class: center, middle # CMPE 30: Lecture 1 Introduction to C --- # Learning Objectives - Describe how C and modern C++ differ - Know the K&R book and why modern C requires `int main(void)` - Compile a C program with `cc` - Use the core `printf` format specifiers - Control width, precision, and zero-fill - Use `scanf` with `%d` and `%s` - Avoid the common `printf`/`scanf` traps --- # Why Learn C? - `cfront` originally translated C++ into C - Modern C++ has drifted far from C - Operating systems, firmware, databases, runtimes are all C - You *will* meet C in a C++ job --- # C++ vs C (1 of 2) .lc[ | C++ | C | |:---|:---| | `std::string` | `char[]` + `'\0'` | | `std::vector` | arrays / `malloc` | | `std::cout` | `printf` | | `std::cin` | `scanf` | | `new` / `delete` | `malloc` / `free` | ] .rc[ - No classes in C - No templates in C - No exceptions in C - No smart pointers in C ] --- # C++ vs C (2 of 2) .lc[ | C++ | C | |:---|:---| | smart pointers | raw pointers | | classes | structs + functions | | references (`&`) | pointers (`*`) | | `bool` built-in | `_Bool` / `<stdbool.h>` | | `// comments` | `/* ... */` (C89) | ] .rc[ - `//` comments OK since C99 - `_Bool` arrived in C99 - `<stdbool.h>` provides `bool` - `bool` a real keyword since C23 - References look like addresses --- confusing ] --- # The Book: K&R .lc[ ```c #include <stdio.h> main() { printf("hello, world\n"); } ``` ] .rc[ - *The C Programming Language* (2nd ed.) --- Kernighan & Ritchie - The classic reference, still the go-to C textbook - **Trap:** that listing is 1988 style --- bare `main()` is an error today - Write `int main(void)` ] --- # Hello, World .lc[ ```c #include <stdio.h> int main(void) { printf("hello, world\n"); return 0; } ``` ] .rc[ - `.c` extension, not `.cpp` - Compile: `cc hello.c` - `printf` lives in `<stdio.h>` - `\n` is yours to add --- `printf` won't ] --- # Core Format Specifiers .lc[ | Spec | For | Example | |:---|:---|:---| | `%d` | `int` | `42` | | `%x` `%X` | `int` hex | `ff` / `FF` | | `%f` | `double` | `3.140000` | | `%e` | `double` sci | `3.140000e+00` | | `%c` | `char` | `A` | | `%s` | `char *` | `hola` | | `%p` | pointer | `0x7ffd...` | | `%zu` | `size_t` | `4` | ] .rc[ - Cast pointer to `void *` before `%p` - `%zu` for `sizeof` and `strlen` - Mismatch between specifier and type is **undefined behavior** ] --- # Width, Precision, Zero-Fill .lc[ ```c printf("%f\n", 98.6); // 98.600000 printf("%.2f\n", 98.6); // 98.60 printf("%02d\n", 3); // 03 printf("%06X\n", 0xFF8800); // FF8800 printf("%d%%\n", 95); // 95% ``` ] .rc[ - `.N` sets precision - Leading `0` pads with zeros instead of spaces - `%%` prints a literal `%` - Same syntax works in `fprintf`, `snprintf` ] --- # printf Traps - **Format string attack:** never write `printf(user_str)` - Always: `printf("%s", user_str)` - Mismatched specifier + type is undefined behavior - `-Wall -Wformat` catches many cases --- use them --- # scanf Basics .lc[ ```c int age; scanf("%d", &age); char name[50]; scanf("%49s", name); ``` ] .rc[ - `scanf` fills variables, so it needs addresses - `&` gets the address of a scalar - Arrays already are addresses - Always cap `%s` with a width ] --- # Reading Multiple Items .lc[ ```c int track; char name[50]; if (scanf("%d %s", &track, name) != 2) { printf("I needed " "a number and a name\n"); } ``` ] .rc[ - `scanf` returns items read - `EOF` when input is exhausted - `%s` stops at the first whitespace - Think of `scanf` as a rough parser, not a safe one ] --- # scanf Traps - Missing `&`: `scanf("%d", age);` --- compiles with a warning, crashes at runtime - Unbounded `%s`: `scanf("%s", name);` can overflow your buffer - Fix: `scanf("%49s", name);` for a 50-byte buffer **Trap:** `scanf` does no bounds checking --- bound it yourself. --- # Live Coding: Try It .lc[ ```c int num; printf("Enter a number: "); scanf("%d", &num); printf("%d %x %X\n", num, num, num); printf("%c\n", num); for (int i = 1; i <= 3; i++) printf("Track %02d\n", i); printf("Pi: %.2f\n", 3.14159); printf("100%% complete\n"); ``` ] .rc[ - Compile with `cc -Wall -Wextra -pedantic` - Intentionally swap `%d` for `%f` and show the warning - The prompt has no `\n` --- the cursor waits on the prompt line; add one, rerun, discuss buffering (preview) ] --- # What does this print? ```c printf("%05d %x\n", 42, 255); ``` 1. `00042 ff` 1. `42 255` 1. ` 42 ff` 1. `00042 FF` 1. Ben got this wrong --- # Where is the bug? ```c int count; scanf("%d", count); ``` 1. `%d` is wrong for `int` 1. Missing `&` before `count` 1. `count` is uninitialized 1. Missing semicolon 1. Ben got this wrong --- # Why pick printf for a log file? 1. printf is slower, which gives prettier output 1. Format strings make the message template visible and easy to translate 1. printf never fails 1. std::cout does not support integers 1. Ben got this wrong --- # Key Points - C and C++ are different languages - `.c` + `cc`; `<stdio.h>` replaces `<iostream>` - Specifiers must match argument types - `scanf` needs `&` on scalars, not arrays - Always cap `%s` with a width - Never `printf(user_string)` **Read:** chapter 3 of *Gorgo C for C++ Programmers*. **Do:** exercises 1-5, 7-10. **Skim:** chapter 2 (Variables) on your own --- types, arrays, `const`, structs are familiar from C++.