Even more C

Relevant Reading

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

1. 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.

Hands-on: Pointer

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

• %p is an output conversion syntax (similar to Java specifiers) for displaying memory address in hex format. See Other Output Conversions for more details.

• Compile and run pointer-1.c

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

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.

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

Pass by Value and Pass by Reference

• Parameters are passed to functions.

• Parameters can be value variables or pointer variables.

• What is the difference?

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

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

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

Answer

• Primitives are passed by value.

• Objects are passed by reference.

2. 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

• 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.

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.

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

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

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

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?

7. 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

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?

Answer

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!

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

8. 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

9. 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

• 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?

Answer

24

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

10. Object in C

• C has no classes or objects.

• Instead, it has struct type (think ancestor of objects) .

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

Challenge

• Modify struct-1.c so that it prints out the address of origin variable.

• What do you learn from the printed out addresses?

Answer

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

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

11. 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).

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.