Even more C

Even more C

:class: tip This lecture will cover contents from Chapter 2 and Chapter 3 of the book.

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## Addresses and pointers


- In Java, you can manipulate the value of a variable via the program but not directly 
in memory (inside the JVM). 
- In C, you can retrieve the address of the location in memory where the variable is 
stored. 
- The operator `&` (reference of) represents the memory address of a variable.



::::{dropdown} Hands-on: Pointer

- Inside the terminal, make suse that you are still inside
`intro-c`, then use `nano` to create `pointer-1.c`with the source code below. 

<script src="https://gist.github.com/linhbngo/d1e9336a82632c528ea797210ed0f553.js?file=pointer-1.c"></script>

- `%p` is an output conversion syntax (similar to Java specifiers) for displaying memory 
address in hex format. See [Other Output Conversions](https://www.gnu.org/software/libc/manual/html_node/Other-Output-Conversions.html#Other-Output-Conversions) for more 
details.
- Compile and run `pointer-1.c`

 

ls gcc -o pointer-1 pointer-1.c ./pointer-1 ```

:::{image} fig/02-c/08.png :alt: Compile and run pointer-1.c :class: bg-primary mb-1 :height: 400px :align: center :::

::::

:::{dropdown} Pointer Definition

• Pointer is a variable that points to a memory location (contains a memory location).
  • We call them pointer variables.
• A pointer is denoted by a * character.
• The type of pointer must be the same as that of the value being stored in the memory location (that the pointer points to).
• If a pointer points to a memory location, how do we get these locations?
  • An & character in front of a variable (includes pointer variables) denotes that variable’s address location.

:::

::::{dropdown} Hands-on: Pointer and Variable’s Addresses

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create pointer-2.cwith the source code below.

• Since p_i is a pointer variable, p_i contains a memory address (hence %p).
• Then, *p_i will point to the value in the memory address contained in p_i.
  • This is referred to as de-referencing.
  • This is also why the type of a pointer variable must match the type of data stored in the memory address the pointer variable contains.
• Compile and run pointer-2.c

:::{image} fig/02-c/09.png :alt: Compile and run pointer-2.c :class: bg-primary mb-1 :height: 150px :align: center ::: ::::

::::{dropdown} Pass by Value and Pass by Reference

• Parameters are passed to functions.
• Parameters can be value variables or pointer variables.
• What is the difference?

::::

::::{dropdown} Hands-on: Pass by value

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create pointer-3.cwith the source code below.

• Compile and run pointer-3.c

:::{image} fig/02-c/10.png :alt: Compile and run pointer-3.c :class: bg-primary mb-1 :height: 150px :align: center ::: ::::

::::{dropdown} Hands-on: Pass by reference

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create pointer-4.cwith the source code below.

• Compile and run pointer-4.c

:::{image} fig/02-c/11.png :alt: Compile and run pointer-4.c :class: bg-primary mb-1 :height: 450px :align: center ::: ::::

• In Java, do you pass by value or pass by reference?

:::{dropdown} Answer

• Primitives are passed by value.
• Objects are passed by reference. :::

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.

::::{dropdown} Dynamic memory allocation

• Unlike Java, you have to do everything!
  • Ask for memory.
  • Return memory when you are done (garbage collection!).
• C function: malloc
  • void *malloc(size_t size);
  • The malloc() function allocates size bytes and returns a pointer to the allocated memory. The memory is not initialized. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().
• C function: free
  • void free(void *ptr);
  • The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if free(ptr) has already been called before, undefined behavior occurs. If ptr is NULL, no operation is performed.

::::

::::{dropdown} Void pointer

• When malloc allocates memory, it returns a sequence of bytes, with no predefined types.
• A pointer that points to this sequence of bytes (the address of the starting byte), is called a void pointer.
• A void pointer will then be typecast to an appropriate type.

::::

::::{dropdown} Hands-on: malloc and type cast

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create malloc-1.cwith the source code below.

• What points to where:
  • void *p = malloc(4);: allocate 4 contiguous bytes. The address of the first byte is returned and assign to pointer variable p. p has no type, so it is a void pointer.
  • int *ip = (int *)p;: The address value pointed to by p is assigned to pointer variable ip. The bytes pointed to be p are now casted to type int.
• Compile and run malloc-1.c

:::{image} fig/02-c/12.png :alt: Compile and run malloc-1.c :class: bg-primary mb-1 :height: 150px :align: center ::: ::::

::::{dropdown} Hands-on: malloc and type cast with calculation

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create malloc-2.cwith the source code below.

