5

In C, if you declare any variable as global or static or declared in heap using calloc , values are zero but if it is declared as local to any function, they are uninitialized. Now it seems that you used the printf in the main function which means all the pointers are declared locally and they are storing some garbage values. Hence when you de-reference the pointer, you get the value stored in that garbage location. But keep in mind that this value is not known to anybody and hence it a bad practice to keep them uninitialized. You should always declare it with NULL

If you write NULL or 0 or nullptr in C++11 , it is guaranteed to give segmentation fault versus if you leave it uninitialized, they may alter/access memory mapped for some other process which in some platform can cause the raise of SIGSEGV signal but while in some other platforms (insure ones) may pose a security threat. Hence the best practice is to initialize them with NULL.