Why doesn't my node context print anything?

Question

I am trying to create a calculator that keeps the order of arithmetic operations. My idea is to convert infix notation to postfix notation so that I can solve it from left to right without worrying about parentheses. Before trying to convert infix into postfix notation I wanted to solve a postfix notation exercise and I tried to use nodes to solve this, but I'm having problems with dividing the numbers and operators into a node. I am new to pointers and structs and that all things confuse me.

Here is the function that tries to divide it:

typedef char* String;
typedef struct node
{
    String       str;
    struct node *next;
} Node;

Node *rpn_divider(String equation, int eq_size)
{
     Node *rpn_parts = node_alloc(1); //pointer to first element in the node
     Node *part_temp = rpn_parts; //pointer to the lattest element in the node
     String temp = malloc(sizeof(char*) * NUM_SIZE);

     int i, j; //i = string equation index, j = string temp index

     for (i = 0, j = 0; i < eq_size; i++)
     {
         if (isNum(equation[i]))
            temp[j++] = equation[i];
         else if (isOper(equation[i]))
         {
              temp[0] = equation[i];
              temp[1] = '\0';
              next_node(part_temp, temp);
         }
         else
         {
              if (temp == '\0') continue;
              temp[j] = '\0';
              next_node(part_temp, temp);
              j = 0;
         }
     }
     free(part_temp->next);
     free(temp);
     return rpn_parts;
}

Here is the next_node function:

void next_node(Node *node, String str)
{
  node->str = str;
     node->next = node_alloc(1);
     node = node->next;
     free(str);
     str =  malloc(sizeof(char*) * NUM_SIZE);
     str[0] = '\0';
}

and when I am trying to print the node context, it doesn't do anything:

Node *ptr;
for (ptr = head; ptr != NULL; ptr = ptr->next);
{
    printf("The Str = %s", ptr->str);
}

Answer 1

In the next_node function, you are allocating memory and assigning it to a local copy of str. This is a memory leak and the caller will never see the new value of str. Instead you can do this:

void next_node(Node *node, String *str)
{
     node->str = *str;
     node->next = node_alloc(1);
     node = node->next;
     free(*str);
     *str =  malloc(sizeof(char*) * NUM_SIZE);
     (*str)[0] = '\0';
}

And use it like this:

next_node(part_temp, &temp);