Question

I am trying to write code to build a stack, but I’m getting compilation errors which don’t make sense to me. Here is my stack.h:

struct StackNode {

    void* previous;
    int   value;
};


struct Stack {

    StackNode* top;
};


Stack* new_stack () {

    StackNode stn = { NULL, 0 };
    Stack* st  = (Stack*) malloc(sizeof(Stack));
    st->top = NULL;
    return st;
}

and my main.c:

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

#include "stack.h"

int main () {

    struct Stack* st = new_stack();

    return 0;

}

gcc throws these errors:

make (in directory: /home/diego/temp/stack) gcc -g -O2 -std=c99 -c

main.c In file included from main.c:4: Compilation failed. stack.h:10:

error: expected specifier-qualifier-list before ‘StackNode’

stack.h:14: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘attribute’

before ‘*’ token main.c: In function ‘main’: main.c:8: warning:

implicit declaration of function ‘new_stack’ main.c:8: warning:

initialization makes pointer from integer without a cast make: *

[main.o] Error 1

EDIT: I found the error. I forgot to put struct before Stack and StackNode in some lines. Always having struct on those lines solves the issue.

Answer 1

Change:

struct Stack {
    StackNode* top;
};

to:

struct Stack {
    struct StackNode* top;
};

and anywhere else StackNode or Stack is used and not preceded by struct. If you wish to not specify struct you could use a typedef.