Why does pointer to custom struct doesn't work here? - c

Question

Why pointer to custom struct doesn't work in that code?
Why I'm getting warning in that line with p->x = x?
Why I'm getting second warning in line with strcpy_s?
#include <stdlib.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
typedef struct sptr {
int x;
char* s;
struct sptr* next;
} ptr;
void add(ptr* p, int x, const char* s) {
ptr* o = p;
p = (ptr*) malloc(sizeof(ptr));
p->x = x; // warning
p->s = (char*)malloc(20 * sizeof(char));
strcpy_s(p->s, 20, (char*)s); // warning
p->next = o;
}
void show(ptr* p) {
ptr* o = p;
while (o != NULL) {
printf("%d %s\n", o -> x, o -> s);
o = o->next;
}
}
int main() {
ptr* p = NULL;
add(p, 5, "xcvxvxv");
add(p, 7, "adadad");
show(p);
return 0;
}

Answer 1

Pointers are values.
add is receiving a copy of the NULL pointer value. Changing the local variable p, in add, to a new pointer value returned by malloc does not change the separate, local variable p in main.
Just as if you wanted to change the value of an int in the caller's scope, you'd use an int * argument:
void change(int *val)
{
*val = 10;
}
int main(void)
{
int a = 5;
change(&a);
}
Changing the value of an int * in the caller's scope would require an int ** argument.
#include <stdlib.h>
void change(int **val)
{
*val = malloc(sizeof **val);
}
int main(void)
{
int *a;
change(&a);
}
This extends to any type.
malloc can fail, and return NULL. Performing indirection on a NULL pointer value is Undefined Behaviour.
You must guard against this happening by checking the return value of malloc.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct node {
int x;
char *s;
struct node *next;
} Node;
void add(Node **p, int x, const char *s) {
Node *new_node = malloc(sizeof *new_node);
if (!new_node) {
perror("allocating node");
exit(EXIT_FAILURE);
}
new_node->s = malloc(1 + strlen(s));
if (!new_node->s) {
perror("allocating node string");
exit(EXIT_FAILURE);
}
new_node->x = x;
strcpy(new_node->s, s);
new_node->next = *p;
*p = new_node;
}
void show(Node *p) {
while (p) {
printf("%d %s\n", p->x, p->s);
p = p->next;
}
}
int main(void) {
Node *list = NULL;
add(&list, 5, "xcvxvxv");
add(&list, 7, "adadad");
show(list);
}

Answer 2

Why pointer to custom struct doesn't work in that code?
TBD
Why I'm getting warning in that line with p->x = x?
Why I'm getting second warning in line with strcpy_s?
2 warnings occur because code de-referenced the pointer from malloc() without first checking if the pointer might be NULL.