pointer changes value without being passed as argument

Question

Following is a code snippet from a code involving some transformations on a binary tree.

void fixPrevPtr(struct node *root)
{
    static struct node *pre = NULL;

    if (root != NULL)
    {
        fixPrevPtr(root->left);
        root->left = pre;
        pre = root;
        fixPrevPtr(root->right);
    }
}

Here 'pre' is initialised in every function as NULL. But when function entered 'if' clause and root->left=pre, was executed, pre that was being assigned was not NULL. It was somehow changed by the function fixPrevPtr(root->left).

My question is that how does it get changed without being even passed into the function.

Thanks in advance.

@KerrekSB... honestly, I am embarassed...I just didn't see the static before node...I work in c++, so I overlook struct keyword used while defining variables as done in c. When, I was going through the code, maybe due to sheer callousness, I overlooked static along with struct. — AvinashK, Aug 10 '13 at 20:40

score 3 · Answer 1 · answered Aug 10 '13 at 10:09

3

This bit is incorrect

Here 'pre' is initialized in every function as NULL.

It only gets initialized once due to the static keyword. If the function changes the value then next time the function has that value instead of null.

answered Aug 10 '13 at 10:09

Ed Heal

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score 2 · Accepted Answer · answered Aug 10 '13 at 10:09

2

pre is static so it keeps it's value from call to call. fixPrevPtr() is recursive (calls itself) so any change to pre "sticks".

answered Aug 10 '13 at 10:09

John3136

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score 2 · Answer 3 · edited Jun 20 '20 at 09:12

static struct node *pre = NULL;

Initialize pre once due to the static keyword.

But the next time you enter in this function, pre will have the last value assigned to it.

I suggest you to read this : Static variable inside of a function in C

In the C standard :

6.2.4 Storage durations of objects

An object whose identifier is declared with external or internal linkage, or with the storage-class specifier static has static storage duration. Its lifetime is the entire execution of the program and its stored value is initialized only once, prior to program startup.

score 0 · Answer 4 · answered Aug 10 '13 at 10:12

0

pre is a static value , it stored in the data segment, and it scope is not only the fixPrevPtr but also the all other function in this process, so of course fixPrevPtr itself can change it.

answered Aug 10 '13 at 10:12

Lidong Guo

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score 0 · Answer 5 · answered Aug 10 '13 at 10:12

you have added static keyword with the struct node * pre = NULL that's why it is retaining a value from a previous function call when pre = root; was executed. remove static and you are good to go

you can read about static variables from this link http://ee.hawaii.edu/~tep/EE160/Book/chap14/subsection2.1.1.6.html

Static automatic variables continue to exist even after the block in which they are defined terminates. Thus, the value of a static variable in a function is retained between repeated function calls to the same function.

pointer changes value without being passed as argument

5 Answers5