Dynamic memory tiny demo

main.c

// I rand this with  gcc -std=c99 main.c && ./a.exe on Windows (cygwin)
// Mac/Linux people would probably prefer gcc -std=c99 main.c && ./a.out

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

int main() {
    printf("The size of an int is %d\n\n", sizeof(int));

    int* ptr1 = malloc(1000*sizeof(*ptr1));
    int* ptr2 = malloc(1000*sizeof(*ptr2));
    printf("ptr1: %p\n", ptr1);
    printf("ptr2: %p\n\n", ptr2);

    printf("Before I set it: ptr1[0] = %d;\n", ptr1[0]);

    ptr1[0] = 1525;
    printf("ptr1[0] = %d;\n", ptr1[0]);

    printf("\nReallocation ptr1 to 4000000 ints...\n");
    void *temp = realloc(ptr1, 4000000*sizeof(*ptr1));
    if (NULL == temp) { printf("couldn't realloc!"); }
    else {
        printf("Realloc succeeded!\n");
        ptr1 = temp;
    }

    printf("ptr1: %p\n", ptr1);
    printf("ptr1[0] = %d;\n", ptr1[0]);

    printf("filling memory that I just allocated.\n");
    for (int i=1; i< 4000000; i++) {
        ptr1[i] = i;
    }

    // notice program memory in TaskManager or Activity Monitor went way up!
    printf("\nPress enter to continue...\n");
    fgetc(stdin);

    printf("Resizing ptr1 back to 1000 members long.\n");
    temp = realloc(ptr1, 1000*sizeof(*ptr1));
    if (NULL == temp) { printf("couldn't realloc!"); }
    else {
        printf("Realloc succeeded!\n");
        ptr1 = temp;
    }

    printf("ptr1: %p\n", ptr1);
    printf("ptr1[0] = %d;\n", ptr1[0]);

    // notice program memory in TaskManager or Activity Monitor went back down!
    printf("\nPress enter to continue...\n");
    fgetc(stdin);

    printf("\nFreeing everything...\n");
    free(ptr1);
    free(ptr2);

    printf("I don't own this: ptr1[0] = %d;\n", ptr1[0]);
    ptr1[0] = 123;
    printf("I changed it anyway: ptr1[0] = %d;\n", ptr1[0]);
}

sample_output.txt

$ gcc -std=c99 main.c && ./a.exe
The size of an int is 4

ptr1: 0x6000484c0
ptr2: 0x600049470

Before I set it: ptr1[0] = 0;
ptr1[0] = 1525;

Reallocation ptr1 to 4000000 ints...
Realloc succeeded!
ptr1: 0x6ffff0b0010
ptr1[0] = 1525;
filling memory that I just allocated.

Press enter to continue...

Resizing ptr1 back to 1000 members long.
Realloc succeeded!
ptr1: 0x60005a430
ptr1[0] = 1525;

Press enter to continue...


Freeing everything...
I don't own this: ptr1[0] = 1525;
I changed it anyway: ptr1[0] = 123;