making a pointer that stores an element of a char array

Question

I've been struggling trying to figure out why I am getting the following warning:
initialization makes pointer from integer without a cast

The highlighted warnings are where I mentioned below. The code I am currently using is just the beginning of creating tree of elements in a linked list fashion. This code seems to be working fine however I get docked points for warnings.

typedef struct Node {
        struct Node *leftChild;
        struct Node *rightChild;
        char data;
} Node;

Node *TreeCreate(int level, const char *data) {
    struct Node *ptr = (struct Node*) malloc(sizeof (Node));
    if (ptr == NULL) {
        // malloc failed
        return 0;
    }
    ptr->data = data;   // WARNING
    ptr->leftChild = NULL;
    ptr->rightChild = NULL;
    return ptr;
}
// TEST CODE IN MAIN
char list[6] = {'A', 'B', 'C','\0'};

// Determines the element
const char *tree = list[0]; // WARNING
ptr = TreeCreate(1, tree);
if (ptr != NULL) {
    sprintf(string, "TreeData: %c\n", ptr->data);
    OledDrawString(string);
    OledUpdate();
}

First [Don't cast the result of `malloc()`](http://stackoverflow.com/a/605858/1983495). Then, please post the declaration of `string`. — Iharob Al Asimi, Mar 02 '15 at 23:51
typedef'ing structs is always a bad idea and should be avoided — user3629249, Mar 03 '15 at 01:35
in C, the returned value from malloc (and family) should not be cast — user3629249, Mar 03 '15 at 01:37

Iharob Al Asimi · Accepted Answer · 2015-03-02T23:57:08.003

Your fundamental mistake is that you are assigning a poitner to a char which is wrong

const char *tree = list[0]; // WARNING

this will not yield the result you expect.

The * in this case is not dereferencing the pointe, you are declaring a poitner and pointeing to a char with it, then when you try to access the pointer, your program tries to read at an invalid memory address causing undefined behavior.

Then you do the opposite thing in

ptr->data = data;

you should enable compiler warnings to avoid this mistakes.

To handle the data you apparently want to handle, first you need to redefine the struct like this

typedef struct Node {
    struct Node *leftChild;
    struct Node *rightChild;
    char *data;
    /*   ^ this should be a char pointer */
} Node;

then in the TreeCreate() function, copy the data by first allocating space and then using memcpy() like this

Node *TreeCreate(int level, const char *data) {
    size_t       length;
    struct Node *ptr;

    ptr = malloc(sizeof (Node));
    if (ptr == NULL) {
        return NULL;
    }
    if (data != NULL)
    {
        length    = strlen(data);
        ptr->data = malloc(1 + length);
        if (ptr->data != NULL)
            memcpy(ptr->data, data, 1 + length);
    }
    else
        ptr->data = NULL;        

    ptr->leftChild  = NULL;
    ptr->rightChild = NULL;

    return ptr;
}

Additionally, it's the other way around at the top of the code. He's assigning a pointer to a char (in the line `ptr->data = data; // WARNING` since ptr->data is a char. — russianfool, Mar 02 '15 at 23:53
@russianfool yes, I think the OP is mixing `char` poitners with `char`'s — Iharob Al Asimi, Mar 02 '15 at 23:55

score 0 · Answer 2 · answered Mar 02 '15 at 23:59

0

I think I understand. The following fixed my warnings. Thanks for the fast response!

const char *tree = &list[0];
ptr->data = *data;

answered Mar 02 '15 at 23:59

Connor

117
1
2
11

Yea, not sure why our professor made us use a pointer input for Node *TreeCreate however maybe it holds more relevance down the line, I've only just started messing with this assignment. – Connor Mar 03 '15 at 00:06

score 0 · Answer 3 · answered Mar 03 '15 at 01:47

the following, a complete program, 
that cleanly compiles
and has the warnings fixed
and eliminates the clutter and unnecessary typedef statements.

#include<stdio.h>
#include<stdlib.h>


struct Node 
{
        struct Node *leftChild;
        struct Node *rightChild;
        char data;
};

struct Node *TreeCreate(int level, const char *data) 
{
    struct Node *ptr = malloc(sizeof (struct Node));
    if (ptr == NULL) 
    {
        // malloc failed
        return NULL ;
    }

    // implied else, malloc successful

    ptr->data = *data;   // WARNING
    ptr->leftChild = NULL;
    ptr->rightChild = NULL;
    return ptr;
}

int main()
{
    struct Node *ptr = NULL;
    char  string[120] = {'\0'};

    // TEST CODE IN MAIN
    char list[6] = {'A', 'B', 'C','\0'};

    // Determines the element
    const char *tree = &list[0]; // WARNING

    ptr = TreeCreate(1, tree);

    if (ptr != NULL) 
    {
        sprintf(string, "TreeData: %c\n", ptr->data);
        //OledDrawString(string);
        //OledUpdate();
    }
    return 0;
}

making a pointer that stores an element of a char array

3 Answers3