返回一个指向 float 的指针会在它到达 main 函数时改变地址

Question

MAIN METHOD

 int x, y;
    printf("Please enter 2 integers separated by a space: ");
    scanf("%d %d", &x, &y);

OTHER METHOD. This is part of the matrix_get function called from the main function

if (key_comp (key, node->key) == 0){
       data_print(node->data);

SOME HEADER FILE

typedef float Data_Item;

Ok so when I return the pointer to the Data_Item (it's a float typedef), the address of it changes and so when I dereference it in the main method it becomes null. Why?

Answer 1

printf("%d", &pdata);

should be

printf("%f", *pdata);

You're trying to be print the address of the pointer instead of the value of the pointer. Trying to print the address with %d is undefined. Use %f for float. Using the wrong format specifier invokes undefined behaviour .

The memory you allocated for the pointer gets leaked, since the pointer's value is overwritten by a local variable's address. Instead you should copy the data in the local variable into the memory allocated for the pointer.

*pdata = node->data;

Answer 2

The line

   pdata=&(node->data);

changes the value of pdata . It is now different from the value returned by malloc . The line that makes more sense, that you have commented out, is:

   //*pdata=node->data;

In the first line, you are assigning pdata to be the address of node->data . In the commented out line, you were trying to set the value of what pdata points to by node->data .

You are missing the return from two of the branches.

Change

if(key_comp (key, node->key) == -1){
      bs_tree_search_h(node->left, key);
}else if(key_comp (key, node->key) == 1){
      bs_tree_search_h(node->right, key);
}

to

if(key_comp (key, node->key) == -1){
      return bs_tree_search_h(node->left, key);
}else if(key_comp (key, node->key) == 1){
      return bs_tree_search_h(node->right, key);
}