COMP26020p1 - Introduction to C

class: center, middle

### COMP26020 Programming Languages and Paradigms Part 1: C Programming
***
# Introduction to C

---
# C: Origin

- C was designed in the early 1970s by Dennis Ritchie at Bell Labs
- Developed alongside Unix: first used for Unix utilities, then to rewrite the Unix kernel with Ken Thompson
--

- Ritchie & Thompson were awarded the ACM Turing Award in 1983 *for their development of generic operating systems theory and specifically for the implementation of the UNIX operating system*

---
# C: Popularity

- Old language, but still **omnipresent** today
- In the top ten of most programming language popularity indexes

---
# C: Characteristics
- **Pros:**
  - *Low-level*: close to the hardware, lots of freedom for the programmer to control the machine
  - Fast
  - Low memory footprint
  - Portable
  - Its syntax has influenced many later languages

--
- **Cons**: the programmer's freedom comes at a cost: **plenty of room for mistakes**
  - Lack of memory safety, risk of undefined behaviour at runtime

**Still extensively used** in: systems software, embedded systems, high-performance computing, etc.

---
# Popular Software Written in C

---
# Programming Mistakes in C

.leftcol[
- Programming mistakes in C are common and lead to:
  - Build failures 😒
  - Crashes 😕
  - Incorrect program behaviour 😠
  - Hard-to-reproduce bugs 😡
  - Security vulnerabilities 😱
- C is generally not recommended for new projects
- Yet a vast amount of legacy C code is still running today, and it is not going anywhere soon
]

.rightcol[
<div style="text-align:center"><img src="include/safety.png" width=370 style="border: 2px solid #ccc; border-radius: 6px; box-shadow: 0 2px 6px rgba(0,0,0,0.3);"/></div>
]

---
# Why Learn C Today?

- Still the default programming language in many domains
- C is not going anywhere soon: vast C codebases are still relevant today
  - C experts will be needed for decades to come
- Proximity to the hardware: knowing C helps in understanding how a computer works
- Knowing C's syntax makes many other programming languages easier to learn

---
# Hello World

```c
#include <stdio.h>

int main() {

printf("hello, world!\n");

return 0;
}

```
.codelink[<a href="src/hello-world.c" download>`03-c-introduction/hello-world.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a>]

- C is a **compiled language**: the source code must be translated into an executable program
- Assuming the code is in `hello-world.c`, to compile and run it on the Linux command
line:

```bash
$ gcc hello-world.c -o hello
$ ./hello
hello, world!
```

---
# Hello World

```c
#include <stdio.h>

int main() {

printf("hello, world!\n");

return 0;
}

- `#include` to get access to functions from external libraries
--

- Program execution starts in the `main` function
--

- It returns an integer (`int`) and takes no parameters
--

- Contains two statements: in C, statements end with `;`
--

- `printf` prints text to the *standard output* (console)
--

- `return` goes back to the calling function; returning from `main` ends the program (`0` indicates success)

---
# Compilation

```bash
$ gcc hello-world.c -o hello
```

---

# Compilation

```c
#include <stdio.h>

*nt main() {
    printf("hello, world!\n");

return 0;
}
```

```bash
$ gcc hello-world.c -o hello             
hello-world.c:3:1: error: unknown type name ‘nt’; did you mean ‘int’?
    3 | nt main() {
      | ^~
      | int
```

.center[**Multiple errors? <u>Fix them in the order they are reported</u>**]

---
# Compilation: Warnings and Errors

- Errors are fatal: compilation fails and no executable is produced
- Warnings do not stop compilation, but *may* indicate a problem

```c
#include <stdio.h>

int main() {
    int x;
    printf("hello, world!\n");

return 0;
}
```

```bash
$ gcc -Wall hello-world.c -o hello
hello-world.c: In function ‘main’:
hello-world.c:4:9: warning: unused variable ‘x’ [-Wunused-variable]
     int x;
         ^
```

---
# Compilation: Warnings and Errors

- Errors are fatal: compilation fails and no executable is produced
- Warnings do not stop compilation, but *may* indicate a problem
- **In practice, warnings almost always indicate a problem: <ins>fix them</ins>**
- Many warnings are disabled by default; enable them with the following GCC flags:
  - `-Wall`: enables most common warnings, recommended
  - `-pedantic`: warns about non-standard C, sometimes too picky

---
# Comments

```c
/* comments.c, illustrate the use of comments */

#include <stdio.h> // necessary to get access to printf

/* this function simply prints out 'hello world' and returns */
int main() {

printf("hello, world!\n"); // here we print ...

/* the line below will not be compiled: */
    // printf("goodbye!\n");

return 0;                  /* ... and return */
}
```
.codelink[<a href="src/comments.c" download>`03-c-introduction/comments.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a>]

```c
/* style 1 */
// style 2
```
.center[**<ins>Use comments</ins> to explain what the code does:<br>uncommented code is hard to understand**]

---
# Summary

- Brief intro to C
- Hello world
- Compilation process
- Comments

----
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