传递到在C函数指针结构即使提前声明不工作

Question

I'm new to C and am learning linked lists. I am using Code Blocks IDE on Linux Mint with a GCC compiler. I am trying to pass in a struct pointer to a function that can help me traverse the the linked list. Here is my present code:

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

void gothru(struct node * Root);

struct node {
    int data; //data in current node
    struct node * next; //pointer to the next node
};

void gothru(struct node * Root){
    struct node * conductor; //this pointer will be used to traverse the linked list
    conductor = Root; //conductor will start at the Node

    /* Conductor traverses through the linked list */
    if(conductor != NULL){
        while(conductor->next != NULL){
            printf("%d->",conductor->data);
            conductor = conductor->next; /*if current position of conductor is not pointing to
                                           end of linked list, then go to the next node */
        }
        printf("%d->",conductor->data);
    }

    printf("\n");

    if (conductor == NULL){
        printf("Out of memory!");
    }

    free(conductor);
}

int main()
{
    struct node * root; //first node

    root = malloc(sizeof(*root)); //allocate some memory to the root node

    root->next = NULL; //set the next node of root to a null/dummy pointer
    root->data = 12; //data at first node (root) is 12

    gothru(root); //traverse linked list


    return 0;
}

Now, I have forward declared my function at the top with the exact same format as when the function is first initialized. But still, I'm getting the following error:

|11|error: conflicting types for 'gothru'

I have tried changing the argument for my "gothru" function to simple variables in which case it works. But the minute I change back to a pointer to the struct, it gives me this error.

Previous answers in Stackoverflow say that we need to forward declare our functions to clear this error. I did exactly that, but it still does not work. Any solutions?

Answer 1

The forward declaration of the gothru function contains the structure type as one of the parameter, so, you need to move the forward declaration for gothru after the structure definition.

Otherwise, the function argument (type) is not known in the forward declaration time.

Something like

struct node {
    int data; //data in current node
    struct node * next; //pointer to the next node
};

void gothru(struct node * Root);  // struct node is known to compiler now

should solve the problem.

Answer 2

Using gcc we get this if we try to compile your code:

vagrant@dev-box:~/tests$ gcc test.c
test.c:4:20: warning: ‘struct node’ declared inside parameter list [enabled by default]
 void gothru(struct node * Root);
                    ^
test.c:4:20: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]
test.c:11:6: error: conflicting types for ‘gothru’
 void gothru(struct node * Root){
      ^
test.c:4:6: note: previous declaration of ‘gothru’ was here
 void gothru(struct node * Root);
      ^

The warning that is shown first is crucial to understanding what is going on here. Here, struct node is first declared inside the function prototype and it's scope is just that line. But it declares the function. Now after defining the struct node you encounter the function definition but in this line struct node means something different from the local struct node encountered in first prototype. And that's why you are getting a conflicting type for the function.

The solution is as already pointed by @SouravGhosh's answer, ie to move the structure definition before the function prototype.