Lecture 2: Introduction to C Programming Language

Key security concept: Pointers / Arrays / Implementation for Memory management

All of the above are unsafe: If your CS61C program contains an error, it might not crash immediately but instead leave the program in an inconsistent (and often exploitable) state.

Now is 2024! We need more!

• Rust, “C-but-safe”: By the time your C is (theoretically) correct w/all necessary checks it should be no faster than Rust.
• Go, “Concurrency”: Practical concurrent programming takes advantage of modern multi-core microprocessors.

C Basics

./a.out and replacements

change the NAME of binary executable file:

gcc –o NAME FILE.c

then you can run it by:

./NAME

Why ./

reason here

Compilation

C compilers map C program directly into architecture-specific machine code (string of 1s and 0s)

• Unlike Java, which converts to architecture-independent bytecode that may then be compiled by a just-in-time compiler (JIT)
• Unlike Python environments, which converts to a byte code at runtime

These differ mainly in exactly when your program is converted to low-level machine instructions (“levels of interpretation”)

Disadvantages

Advantages

Command-line arguments

Combined, argc and argv get the main function to accept arguments.

• argc will contain the number of strings on the command line (the executable counts as one, plus one for each argument). Here argc is 2:

  $ ./a.out myFile
  

  The number 2 includes ./a.out itself (really interesting)
• argv is a pointer to an array containing the arguments as strings (more later re: pointers and strings).

Example

#include<stdio.h>

int main(int argc, char* argv[])
{
    printf("Hello World!\n");
    return 0;
}

Printf() formats

%s: string
%d: decimal numeral
%x: hexadecimal numeral

Basic Types

1. sizeof(...)
2. intN_t and uintN_t

Consts, Enums, #define

Constant, const is assigned a typed value once in the declaration.

• const Value can't change during entire execution of program.

const float π = 3.14;
const int day_in_week = 7;
const double the_law = 2.99792458e8;

#define is a preprocessor directive that defines a macro. It's a CPP (C PreProcessor) Macro(宏).

• Prior to compilation, preprocess by performing string replacement in the program based on all #define macros.
• Replace all PI with (3.14159)àIn effect, makes PI a “constant”

Enums: a group of related integer constants.

enum cardsuit {CLUBS, DIAMONDS, HEARTS, SPADES};
enum color {RED, GREEN, BLUE};

Typedef and struct

1) typedef: create a new name for an existing type, "alias".

typedef uint8_t BYTE;
BYTE b1, b2;

2) struct: create a new type that are structured groups of variables.

There are 3 ways to clarify a struct, you can see here

CPP Macros

Appendix

1. linux下很多可执行文件（二进制文件）并不需要特定的后缀

2. linux系统依赖文件的权限和内容来识别和处理文件，而不是依赖文件的扩展名（chmod...）

3. Windows经常使用扩展名标识文件类型（.exe 表示可执行文件），这是windows自身“小众化”的问题，大多数系统都是不需要的