How to fix: initialization from incompatible pointer type

Question

When I compile and run the program, it says: In function 'addToTheEnd': [Warning] initialization from incompatible pointer type //and it points to this line -> knot_t *current = start;

How to fix it? I'm new to C, so I don't understand what should be changed. I tried to understand it but I couldn't. My goal is to get some output out of this program when I run it, but nothing displays.

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

typedef struct knot {
    int number;
    struct knot *next;
} knot_t;

int addToTheEnd(knot_t **start, int value) {
    knot_t *new_ = (knot_t *)malloc(sizeof(knot_t));
    if (new_ == NULL) {
        return 1;
    }
    new_ -> number = value;
    new_ -> next = NULL;
   
    if (start == NULL) {
        *start = new_;
    } else {
        knot_t *current = start;
        while (current -> next != NULL) {
            current = current -> next;
        }
        current -> next = new_;
    }
   
    return 0;
}

void printList(knot_t *start) {
    knot_t *current = start;
    while (current != NULL) {
        printf("%d ", current->number);
        current = current -> next;
    }
}

void clearList(knot_t *start) {
    knot_t *current = start;
    while (current != NULL) {
        knot_t* trenutni = current;
        current = current -> next;
        free(trenutni);
    }
}

int main() {
    knot_t *start = NULL;
    int i = 0;
    for (i = 0; i < 10; i++) {
        if(addToTheEnd(&start, i)) {
        printf("Fail\n");
return EXIT_FAILURE;
}
    }
   
    printList(start);
    clearList(start);
   
    return EXIT_SUCCESS;
}

Answer 1

The function has the parameter start of the type knot_t **.

int addToTheEnd(knot_t **start, int value) {

While the local variable current has the type knot_t *.

knot_t *current = start;

So as there is no implicit conversion from the type knot_t ** to the type knot_t * the compiler issues an error.

It seems you mean

knot_t *current = *start;

Also the condition of the if statement

 if (start == NULL) {
        *start = new_;

must be

 if (*start == NULL) {
        *start = new_;

Also it would be more logical consistent when the function in the case of success would return 1 instead of 0.

The function can be defined the following way

int addToTheEnd( knot_t **start, int value ) 
{
    knot_t *new_knot = malloc( sizeof( knot_t ) );
    int success = new_knot != NULL;

    if ( success )
    {
        new_knot -> number = value;
        new_knot -> next = NULL;
   
        while ( *start != NULL ) start = &( *start )->next;

        *start = new_knot;
    } 
   
    return success;
}

And as the function printList does not change pointed nodes it should be declared at least like

void printList( const knot_t *start) {
    const knot_t *current = start;
    while (current != NULL) {
        printf("%d ", current->number);
        current = current -> next;
    }