Lecture 1 - Introduction, Software Setup, Hello World¶

0. Introduction¶

Welcome to CMSC 14300 / CS143 - Systems Programming
What is "systems programming"?
Getting closer to the machine than Python lets you
Bits, bytes, pointers, memory - making the computer's operations transparent
Why C?
The lingua franca of programmers; foundational, historic, still everywhere
Quirky and flawed, but one of the great practical language designs
You already program (Python, data structures) - now we go a level down
Course arc (Weeks 1-8): | Wk | Topics | |----|--------| | 1 | Basics, the terminal, control structures, functions, loops | | 2 | Operators, bits, binary encoding | | 3 | Pointers, allocation, arrays | | 4 | Strings, structs, unions | | 5 | Linked lists, hash tables | | 6 | pthreads, locks | | 7 | Trees | | 8 | Sorting, graphs |

1. Syllabus & Logistics¶

Walk through the course website live (site/) and Ed
Instructor: Suhail Rehman - JCL158, office hours by appointment
Meetings: Mon & Wed, 6:00-8:30 PM, RY277
Textbook: K&R, The C Programming Language - recommended, not required
Grading: 50 / 50
50% programming assignments
50% quizzes + final exam
Guarantees: 92 -> A-, 82 -> B-, 72 -> C-, 65 -> D- (boundaries may drop, never rise)
Big quiz/assignment discrepancies -> handled as special cases
Final exam: in class, Wed Aug 5 (last lecture)
Late policy: 3 late days total (Gradescope, per-minute); then 4% of total course grade per day; no assignment > 3 days late
Quizzes: weekly, ~20-30 min, start Week 3 (Mondays); cover prior week
Ungraded PSets: released Wednesdays, done before next class, discussed in class, used to prep for quizzes
Academic integrity: don't copy / don't be copied; document collaboration; cite sources (even small ones, as code comments)
Generative AI policy - read carefully:
No LLMs to write CS143 code
No pasting your CS143 code into an LLM for suggestions/fixes
No uploading course material (syllabus, assignments) to an LLM
Make sure your work is actually your work

2. Computer Systems Overview¶

A few Basics - Input, Processing, Output
Hardware vs. Software
Hardware: the physical components of a computer (CPU, memory, storage)
Software: the programs and instructions that run on the hardware
Von Neumann Architecture
The stored-program concept: instructions and data are stored in the same memory
CPU fetches instructions, decodes them, and executes them
This architecture is the foundation of most modern computers
Low-level vs High-Level Programming
Low-level: closer to the machine, involves direct hardware manipulation
High-level: abstracts away hardware details, easier to write and understand
This distinction is not absolute; think of it as a spectrum
The absolute lowest level is machine code and/or assembly language, which is the pure representation of the computer's instructions that run on a CPU - these are in Binary (more on binary in the next few lectures). Assembly language is a machine code that has been made human-readable using mnemonics.
Python is a high-level programming language, which abstracts away many low-level details.
C is a lower-level programming language, which provides more direct control over hardware and memory.
Compiled vs. interpreted
Python: interpreted, dynamic typing, runs via an interpreter
Python code is converted to bytecode (an intermediate representation)
C: compiled ahead of time to native machine code
The pipeline: source code -> compiler -> machine code -> execution
What the machine actually has (teaser, depth comes later):
CPU executes instructions; memory holds code + data
Stack vs. heap distinction -> revisited Week 3 (pointers/allocation)
Static typing in C
Types are mandatory; the program must type-check to compile
Types guide memory layout and catch errors before the program runs

3. Intro to the C Language¶

History/lineage: Kernighan & Ritchie, Bell Labs, Unix - the "K&R" book
Our toolchain: clang as the compiler
Anatomy of a minimal C program:
#include <stdio.h> - bring in declarations (here, printf)
int main(void) - the entry point; returns an int status
statements end with ;
return 0; - 0 means success
The edit -> compile -> run loop

Live-coding break - Hello World¶

Write hello.c together, compile with clang, run it
Full walkthrough in the handout: hello_world.md
Deliberately break it (drop a ;, drop the #include) -> read the errors

4. C Programming Basics - Syntax¶

Coming from Python: the biggest visible changes
Code blocks are delimited by braces { }, not by indentation
Statements must end with a semicolon ;
Whitespace/indentation is for humans - the compiler ignores it (so indent anyway, for readability)
Comments: // single line and /* block comment */
Everything lives inside functions; execution starts at main
A program is a sequence of declarations and statements; order matters
Common beginner errors (preview them now, you'll meet them again):
missing ;, mismatched { }, calling something before it's declared

