Dynamic Memory Allocation (Solutions)

Originally created by Jerrett Longworth and Idel Martinez in Fall 2020.

Let’s take a look at one of the craziest concepts we have in our toolkit to date: dynamically-allocated memory.


Dynamically Allocated Structs

  1. Is this program managing memory correctly?
typedef struct Computer
{
  int memory;
  int processes;
  float power;
} Computer;

int main(void)
{
  Computer *my_PC = malloc(sizeof(Computer));

  printf("Hello, world!\n");

  return 0;
}

Answer: Nope, we’re missing free(my_PC) so memory is getting leaked!

  1. Are these two structs equal? In other words, is the condition inside the if statement true? What will print out in this program?
typedef struct Computer
{
  int memory;
  int processes;
  float power;
} Computer;

Computer *copy_struct(Computer *pc)
{
  Computer *temp = malloc(sizeof(Computer));
  temp->memory = pc->memory;
  temp->processes = pc->processes;
  temp->power = pc->power;

  return temp;
}

int main(void)
{
  Computer *my_PC = malloc(sizeof(Computer));
  Computer *my_other_PC;

  my_PC->memory = 10000;
  my_PC->processes = 29;
  my_PC->power = 79.81;
  my_other_PC = copy_struct(my_PC);

  if (my_PC == my_other_PC)
  {
    printf("Now my PC and my other PC are equal!\n");
  }
  else
  {
    printf("MEM: %d vs %d\n", my_PC->memory, my_other_PC->memory);
    printf("PRO: %d vs %d\n", my_PC->processes, my_other_PC->processes);
    printf("POW: %.2f vs %.2f\n", my_PC->power, my_other_PC->power);
  }

  free(my_PC);
  free(my_other_PC);

  return 0;
}

Answer: The structs are equal, but the conditional is false since they are NOT pointing to the same address. You will see that the values that print out will be the same for my_PC and my_other_PC, since they are equal but independent - this is why we free twice.

  1. Are these two structs equal? In other words, is the condition inside the if statement true? Also, what is missing from this program regarding memory management?
typedef struct Computer
{
  int memory;
  int processes;
  float power;
} Computer;

Computer *copy_struct(Computer *pc)
{
  Computer *temp = pc;

  return temp;
}

int main(void)
{
  Computer *my_PC = malloc(sizeof(Computer));
  Computer *my_other_PC = copy_struct(my_PC);

  if (my_PC == my_other_PC)
    printf("Now my PC and my other PC are equal!\n");

  return 0;
}

Answer: Notice how the copy_struct function, different from the previous exercise, is setting one pointer to be my_PC. Therefore, both pointers will be pointing to the same address and the conditional is true. Notice also that we are missing the free… oh no!

  1. Is this the correct way to handle memory for these pointers to a struct?
typedef struct Computer
{
  int memory;
  int processes;
  float power;
} Computer;

Computer *copy_struct(Computer *pc)
{
  Computer *temp = pc;

  return temp;
}

int main(void)
{
  Computer *my_PC = malloc(sizeof(Computer));
  Computer *my_other_PC = copy_struct(my_PC);

  free(my_PC);
  free(my_other_PC);

  return 0;
}

Answer: Similar to the previous exercise, my_PC and my_other_PC are pointing to the same address (they are equal to each other). This means that when we free one of the pointers, we freed the memory they were pointing to and it would be incorrect to try to free again, since it had been freed before.

  1. What is the value of power for both my_PC and my_other_PC?
typedef struct Computer
{
  int memory;
  int processes;
  float power;
} Computer;

Computer *copy_struct(Computer *pc)
{
  Computer *temp = pc;
  temp->power = 100;

  return temp;
}

int main(void)
{
  Computer *my_PC = malloc(sizeof(Computer));
  my_PC->power = 79.81;
  Computer *my_other_PC = copy_struct(my_PC);

  free(my_PC);
  free(my_other_PC);

  return 0;
}

Answer: Once again, copy_struct is setting the pointers equal to each other so that they are pointing to the same address. This means that any changes to one pointer will also be reflected on the other pointer, so the value of power is 100 for both. Notice also that we should not be using free() twice. Can you see why?


