This code defines a struct for a dog, creates a new dog using a function, and then frees the memory used by the dog using another function.

The struct named dog defines the information that a dog should have, which includes the dog's name, age, and owner's name. A typedef statement is used to create a new type called dog_t that can be used in place of the struct dog.

The code then defines three functions: new_dog(), _strlen(), and _strcpy().

_strlen() takes a string as input and calculates its length by iterating through each character in the string and incrementing a counter. It returns the length of the string.

_strcpy() takes two strings as input and copies the contents of the second string into the first string. It does this by iterating through each character in the second string and copying it into the corresponding position in the first string. The function then adds a null terminator at the end of the copied string and returns a pointer to the destination string.

new_dog() creates a new dog using the input provided. It takes the name, age, and owner's name as arguments and checks to make sure that the name and owner's name are not null pointers, and that the age is greater than or equal to zero. It returns null if any of these conditions are not met. Otherwise, it allocates memory for a new dog_t struct using the malloc() function. If memory allocation is unsuccessful, it returns null. If successful, it then allocates memory for the name and owner using malloc() based on their length, then uses _strcpy() to copy the input name and owner into the allocated memory. Finally, it sets the dog's age and returns a pointer to the new dog.

free_dog() takes a pointer to a dog_t struct as input and frees the memory allocated for the struct's name, owner, and the struct itself. It first checks to see if the input pointer is null and returns if it is. Otherwise, it frees the memory allocated for the name, owner, and then the struct itself using the free() function.

The main() function demonstrates how to use the new_dog() and free_dog() functions. It creates a new dog named "Poppy" with an age of 3.5 and an owner named "Bob", prints out the dog's name and age, and then frees the memory used by the dog.