Why Do I get a segmentation fault 11 using malloc for a struct?

Question

I'm new to C and work myself throug the MIT Open Courseware for practical C programming (MIT OCW Homepage) in order to learn some C basics.

In assignment 5 we are supposed to implement a small binary tree library for allocation, deallocation, display and traversal.

While testing the following implementation

#include <stdlib.h>

struct TreeNodeStruct {
        int data ;
        struct TreeNodeStruct *left, *right ;
} ;

typedef struct TreeNodeStruct TreeNode ;

TreeNode* talloc(int data)
{

    /* variables */

    TreeNode *p ;

    /* logic */

        p = malloc(sizeof(struct TreeNodeStruct)) ;
        p->data = data ;

    /* return */

    return p ;

}

TreeNode* addnode(TreeNode* root ,int data)
{

    /* logic */

        if(root == NULL) return talloc(data) ; // allocate node and return as new root
        else if(data < root->data) root->left = addnode(root->left, data) ;
        else root->right = addnode(root->right, data) ;

    /* return */

    return root ;

}

in a very simple test case

printf("\nProblem 5.2\n") ;

tree = talloc(0) ;

printf("talloc(0): %s\n", tree == NULL ? "failure" : "success") ;

for(int i1 = 0, i2 = 10; ++i1 < i2 ; ) addnode(tree, i1) ;

I run into a segmentation fault 11 error. Is this an artefact of my test case or an error in my implementation? If it is, what am I doing wrong?

Use calloc instead of malloc. It will initialize the members of the node with zeros. And use sizeof *p instead of sizeof(struct TreeNodeStruct). sizeof is compile-time. — a_guest
– a_guest, Commented Mar 19, 2015 at 15:40
Alright, simple problem, simple solution. Thanks for the quick answers! — FK82
– FK82, Commented Mar 19, 2015 at 15:42

R Sahu · Accepted Answer · 2015-03-19 15:40:43Z

2

malloc doesn't initialize the members of the struct. Add code to do that.

p = malloc(sizeof(struct TreeNodeStruct)) ;
p->data = data ;
p->left = NULL;
p->right = NULL;

You can also use calloc if you don't want to have those lines.

p = calloc(1, sizeof(struct TreeNodeStruct)) ;
p->data = data ;

answered Mar 19, 2015 at 15:40

R Sahu

207k15 gold badges168 silver badges291 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Lundin · Accepted Answer · 2015-03-19 15:40:01Z

2

In talloc you need to set left and right to NULL.

When your code iterates through the binary tree, it will continue to call itself recursively until it finds NULL, which indicates you are on a "leaf". But since left and right contain garbage, it won't find NULL.

answered Mar 19, 2015 at 15:40

Lundin

220k47 gold badges282 silver badges446 bronze badges

2 Comments

FK82 Over a year ago

Ok, yeah i get it now. left and right weren't initialized. Why aren't they initialized to 0 though? Or, more to the point, why do they point to "garbage"?

Lundin Over a year ago

@FK82 Because malloc does nothing but allocating a chunk of memory as fast as possible. It does not initialize the allocated memory to any known values. Unlike calloc, which is guaranteed to initialize the allocated memory to zero.

timrau · Accepted Answer · 2015-03-20 00:55:57Z

2

You didn't initialize left and right to NULL in talloc(). addnode() cannot identify non-existing subtree correctly.

You should note that malloc() does not initialize anything in the allocated memory space. To initialize it into 0 automatically, you should use calloc().

p = calloc(1, sizeof(TreeNode));

edited Mar 20, 2015 at 0:55

answered Mar 19, 2015 at 15:39

timrau

23.1k4 gold badges55 silver badges67 bronze badges

Collectives™ on Stack Overflow

Why Do I get a segmentation fault 11 using malloc for a struct?

3 Answers 3

Comments

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related