CHALLENGE PROBLEMS:

  1. Below we have a Pixel struct with the red, green, and blue components. (1) Create an array of 1024 pixels dynamically, (2) assign values to each pixel, and (3) free the memory associated with the array.

Answer:

typedef struct Pixel
{
  int r;
  int g;
  int b;
} Pixel;

int main(void)
{
  Pixel *scan_line;
  int n = 1024;

  srand(time(NULL));

  // Create an array of pixels. Your code goes here
  scan_line = malloc(sizeof(Pixel) * n);

  for (int i = 0; i < n; i++)
  {
    // Your code goes here
    scan_line[i].r = rand() % 256;
    scan_line[i].g = rand() % 256;
    scan_line[i].b = rand() % 256;
  }

  // Time to free. Your code goes here
  free(scan_line);

  return 0;
}
  1. I keep forgetting my name, age, and birthday, so I want to create this program that stores it for me. But I’m surely not the only one with this problem, so you should use it as well. Can you help me create this program? Remember to allocate memory to a person (you), and assign a name, age, and birthday - plus free all the memory!

Answer:

typedef struct Birthday
{
  int day;
  int year;
  char month[10];
} Birthday;

typedef struct Person
{
  char *name;
  int age;
  Birthday birthday;
} Person;

int main(void)
{
  Person *human = malloc(sizeof(Person));
  int name_length = 14; // or better yet: strlen("Adele Goldberg")

  human->name = malloc(sizeof(char) * (name_length + 1));

  // Your code goes here
  strcpy(human->name, "Adele Goldberg");
  human->age = 75;

  human->birthday.day = 22;
  human->birthday.year = 1945;
  strcpy(human->birthday.month, "July");

  printf("Hello, I am %s, a %d-year-old.\n", human->name, human->age);
  printf("I was born in %s, %d, %d\n", human->birthday.month, human->birthday.day, human->birthday.year);

  free(human);

  return 0;
}

Crazy advanced answer using user input and more pointers:

typedef struct Birthday
{
  int day;
  int year;
  char month[10];
} Birthday;

typedef struct Person
{
  char *name;
  int age;
  Birthday *birthday;
} Person;

Person *create_person(char *name, int name_length)
{
  Person *person = malloc(sizeof(Person));

  person->name = malloc(sizeof(char) * (name_length + 1));
  strcpy(person->name, name);
  person->age = 100;

  person->birthday = malloc(sizeof(Birthday));
  person->birthday->day = 1;
  person->birthday->year = 1900;
  strcpy(person->birthday->month, "January");

  return person;
}

void free_person(Person *person)
{
  if (person == NULL)
    return;

  free(person->name);
  free(person->birthday);
  free(person);
}

int main(void)
{
  Person *human = NULL;
  int name_length;
  char buffer[1024];

  printf("What is your name? ");
  scanf("%s", buffer);

  name_length = strlen(buffer);
  human = create_person(buffer, name_length);

  printf("Hello, I am %s, a %d-year-old.\n", human->name, human->age);
  printf("I was born in %s, %d, %d\n", human->birthday->month, human->birthday->day, human->birthday->year);

  free_person(human);

  return 0;
}

Using calloc()

  1. What do lines 13 and 14 print out?
typedef struct Computer
{
  int memory;
  int processes;
  float power;
} Computer;

int main(void)
{
  Computer *my_PC = calloc(1, sizeof(Computer));
  Computer *my_other_PC = malloc(sizeof(Computer));

  printf("%d\n", my_PC->memory + my_PC->processes + my_PC->power);
  printf("%d\n", my_other_PC->memory + my_other_PC->processes + my_other_PC->power);

  free(my_PC);
  free(my_other_PC);

  return 0;
}

Answer: Because calloc sets initial values to the struct, my_PC members are all 0. malloc doesn’t do anything to do values and can can be garbage values. So, line 13 prints 0 and line 14 prints garbage values.