C problem, left of '->' must point to class/struct/union/generic type?

Question

Trying to understand why this doesn't work. I keep getting the following errors: left of '->nextNode' must point to class/struct/union/generic type
(Also all the lines with a -> in the function new_math_struct)

Header file

#ifndef MSTRUCT_H
#define MSTRUCT_H

    #define PLUS 0
    #define MINUS 1
    #define DIVIDE 2
    #define MULTIPLY 3
    #define NUMBER 4

    typedef struct math_struct
    {
        int type_of_value; 
        int value;
        int sum;
        int is_used;
        struct math_struct* nextNode;
    } ;

    typedef struct math_struct* math_struct_ptr;
#endif

C file

int get_input(math_struct_ptr* startNode)
{
    /* character, input by the user */ 
    char input_ch; 
    char* input_ptr;

    math_struct_ptr* ptr;
    math_struct_ptr* previousNode;

    input_ptr = &input_ch;
    previousNode = startNode;

    /* as long as input is not ok */ 
    while (1)
    {             
        input_ch = get_input_character();
        if (input_ch == ',') // Carrage return
            return 1;
        else if (input_ch == '.') // Illegal character
            return 0;


        if (input_ch == '+')
            ptr = new_math_struct(PLUS, 0);
        else if (input_ch == '-')
            ptr = new_math_struct(MINUS, 0);
        else if (input_ch == '/')
            ptr = new_math_struct(DIVIDE, 0);
        else if (input_ch == '*')
            ptr = new_math_struct(MULTIPLY, 0);
        else
            ptr = new_math_struct(NUMBER, atoi(input_ptr));

        if (startNode == NULL)
        {
            startNode = previousNode = ptr;
        }
        else
        {
            previousNode->nextNode = ptr;
            previousNode = ptr;
        }
    } 
    return 0;
}

math_struct_ptr* new_math_struct(int symbol, int value)
{
    math_struct_ptr* ptr;
    ptr = (math_struct_ptr*)malloc(sizeof(math_struct_ptr));
    ptr->type_of_value = symbol;
    ptr->value = value;
    ptr->sum = 0;
    ptr->is_used = 0;    
    return ptr;
}

char get_input_character()
{
    /* character, input by the user */ 
    char input_ch; 

    /* get the character */ 
    scanf("%c", &input_ch);     

    if (input_ch == '+' || input_ch == '-' || input_ch == '*' || input_ch == '/' || input_ch == ')')
        return input_ch; // A special character
    else if (input_ch == '\n')
        return ','; // A carrage return
    else if (input_ch < '0' || input_ch > '9')
        return '.'; // Not a number          
    else
        return input_ch; // Number
}

The header for the C file just contains a reference to the struct header and the definitions of the functions. Language C.

Solution 1

Since your math_struct_ptr already contains the pointer declarator, you don't need to specify it at the point of use. Drop the *:

math_struct_ptr ptr;

Or write

struct math_struct *ptr;

Solution 2

math_struct_ptr* is a math_struct** - you're creating a pointer to a pointer with nested stars there. The advantage of your typedef is that you don't have to put a * after math_struct_ptr, so you can just leave that out

Solution 3

You typedef

 typedef struct math_struct* math_struct_ptr;

So if you use math_struct_ptr*, you get a pointer to a pointer. You probably just want math_struct_ptr in that case. I would not hide the pointer with typedef, but I see many people do that. I find it just confusing. It's probably a matter of taste.

Comments

[jamesdlin]

It's more than a matter of taste. const math_struct_ptr and const math_struct* are two different things.