Conflicting types error message in linked lists program using C pointers and struct's

Question

Description

I'm trying to understand pointers, linked lists, structures and such in C. As a learning experience, I have written this small program, which:

• defines some basic structures and creates links between them, using structure pointers.
• iterates over the whole linked lists and prints out all variables.
• creates another structure, but doesn't insert it into the linked list.
• calls a function insertEntry which inserts a given struct between one element of the linked list and his direct follower.

What I've done so far:

• This Stack Overflow answer says: "That actually means that there is another function/declaration [..] elsewhere in your source code structure that has a different function signature."

  • I've checked for typos in the function definition, declaration and its call.
  • I've checked the amount and type of the parameters. Both parameters of insertEntry are always two structs of the same type.

• This different Stack Overflow answer says: "You have forgotten to #include "client.h", so the definition", but I've also checked that. Both the actual filename on the filesystem and the #include.

===> I don't get, where my error is.

---

ex1_insertStructure_linkedList.h:

void insertEntry(struct entry, struct entry);

---

ex1_insertStructure_linkedList.c:

#include <stdio.h>
#include "ex1_insertStructure_linkedList.h"

struct entry {
    int value;
    struct entry *next;
};

// clangtidy: conflicting types for 'insertEntry' [clang-diagnostic-error]
void insertEntry(struct entry given_entry, struct entry entry_to_insert) {

    printf("Print inside insertEntry method: %i\n", given_entry.value);
    struct entry *second_pointer = (given_entry).next;

    // entry_to_insert is now the element in the middle
    given_entry.next = &entry_to_insert;

    // the third element
    entry_to_insert.next = second_pointer;

    return;
}

int main(int argc, char *argv[]) {
    struct entry n1, n2, n3;

    n1.value = 1;
    n1.next = &n2;

    n2.value = 32;
    n2.next = &n3;

    n3.value = 34242;
    n3.next = (struct entry *)0;

    struct entry *list_pointer = &n1;

    while (list_pointer != (struct entry *)0) {
        int printValue = (*list_pointer).value;
        list_pointer = (*list_pointer).next;
        printf("%i\n", printValue);
    }

    printf("--------------------\n");
    list_pointer = &n1;

    struct entry a;
    a.value = 999999;
    a.next = (struct entry *)0;

    // clangtidy: argument type 'struct entry' is incomplete [clang-diagnostic-error]
    insertEntry(n1, a);

    while (list_pointer != (struct entry *)0) {
        int printValue = list_pointer->value;
        list_pointer = list_pointer->next;
        printf("%i\n", printValue);
    }

    return 0;
}

Answer 1

You should place struct entry declaration in ex1_insertStructure_linkedList.h :

struct entry {
    int value;
    struct entry *next;
};

void insertEntry(struct entry, struct entry);

Answer 2

You need to "forward-declare" the struct entry in your file ex1_insertStructure_linkedList.h, i.e. before the function declaration: void insertEntry(struct entry, struct entry);, that is put the following struct entry; before that function declaration.

The reason for this is because when the compiler encounters insertEntry(struct entry, struct entry);, it doesn't know anything about struct entry. By forward declaring struct entry, you "ensure" to the compiler that the struct entry is defined somewhere in a source file.