Operating Systems: Introduction to C

1. What is C?

• Developed by Dennis Ritchie at Bell Labs
• First public released in 1972.
• The book: *The C programming languange” by Dennis M. Ritchie and Brian W. Kernighan. Prentice Hall 1988.

2. How to learn C (now that you already know Java)?

• C for Java programmers
• C programming vs. Java programming

3. Scary stuff ahead …

• C is much less supportive for programmers than Java.
• (Much) easier to make mistake, and (much) harder to fix.

4. But it is exciting …

• C requires less memory resources than Java.
• C, in many instances, runs faster than Java.
• Knowing C will make you a better programmer overall.

5. Similarities (or mostly similar) between C and Java

• Values, types, literals, expressions
• Variables
• Control flow (if, switch, while, for, do-while)
• Call-return: parameters, arguments, return values
• Arrays (mostly)
• Primitive and reference types
• Type casting.
• Library usage.

6. Differences between C and Java

• C has no classes or objects (but something similar)
• C is not object-oriented.
• C arrays are simpler:
  • No boundary checking.
  • No knowledge of array’s own size.
• String operations are limited.
• No collections, exceptions, or generics.
• No automatic memory management.
• Pointers!!!

7. How Java programs run

8. How C programs run

9. Hands-on: Getting started

• Open a terminal (Windows Terminal or Mac Terminal).
• Reminder: It is podman on Windows and docker on Mac. Everything else is the same!.
• Launch the container:

Windows:

$ podman run --rm --userns keep-id --cap-add=SYS_PTRACE --security-opt seccomp=unconfined -it -p 2222:22 -v /mnt/c/csc231:/home/$USER/csc231:Z localhost/csc-container /bin/bash

Mac:

$ podman run --rm --userns keep-id --cap-add=SYS_PTRACE --security-opt seccomp=unconfined -it -p 2222:22 -v /Users/$USER/csc231:/home/$USER/csc231:Z localhost/csc-container /bin/bash

10. Hands-on: Setup directory

Create a directory named intro-c inside your home directory, then change into that directory.

$ cd ~/csc231
$ pwd
$ mkdir intro-c
$ ls
$ cd intro-c
$ pwd

11. Hands-on: Create hello.c

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

$ pwd
$ nano intro-c

• Once finished editing in nano:
  • first hit Ctrl-X (same for both Windows and Mac).
  • Next hit Y to save modified buffer (new contents).
  • Hit Enter to save to the same file name as what you opened with.
• Memorize your key-combos!.

12. What’s in the code?

• Line 1: Standard C library for I/O, similar to Java’s import.
• Line 2-4: Function declaration/definition for main:
  • Line 2:
    • return type: int
    • function name: main
    • parameter list:
      • argc: number of command line arguments.
      • *argv[]: pointers to array of command line argument strings.
  • Line 3: Invoke builtin function printf to print out string Hello world! with an end-of-line character \n. This is similar to System.out.printf.
  • Line 4: Exit a successfully executed program with a return value of 0.

13. Hands-on: Simple compile and run

• Similar to javac, we use gcc to compile C code.
• Before compile, make sure that you are still inside intro-c in the terminal.

$ ls
$ pwd
$ gcc -o hello hello.c
$ ls
$ ./hello

14. Hands-on: Compile and show everything

• There are a number of steps from C codes to executable binaries.

$ ls -l
$ gcc -save-temps -o hello2 hello.c
$ ls -l
$ ./hello2

15. What are those?

• hello.i: generated by pre-processor
• hello.s: generated by compiler.
• hello.o: generated by assembler.
• hello (or hello2): executable, generated by linker.

16. Hands-on: View files

• For hello.i and hello.s, they can be view on the editor.
• Run the following command to view hello.i

$ cat -n hello.i

• Run the following command to view hello.s

$ cat -n hello.s

• For hello.o and hello, we need to dump the binary contents first.

$ xxd -b hello.o > hello.o.txt
$ xxd -b hello > hello.txt
$ ls -l

• 

• Run the following command to view hello.o.txt

$ cat -n hello.o.txt

• Run the following command to view hello.txt

$ cat -n hello.txt

17. Challenge:

The usage of C’s printf is similar to Java’s System.out.printf. Find out how to modify hello.c so that the program prints out Hello Golden Rams! with each word on a single line. The program should use exactly one printf statement.

Answer

18. Variables, 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.

19. 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

20. Pointer Definition

• Pointer is a variable that points to a memory location (contains a memory location).
  • We can 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.

21. 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

22. Pass by Value and Pass by Reference

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

23. 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

24. 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

25. Question

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

Answer

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

26. 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.

27. 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.

28. 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.

29. 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

30. 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

31. 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

32. 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?

33. Hands-on: What does an array in C look like?

• 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

34. 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

35. 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!

36. 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

37. Hands-on: Dynamic array creation with 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

38. Hands-on: 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.

39. Hands-on: Array of strings

• Inside the terminal, make suse 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

40. Object in C

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

41. 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

42. 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

43. 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).

44. 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.