Dynamic Memory Allocation in C using malloc(), calloc(), free() and realloc() Functions

/*
 * malloc_not_ini.c -- program displays memory allocated by malloc is not
 * initialized
 */
#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    char *pc, chars, i;
 
    puts("**Program displays malloc() returns uninitialized block**");
    printf("Specify no. of characters to which memory to allocate,\n");
    scanf("%d", &chars);
 
    /* now allocate memory */
    pc = (char *)malloc(chars * sizeof(char));
 
    /* verify if allocation successful */
    if (pc == NULL) {
            puts("Error: Allocation Failed!");
            exit(EXIT_FAILURE);
    }
 
    /* let us see the contents of allocated block */
    i = 0;
    printf("Allocated block, an array of %d chars, contains values:",
            chars);
    while (i < chars) {
            printf("%2c", *(pc + i));
            i++;
    }
 
    /* Now we free allocated block back to pool of available memory */
    free(pc);
 
    puts("");
    return 0;
}

**Program displays malloc() returns uninitialized block**
Specify no. of characters to which memory to allocate,
25
Allocated block, an array of 25 chars, contains values:

/*
 * calloc_ini.c -- program displays memory allocated by calloc is
 * initialized to 0
 */
#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    int *pi, ints, i;
 
    puts("**Program displays calloc() initializes memory"
         " It allocates**");
    printf("Specify no. of integers to which memory to allocate,\n");
    scanf("%d", &ints);
 
    /* now allocate memory */
    pi = (int *)calloc(ints, sizeof(int));
 
    /* verify if allocation successful */
    if (pi == NULL) {
            puts("Error: Allocation Failed!");
            exit(EXIT_FAILURE);
    }
 
    /* let us see if allocated block is initialized with zeroes */
    i = 0;
    printf("Allocated block, an array of %d ints, is initialized with:",
            ints);
    while (i < ints) {
            printf("%2d", *(pi + i));
            i++;
    }
 
    /* free block up back to pool of available memory */
    free(pi);
 
    puts("");
    return 0;
}

**Program displays calloc() initializes memory It allocates**
Specify no. of integers to which memory to allocate,
6
Allocated block, an array of 6 ints, is initialized with: 0 0 0 0 0 0

/*
 * realloc.c -- Let's get resize previously allocated block and
 * free that up for reuse
 */
/* void *realloc(void *, size_t num_bytes) */ /* realloc resizes */
#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    int *pi, i, num = 0;
    int *pi1 = 0;   /* pointer set to NULL */
 
    puts("\nLet's try to get block of memory for integers u want...\n");
    scanf("%d", &num);
 
    /* assigned to int * type */
    pi = malloc(num * sizeof(int));
 
    /* confirm if allocation is done */
    if (pi == NULL) {
            puts("Error: System short of memory!");
            exit(1);
    }
 
    puts("Good! Now, let's use dynamic memory...\n");
    printf("User type in %d integers one by one...\n", num);
    for (i = 0; i < num; i++)
            scanf("%d", (pi + i));
 
    puts("");
 
    printf("User, let's see what had u typed...\n");
    for (i = 0; i < num; i++)
            printf("%3d", *(pi + i));
 
    puts("");
 
    puts("Yeah, I want to resize the previously allocated chunk,\n"
            "specify another value for integers you want now...");
    scanf("%d", &num);
    /* reallocate block using realloc() function */
    pi1 = realloc(pi, num * sizeof(int));
 
    /* confirm if allocation is done */
    if (pi1 == NULL) {
            puts("Error: System short of memory!");
            exit(1);
    }
 
    printf("Let's see the contents of resized block of %d integers.\n",
             num);
    for (i = 0; i < num; i++)
           printf("%3d", *(pi1 + i));
 
    puts("");
 
    puts("Good! Now, let's use new chunk...\n");
    printf("User type in %d integers one by one...\n", num);
    for (i = 0; i < num; i++)
            scanf("%d", (pi1 + i));
 
    puts("");
 
    printf("User, let's see what had u typed...\n");
    for (i = 0; i < num; i++)
           printf("%3d", *(pi1 + i));
 
    puts("");
    puts("Let's return allocated memory to system for reuse.");
    free(pi1);      /* remember don't double free up memory */
 
    puts("Thank you!\n");
    return 0;
}

Let us try to get block of memory for integers u want...
 
6
Good! Now, let's use dynamic memory...
 
User type in 6 integers one by one...
1
2
3
4
5
6
 
User, let us see what had u typed...
  1  2  3  4  5  6
Yeah, I want to resize the previously allocated chunk,
specify another value for integers you want now...
5
Let us see the contents of resized block of 5 integers.
  1  2  3  4  5
Good! Now, let's use new chunk...
 
User type in 5 integers one by one...
5
6
7
8
9
 
User, let us see what had u typed...
  5  6  7  8  9
Let's return allocated memory to system for reuse.
Thank you!

        void free(void *);