Structure having an array of pointers in c

Question

I am implementing multibit trie in C. I am getting segmentation fault error. When I run the program. I don't know what is going wrong?

The node of Multi bit trie is like that:

struct MtNode{
        /* nodes is an array 8 elements. Each element is a pointer to its child node.*/
        MtNode* nodes[8];  // 2^stride = 2^3 = 8
        int   nexthop;
    };

Each node is initialized as following:

typedef MtNode node;
node *init_mtnode(){
    node *ret = (node*) malloc(sizeof(node));   
    int size = (int)pow(2,STRIDE);
    for (int i=0; i<size ; ++i)
    {
           ret->nodes[i] = NULL;
    }    
    ret->nexthop = -1;
    return ret;
}

Is anything wrong in init_mtnode method?

Are you sure it's c? In [tag:c] the declarations you have would be wrong, they are valid in [tag:c++] though. Also, don't cast `pow` to int like that, and you don't need to cast `malloc()` either, again *unless it's [tag:c++]*. — Iharob Al Asimi, Oct 03 '15 at 01:38
If i dont cast malloc, I get compilation error. @iharob. and actually i use c++ compiler cause i need some library of c++ such maps.. but the work is just a procedual programming...that's why i put c tag — explorer, Oct 03 '15 at 01:41
@BLUEPIXY...i tried with struct MtNode* nodes[8];....same error..segfault — explorer, Oct 03 '15 at 01:42
No, you should put the tag corresponding to the programming language you use, for example you should use `new` instead of `malloc()` and also, `maps` is not a library they are part of `stl` from standard c++. — Iharob Al Asimi, Oct 03 '15 at 01:42
It is, the help you get depends on that. And c solutions might be a bad choice in c++ and c++ solutions will not generally work in c. Also, there are some situations where a c solution will not work in c++. — Iharob Al Asimi, Oct 03 '15 at 01:46

Iharob Al Asimi · Accepted Answer · 2015-10-03T02:05:01.560

1

There are multiple possible reasons:

You don't check malloc() for NULL, unlike new which would throw an exception, malloc() will return NULL if an error happens.
You calculate the value of size and then loop size times, but your array can only hold 8 pointers.

Try this

typedef MtNode node;
node *init_mtnode()
{
    node *ret;
    int size;
    ret = static_cast<node *>(malloc(sizeof(node)));
    if (ret == NULL) /* check for NULL */
        return NULL;
    size = 2 << STRIDE;
    if (size >= 8)
        size = 7; /* maximum possible value */
    for (int i = 0 ; i < size ; ++i)
        ret->nodes[i] = NULL;
    ret->nexthop = -1;

    return ret;
}

edited Oct 03 '15 at 02:05

answered Oct 03 '15 at 01:44

Iharob Al Asimi

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Then the error is not in this code. What is the value of `STRIDE`? – Iharob Al Asimi Oct 03 '15 at 01:49
its 3..#define STRIDE 3 – explorer Oct 03 '15 at 01:50
See how I fixed your code, it will not take away the segmentation fault, but this is the correct way to do it. – Iharob Al Asimi Oct 03 '15 at 01:52
Ok...i see..but for now, i got error called "nullptr is not declared in scope" ..I am using g++ in linux now to compile my code – explorer Oct 03 '15 at 01:56
it compiles...but the segfault is ther! – explorer Oct 03 '15 at 02:01
1

Please post the rest if the code, build the smallest possible program that will reproduce this behavior. – Iharob Al Asimi Oct 03 '15 at 02:05
@iharob...thanks for the help...there is nothing wrong in this piece of code!!...i will check rest! – explorer Oct 03 '15 at 16:04
...Can you please help me further? ..http://stackoverflow.com/questions/32936609/trie-implementation-in-c-c – explorer Oct 04 '15 at 18:27

Structure having an array of pointers in c

1 Answers1