Basics of C

A Crash Course in C
GDSC Reading Week Workshop 3

Admin Stuff
- A CSC209 disclaimer
- Getting the demo code
- SSH into the DH2020 Lab machines using the Remote-SSH VSCode extension
- Open terminal (Ctrl + `) in VSCode, and run:
- git clone https://github.com/utmgdsc/2023-c-workshop
- This should download the GitHub repository, and you should have access to the
ﬁles

What is C?
- A statically-typed, compiled programming language
- Developed by Ken Thompson and Dennis Ritchie for UNIX

The Impact of C
- Implementation language of numerous software tools
- All modern operating systems (Linux, Windows, MacOS)
- The Python interpreter
- Java Virtual Machine
- Anything low-level, really
- Inﬂuenced numerous other programming languages

Python vs. C
Python
- Runtime:
- Interpreted
- Type System:
- Dynamic, strong
- Paradigm:
- Object-oriented, functional
- Easy to use, beginner-friendly
- Sloooooow
C
- Runtime:
- Compiled
- Type System:
- Static, weak
- Paradigm:
- Structural/Imperative
- Difﬁcult to master, memory
management is a pain
- Blazing fast

Anatomy of a C Program
Import Statement
Entry Point
Declaration
Return Value
Return Statement
Print Statement

Control Flow and Data Types
- Conditional Statements
- If-Else, Switch-Case
- Loops
- For and While Loops
- Function Calls
- Type Casting
- Various Flavors of Integers
- Int, unsigned int, long, long long
- Floats and Doubles
- Char (characters)
- Booleans (with a caveat!)
- Arrays
- Structs (create your own type!)
- Pointers
- Strings…?
C is a very simple language,
but very hard to master!

Pointers: A Motivating Example
How many bytes do data types use?
- Integers
- Default 4 bytes (32 bits), longs are 8 bytes (64 bits)
- Characters
- Always 1 byte (8 bits)
- Booleans
- Always 1 byte
- Arrays
- However big the programmer declares them to be
- Size is known at compile time

How Big is a String?
- “Hello World!” has 12 characters, so 12 bytes
- “I like Dan Zingaro’s cows” has 25 characters, so 25 bytes
- A string can be 1 byte, 20 bytes, 420 bytes, 32,000 bytes…
We don’t know how big a string is!
(Rather, strings have arbitrary size that is not always known at compile time)

Pointers (for real this time)
- Stores the memory address of an object
- Rather than the object itself
- “Points” to the object
- Closest analogue: object IDs in Python
- Pointers are 4 bytes (32 bits) or 8 bytes (64 bits), depending on the system
- Indicated by an asterisk * in C
- e.g. int *c declares a pointer (named c) pointing to an integer
- Pointer operators
- ‘&’ (The reference operator)
- ‘*’ (The dereference operator)
- How strings are stored in C!
- char *, or char[]

Strings in Memory
H E L L O ! 0
0x45f24a 0x45f24b 0x45f24c 0x45f24d 0x45f24e 0x45f24f 0x45f250
char *string

C’s Memory Model
The Stack (a.k.a the call stack)
- Most sized variables live here
- Integers
- Pointers
- Predeﬁned arrays
- Etc etc.
- Most items get allocated and
deallocated automatically when a
function returns
- Size of stack variables cannot be
changed
The Heap
- Used for storing variables of
unknown size
- e.g. strings
- Size of heap variables can be
changed
- Always interacted with through
pointers
- Must be managed manually by the
programmer
- The reason why memory
management is so important!

Memory Management
Memory Allocation (malloc)
- Allocates a block of memory on the heap
- Returns a pointer to the allocated block
- Takes one argument specifying how
many bytes the programmer wants
- Comes in various ﬂavors:
- calloc - zeroes out the memory before
being used
- realloc - resizes an existing block of
memory
- If allocation fails, returns NULL
Memory Deallocation (free)
- Deallocates the memory being
pointed to by a given pointer and
frees it up for other use
- Takes one argument, a pointer that
points to the block of memory to be
deallocated
- The block to be deallocated must
have been previously allocated, if not
bad things happen!

Memory Management Best Practices
- The GOLDEN RULE of Memory
Management:
- Anything allocated must always be
freed after use.
- For every malloc there must exist
one (and only one) free!
- Avoid aliasing
- Avoid dangling pointers
- Any dangling pointers should be set
to NULL
- Watch out for out-of-bounds errors!
What can go wrong?
- Segfaults
- Out-of-bounds errors
- Use-after-free
- Usually results from aliasing
- Buffer Overﬂow
- Usually from writing to memory that
is not allocated
- Security Vulnerabilities!

Structs and Typedefs
Structs
- Allow programmers to create custom
data types
- Provide ways of grouping related
data under a single name
- This is done by defining fields within
a struct
Typedefs
- Allow programmers to create type
aliases for existing types
- Can be used to redefine existing
types to a more relevant name, or
define a struct to use more succinct
syntax

Pointers and Structs
- Pointers are often used with structs
- C is a pass-by-value language, so pointers allow pass-by-reference
- Common syntax includes:
- struct some_struct *var_name;
- typedef struct somestruct *structptr;
- structptr ptr = malloc(sizeof(some_struct));
- Same memory management rules apply!

Putting It All Together
A Linked List!
- What we will implement:
- Append
- Push
- Remove
- Index

Homework!
- Figure out recursion
- Implement linked list operations recursively
- Implement a recursive ﬁbonacci number calculator
- Implement a dynamic string read function
- Implement a binary search tree
- Implement one of your existing (smaller) projects in C
- Do some reading
- History of C and UNIX
- How a C program is compiled
- How malloc and free work
- “The C Programming Language” by Brian Kernighan

Basics of C

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