COMP26020p1 - Introduction to C

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

---
# C: Origin

- C was designed in the 70s by Dennis Ritchie at Bell Labs
- Originally used to develop applications for Unix, then to reimplement its OS kernel with Ken Thompson
--

- Ritchie & Thompson 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

.leftsmallcol[
- Old language, but still **omnipresent** today
- In the top ten of most programming languages popularity indexes
]

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<div style="text-align:center"><img src="include/Redmonk.png" width=575 /></div>
]

---
# C: Characteristics
- **Pros:**
  - *Low-level*: close to the hardware, lots of freedom for the programmer to manipulate the machine
  - Fast, low memory footprint
  - Portable
  - Established a popular syntax

--
- **Cons**: programmer freedom comes at a cost: **a large area for making mistakes**
  - Lack of memory safety, risks of undefined behaviour at runtime

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

---
# Popular Software Written in C

---
# Programming Mistakes in C

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- Programming mistakes in C are common and lead to:
  - Build failing 😒
  - Crashes 😕
  - Program misbehaving 😠
  - Hard to reproduce bugs 😡
  - Security vulnerabilities 😱
- C is not particularly recommended for new projects
- Still a gigantic amount of legacy C code running today that not going anywhere anytime soon
]

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<div style="text-align:center"><a href="https://www.whitehouse.gov/wp-content/uploads/2024/02/Final-ONCD-Technical-Report.pdf"><img src="include/white-house.png" width=370/></a></div>
]

---
# Why Learn C Today?

- Still the default programming languages in many different domains
- C is not going anywhere anytime soon, vast amount of C codebases still relevant today
  - We will need C experts in the decades to come
- Proximity to the hardware: knowing C helps understanding how a computer works
- Knowing C's syntax will make it easy to lean many other programming languages

---
# 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><a href="https://www.programiz.com/online-compiler/1jRWpUU5Tv5RG" target="_blank" style="text-decoration: none"><img src="include/programiz-logo.png" style="height: 1em"></a>]

Assuming the code is in `hello-world.c`, to compile and run on the Linux command
line:
```sh
$ 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
--

- `main` will run first when the program is executed
--

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

- Contains two statements, in C statements ends with `;`
--

- `printf` used to print text on the *standard output* (console)
--

- `return` to go back to the calling context, and exit if returning from `main`

---
# Compilation

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

---

# Compilation

```c
#include <stdio.h>

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

return 0;
}
```

```sh
$ 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 issues? fix them in the order they are reported**]

---
# Compilation: Warning and Errors

- Errors are unrecoverable
- Warnings *may* indicate a problem

```c
#include <stdio.h>

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

return 0;
}
```

```sh
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: Warning and Errors

- Errors are unrecoverable
- Warnings *may* indicate a problem
- **Warnings almost always indicate a problem and <ins>you should fix them</ins>**
- Some warnings (picky ones) are disabled by default, enable them with the `-Wall -pedantic` flags

---
# Comments

```c
/* listing2.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><a href="https://www.programiz.com/online-compiler/1vC3YZNHza1eR" target="_blank" style="text-decoration: none"><img src="include/programiz-logo.png" style="height: 1em"></a>]

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

---
# Summary

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

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