从'void *'赋值给'int'使得integer从指针没有强制转换

Question

I am trying to make a linked list stack, and I followed a tutorial online, however I get this warning, and the author does not.

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

typedef struct stack {
    int data;  
    struct stack *next;

} stack;

stack *top;


void Initialize(stack *s){
    s=malloc(sizeof(stack));
    s->data=NULL;
    s->next=NULL;
    top=s;

    
    return;
}

During debugging I get a segmentation fault, if I don't use malloc in the initialize function. MINGW-W64 compiler.

Answer 1

The warning is due to the fact the the NULL macro is defined (by most modern compilers) as ((void *)0) , as it is intended to be used only for pointers. Assigning this value to the data member of your structure causes the warning.

To remove this warning, use s->data=0; in place of s->data=NULL; . The author of the tutorial is either using an older compiler or has disabled this particular warning.

Also, the pointer ( s ) passed to your Initialize function will be a copy of whatever variable you use as an argument when you call that function and, as such, its value is not updated in the calling code. You haven't specified how you intend to use that function, but here is a (perhaps) better implementation:

stack* Initialize(void) { // Don't need an argument - just return the 'answer'
    stack* made = malloc(sizeof(stack));
    if (made) { // Don't attempt the initialization if "malloc" failed!
        made->data = 0;
        made->next = NULL;
    }
    return made;
}

And then, when you call that function, you can assign its returned value to your 'global' top pointer:

//...
top = Initialize();
// You should check that 'top' is not NULL before continuing!

Answer 2

In C the macro NULL is defined like

( void * )0

that is the type of the expression is a null pointer of the type void * .

Thus in this statement

s->data=NULL;

a pointer is assigned to an object of the type int and the compiler issues a message saying that you are doing something wrong.

The function Initialize as it is written does not make a sense. Apart of all for example this statement

s=malloc(sizeof(stack));

does not change the original pointer top used as the function argument.

In fact the function is redundant.

Nevertheless if to write such a function then it can look either like

stack * Initialize( void )
{
    return NULL;
}

and called like

stack *top = Initialize();

or like

void Initialize( stack **top )
{
    *top = NULL;
}

Or for a new created node it can be declared and defined like

void Initialize( stack *s, int data, stack *next )
{
    s->data = data;
    s->next = next;
}