C Language Zemi

NULL --Null pointer constant

NULL is called a null pointer constant and is defined in "stddef.h" as "integer 0" or "integer 0 cast to (void *)".

Substituting NULL for pointer is called a null pointer. NULL can be assigned to any pointer type.

// Null pointer
void * ptr = NULL;
int32_t * int32_ptr = NULL;
float * float_ptr = NULL;

NULL sample

This is a null sample.

Check memory allocation

In the sample below, NULL is returned if the memory allocation of the calloc function fails. Therefore, it can be used to check that memory is allocated.

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

int main (void) {
  // Pointer
  int32_t * nums = calloc (sizeof (int32_t), 10);
  
  // Pointer is not null
  if (nums! = NULL) {
    printf("Success\n");
  }
  else {
    printf("Fail\n");
  }
  free (nums);
}

Output result

Success

Is there a child element in the node of the tree?

This is a sample of checking if a child element exists in a node of a tree.

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

struct myapp_node {
  struct myapp_node * first_child;
  struct myapp_node * last_child;
};;

int main (void) {
  
  // Parent node
  struct myapp_node * parent = calloc (sizeof (struct myapp_node), 1);
  
  // Child node
  struct myapp_node * first_child = calloc (sizeof (struct myapp_node), 1);
  
  // Assign to the first child of the parent node
  parent->first_child= first_child;
  
  if (parent->first_child! = NULL) {
    printf("Have First Child\n");
  }

  if (parent->last_child== NULL) {
    printf("Don't Have Last Child\n");
  }
  
  free (parent);
  free (first_child);
}

Output result

Have First Child