### Pointers and memory allocation
- How does C request dynamic memory when you don't know at compile-time exactly what you will need?
- How does C allocate memory?
- Automatic: compile arranges for memory to be allocated and initialized for local variables when it is in scope.
- Static: memory for static variables are allocated once when program starts.
- Dynamic: memory is allocated on the fly as needed.
### Dynamic Memory Allocation in C
- Conceptually, it is similar to Java
- But you have to do everything!
### malloc, free
https://linux.die.net/man/3/malloc
### Hands-on
- Start virtualbox
- Start the *csc-331* VM
### Setup directory
- If you shutdown your VM properly, you should still have the `intro-c` directory.
- Confirm that you are in your `home` directory by typing `pwd` and confirm that you are in `/home/student`.
- Confirm that you still have `intro-c` using `ls`.
- Change into `intro-c`.
- If you don't have `intro-c`, create one (take a look at Tuesday's lecture for instruction.)
```
$ ls
$ cd intro-c
```
### What points to where!!!
- `void *p = malloc(100);`
- Allocate and assign the address of 100 bytes of unknown type to pointer variable `p`.
- `int *ip = (int *)p;`
- Assign the address of the 100 bytes above to pointer variable `ip`, and cast `ip` to type int.
- Now you can use `*ip` to manipulate these 100 bytes.
### Compile and Run: Simple Compilation
- Make sure that you are still inside `/home/student/intro-c` using `pwd`.
```
$ pwd
$ gcc malloc.c
$ ./a.out
```
### So wasteful!!!
- An `int` is only 4 to 8 bytes.
- How do we know how much to ask for?
### Array size determination
- In Java, you can allocate storage for an array at any time in the program, not just when a variable is first initialized.
- In C, you need to specify the array size upfront when using the standard [] notation (*rare exception* with C99 standards).
- What to do?
### What is an array in C?
- In Java, you can allocate storage for an array at any time in the program, not just when a variable is first initialized.
- In C, you need to specify the array size upfront when using the standard [] notation (*rare exception* with C99 standards).
- What to do?
### Exercise
- Create a copy of `array.c` called `array-2.c`
- Change type of `numbers` to `double`.
- What is the address step now?
### An array variable ...
- ... is in fact pointing to an address containing a value.
- ... without the bracket notation and with an index points to the corresponding address of the value at the index.
### An array variable ...
- ... is quite similar to a pointer.
### Array of Strings
- String is an array of characters.
- What then is an array of strings?
### Object in C
- C has no classes or objects
### Object in C
- C has **struct** type (think ancestor of object)
### Composite struct
Just like Java
```
struct line {
struct point start;
struct point end;
};
....
struct line line1;
line1.start.x = 1;
line1.start.y = 2;
line1.end.x = 3;
line1.end.y = 4;
```
### Almost the same as methods in Java, except for one tiny difference ...
## Everything is a pointer!!!
## How do you replicate the fundamental behavior of object and methods in Java with struct, function, and pointer in C?