• Only ask for exactly what you need!
• Compile and run malloc-2.c

:::{image} fig/02-c/13.png :alt: Compile and run malloc-2.c :class: bg-primary mb-1 :height: 150px :align: center ::: ::::

::::{dropdown} Hands-on: Safety

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create malloc-3.cwith the source code below.

• Return and free memory after you are done!
• Compile and run malloc-3.c

:::{image} fig/02-c/14.png :alt: Compile and run malloc-3.c :class: bg-primary mb-1 :height: 150px :align: center ::: ::::

::::{dropdown} Dynamic memory allocation

• Critical to support complex data structures that grow as the program executes.
• In Java, custom classes such as ArrayList and Vector provide such support.
• In C, you have to do it manually: How?
• Let’s start with a simpler problem:
  • How can we dynamically allocate memory to an array whose size is not known until during run time? ::::

Array in C

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create array-1.cwith the source code below.

• What is the distance between addresses? Why?
• Compile and run array-1.c

:::{image} fig/02-c/15.png :alt: Compile and run array-1.c :class: bg-primary mb-1 :height: 250px :align: center :::

:::::{dropdown} Exercise

• Create a copy of array-1.c called array-2.c.
• Change the type of numbers to double.
• What is the address step now?

::::{dropdown} Answer

:::{image} fig/02-c/16.png :alt: Compile and run array-2.c :class: bg-primary mb-1 :height: 250px :align: center ::: :::: :::::

::::{dropdown} An array variable

• … is in fact pointing to an address containing a value.
• … without the bracket notation and an index points to the corresponding address of the value at the index.
• … is quite similar to a pointer! ::::

::::{dropdown} Hands-on: Array as pointer (or vice versa …)

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create array-3.cwith the source code below.

• Compile and run array-3.c

:::{image} fig/02-c/17.png :alt: Compile and run array-3.c :class: bg-primary mb-1 :height: 250px :align: center ::: ::::

Command line arguments.

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create array-4.cwith the source code below.

• In C, the command line arguments include the program’s name. The actual arguments start at index position 1 (not 0 like Java).
• Compile and run array-4.c

:::{image} fig/02-c/18.png :alt: Compile and run array-4.c :class: bg-primary mb-1 :height: 250px :align: center :::

String

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create string-1.cwith the source code below.

• Compile and run string-1.c

:::{image} fig/02-c/19.png :alt: Compile and run string-1.c :class: bg-primary mb-1 :height: 150px :align: center :::

• In C, string is considered an array of characters.
• How many characters were printed out on the second line in the terminal?
• Hint: Can you see all of them?

:::{dropdown} Answer
24 :::

::::{dropdown} Hands-on: Array of strings

• Inside the terminal, make sure that you are still inside intro-c, then use nano to create string-2.cwith the source code below.

• Compile and run string-2.c

:::{image} fig/02-c/20.png :alt: Compile and run string-2.c :class: bg-primary mb-1 :height: 150px :align: center ::: ::::

Object in C

• C has no classes or objects.
• Instead, it has struct type (think ancestor of objects) .

::::{dropdown} Hands-on: Struct in C

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create struct-1.cwith the source code below.

• Compile and run struct-1.c

:::{image} fig/02-c/21.png :alt: Compile and run struct-1.c :class: bg-primary mb-1 :height: 150px :align: center ::: ::::

::::{dropdown} Challenge

• Modify struct-1.c so that it prints out the address of origin variable.
• What do you learn from the printed out addresses?

:::{dropdown} Answer
Insert printf("The address of the origin is: %p\n", &origin); between the existing printf calls.
::: ::::

::::{dropdown} Hands-on: Struct of structs in C

• Inside the terminal, make suse that you are still inside intro-c, then use nano to create struct-2.cwith the source code below.

• Compile and run struct-2.c

:::{image} fig/02-c/22.png :alt: Compile and run struct-2.c :class: bg-primary mb-1 :height: 150px :align: center ::: ::::

Function in C

• Almost the same as methods in Java, except for one small difference.
• They need to either be declared, or must be defined prior to being called (relative to written code position).

::::{dropdown} Hands-on: Functions in C - definition and declaration

• Create three C files, function-1.c, function-2.c, and function-3.c, with the source codes below:

• Compile and run these files.

:::{image} fig/02-c/23.png :alt: Compile and run function-1.c, function-2.c, function-3.c :class: bg-primary mb-1 :height: 250px :align: center ::: ::::