5. Data Types & Variables¶

Static typing: every variable has a fixed type, declared before use
int count; then count = 5; - or together: int count = 5;
Contrast with Python, where a name can be rebound to any type
Core built-in types (sizes are typical, not guaranteed - more in Week 2): | Type | Holds | Example literal | |------|-------|-----------------| | int | whole numbers | 42, -7 | | char | a single byte / character | 'A', '\n' | | float | single-precision real | 3.14f | | double | double-precision real | 3.14159 |
char is really a small integer - 'A' is 65 (teaser for binary/ASCII week)
Integer division truncates: 7 / 2 is 3, not 3.5 (a classic bug)
Mix in a double to get real division: 7 / 2.0 is 3.5
Variables are uninitialized garbage until you assign them - unlike Python

6. Control Structures¶

Conditionals:
if (cond) { ... } else if (cond) { ... } else { ... }
Conditions are integers: 0 is false, anything non-zero is true
Loops:
while (cond) { ... }
for (init; cond; update) { ... } - e.g. for (int i = 0; i < n; i++)
do { ... } while (cond); - runs the body at least once
switch / case / break / default - multi-way branch on an integer
Jump statements: break (leave a loop), continue (next iteration)
Watch out: if (x = 5) assigns instead of comparing - use == for equality

7. `printf` & Format Specifiers¶

printf prints formatted text; the first argument is a format string
Format specifiers are placeholders filled by the remaining arguments: | Specifier | Prints | |-----------|--------| | %d | int | | %f | float / double | | %c | single character | | %s | string (Week 4) | | %% | a literal % |
Examples:
printf("count = %d\n", count);
printf("%d C = %f F\n", c, f);
\n is a newline; without it, output runs together
The specifier must match the argument's type - %d with a double is undefined behavior (no error, just wrong output) - a recurring theme in C

Reading input with `scanf`¶

scanf is the mirror image of printf: it reads formatted input from the user instead of writing it
Same specifiers (%d, %f, %c, ...), but there's one crucial difference:
You must pass the address of the variable, using &
scanf("%d", &n); - read an int into n
scanf("%lf", &temp); - read a double (note: %lf, not %f, for scanf)
The & ("address-of") gives scanf somewhere to store the result; without it you'll get garbage or a crash. Why this is needed becomes clear once we reach pointers (Week 3) - for now, just remember: scanf needs &

Always prompt first so the user knows you're waiting:

int n;
printf("Enter a number: ");
scanf("%d", &n);

In-class exercise break¶

1. Celsius -> Fahrenheit. Read a Celsius value and print the Fahrenheit equivalent. Discuss why we write 9.0 / 5.0 (real division) and not 9 / 5 (which truncates to 1).

#include <stdio.h>

int main(void) {
    double celsius;

    printf("Enter temperature in Celsius: ");
    scanf("%lf", &celsius);

    double fahrenheit = celsius * 9.0 / 5.0 + 32.0;
    printf("%.1f C = %.1f F\n", celsius, fahrenheit);

    return 0;
}

Point out %.1f - a format modifier that prints one digit after the decimal.

2. The Count's Counting Program. In the spirit of Count von Count from Sesame Street: read a number n from the user, then count from 1 up to n, one per line, and finish with a triumphant total. A simple for loop over a couple of int variables.

#include <stdio.h>

int main(void) {
    int n;

    printf("How high shall we count today? ");
    scanf("%d", &n);

    for (int i = 1; i <= n; i++) {
        printf("%d! Ah-ah-ah!\n", i);
    }

    printf("%d wonderful numbers! Mwa-ha-ha-ha!\n", n);

    return 0;
}

Discuss the loop anatomy: init; cond; update, and what happens when n is 0 or negative (the loop body never runs - a good edge case to mention).

8. Software Setup Overview¶

Need a Linux or macOS machine (Windows: WSL)
Tools we install this quarter (all free / open-source, industry-standard):
clang - compiler
git - version control
lldb - debugger
valgrind - memory analysis
Detailed setup is self-serve this week - follow the posted instructions; bring problems to Ed / office hours

9. IDEs, Editors & Command-Line Tools - opinions¶

Editor vs. IDE - what's the difference, and does it matter now?
Why we start at the terminal + compiler directly:
You learn what the IDE is doing for you
The skills transfer to any machine / any future job
Quick survey: VS Code, vim/neovim, emacs, nano
Pick one you can be productive in; don't fight your tools
If you haven't learned a text editor yet, I am partial to vim.