invalid application of ‘sizeof’ to incomplete type

Question

This is my makefile file all: trie

trie: trie.o main.o
    gcc trie.o main.o -o trie -std=c11 -g -Wall

trie.o: trie.c trie.h
    gcc -c trie.c -o trie.o -std=c11 -g -Wall

main.o: main.c trie.h
    gcc -c main.c -o main.o -std=c11 -g -Wall

clean:
    rm -f *.o trie

and header file

#ifndef TRIE_H
#define TRIE_H

struct node;
typedef struct node node;

//insert a word in a leaf
void insert(char* word, node* leaf);

#endif //TRIE_H

and trie.c file

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

#include "trie.h"

struct node {
  char* data;
  node* child[127];
};

void insert (char* word, node* leaf) {
  node* curr = leaf;
  for (size_t i = 0; i < strlen(word); i++) {//start from beginning of char to end
    if (curr == NULL) {
      curr = (node*)malloc(sizeof(node)); // if it's null, creating new node
      curr->data = "";
    }
    curr = curr->child[(int) word[i]];
  }
  curr->data = word; // set last node stored the word
}

it occurs error message in the main file

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

#include "trie.h"

int main() {
  node* x = (node*) malloc(sizeof(node));
  insert("hi", x);
  return 0;
}

and this is error message:

main.c: In function ‘main’: main.c:7:35: error: invalid application of ‘sizeof’ to incomplete type ‘node {aka struct node}’ node* x = (node*) malloc(sizeof(node));

have any idea why my code has an error?

Answer 1

Your main.c doesn't have a definition of node, just a declaration of the name without defining the structure. You either need to include the definition in the .h file so both trie.c and main.c can see it, or you need to provide an allocator method (declared in trie.h, defined in trie.c) that can perform a definition aware allocation (and possibly initialization) of a node in a place that has access to the definition of the otherwise opaque type.