Introduction to C

You can access the slides 🖼️ for this lecture. All the code samples given here can be found online, alongside instructions on how to bring up the proper environment to build and execute them here. You can download the entire set of slides and lecture notes in PDF from the home page.

Here we do a quick introduction to the C Programming language. We also learn how to write, compile and execute a few simple C programs.

C: Origin

C is a relatively old programming language, it was developed in the 70s by Dennis Ritchie, working at the time at Bell Labs. The language was oringally used to develop applications running on top of the Unix operating system, developed by Ken Thompson, with Dennis Ritchie among others. C ended up also being used to reimplement Unix's operating system kernel, which was originally written mostly in assembly, significanly increasing its portability accross various architectures.

Ritchie and Thomson received in 1983 the Turing award for their contributions to operating system, and in particular for the development of Unix. On the picture below you can see Thompson on the left and Ritchie on the right:

C: Popularity

C is still today one of the most favoured programming language. It can be found in the top-10 of most programming languages popularity indexes (e.g. TIOBE, RedMonk, or PYPL).

The graph below is taken from the RedMonk page, and present the popularity rank of languages on StackOverflow on the Y axis, and the popularity rank on GitHub on the X axis: C is up there in the top right corner.

C: Characteristics

In terms of pros, C has a very simple and extremely popular syntax. Many newer languages such as Java or Go take much inspiration from the syntax of C. C is also a low level language that efficiently maps to machine code and integrates well with assembly. It gives a high degree of control over the hardware, for example contrary to many languages C lets the programmer write at arbitrary virtual addresses in memory. Because of its simplicity C is also very fast, and also allows programs to have a low memory footprint. This all comes at a relatively high cost which is a very large area to make mistakes, resulting from the high degree of control over the hardware. The compiler will not catch every mistake. And some errors may lead to undefined behaviour that is sometimes hard to debug. Still, the benefits of may make that it is the default programming language in many domains, including systems software such as operating systems or virtual machine monitors, but also high performance computing, embedded systems, among others.

A lot of extremely popular software is written in C:

Examples include:

  • The Linux kernel, the most popular OS in HPC, cloud/servers, embedded systems
  • The Xen hypervisor, one of the most popular virtual machine monitors.
  • The GNU Libc standard library, used by most programs in many Linux distributions to interface with the OS.
  • Apache, NGINX, and Redis, that are among the most popular web / key-value store servers.
  • Git, which is by far the most popular version control system.

Programming Mistakes in C

Programming mistakes are quite common in C, and can lead to:

  • Compilation failing: the C compiler used to transform sources into executables can detect many errors and will refuse to build any program that contain clear programming mistakes.
  • Crashes: the fact that a C program builds does not mean it is correct. Crashes at runtime can be annoying but at least it is clear that something is wrong and that the program needs to be fixed.
  • Program misbehaving: this is also generally a clear indication that something needs to be fixed.
  • Hard to reproduce bugs: certain crashes or forms of program misbehaviour can be hard to reproduce because they require very specific execution conditions. So it is not uncommon for certain bugs to require running the program for a very long time or a very high number of times to trigger the bug. The problem with these bugs is that without proper testing the developpers can simply miss it.
  • Security vulnerabilities: certain bugs lead to much more dire consequences than crashes/misbehaviour, and can be exploited by attackers to take over the program or the computer it executes upon, leak critical data, etc.

Why Learn C Today?

It is still very relevant to learn C today. As we have seen, it is still the default programming language in many different domains. C is also not going anyway anytime soon: there is an enormous amount of existing/legacy C codebases, and it is likely that we will need programmers with knowledge of C for decades. Also, because of certain characteristics of the language, learning C will have beneficial side-effect on your knowledge:

  1. Because of its proximity to the hardware, learning C and how a C programm executes helps understanding how a computer works.
  2. C has established an extremely popular syntax, and learning C will make it easier to learn the many other languages sharing that syntax (e.g. Java).

Hello World in C

This C program simply writes hello world to the console, also named the standard output:

#include <stdio.h>
int main() {
    printf("hello, world!\n");
    return 0;
}

Before studying it more in details we can build and run it on the Linux command line as follows:

$ gcc listing1.c -o listing1
$ ./listing1
hello, world!

The first line with #include allows us to get access to functions from external libraries. We need the printf function from stdio.h to be able to print to the standard output.

Next we have a function named main. In C functions are implemented as follows. We first have the return type, here int, followed by the function name, main. Then between braces we have the function body, containing the function's code. In C main represents the program's entry point, i.e. the first function to run when the program is launched.

main's body is composed of two statements. In C statements ends with a semicolon. The first statement is a function call: the printf function is called with a parameter between parentheses. printf is used to print something on the standard output, and that something is passed as a parameter. It is a character string, delimited by double quotes. '\n' is a special character defining a carriage return. Finally, the second statement uses the return keyword that returns from the current function to the calling context. We return the integer value zero, a common return code indicating successful execution. Because we return from main, this actually triggers the exit of the program with return code 0.

Compilation

The compilation is the process of creating an executable program from the source code:

For this we use a tool named the compiler. The C compiler we will use is named the GNU C compiler, GCC.

A compiler takes the source code as input, here it's listing1.c. It generates the machine code ready for execution on the CPU and puts it into an executable, here listing1. .c is a common file name extension for C source code. Executables generally don't have a file name extension on Linux.

Concretely, we call the compiler as follows on the command line: gcc, followed by the name of the source file listing1.c, followed by -o to indicate the name of the output program, listing1:

$ gcc listing1.c -o listing1

Compilation: Warnings and Errors

The compiler will check and report some programming mistakes. There are two levels of issues reported by the compiler:

  • An error is unrecoverable, and the compilation process will stop upon detecting it
  • On the other hand, warnings may or may not be indicative of an issue and the compilation continues.

Compilers are very advanced today and warnings are almost always indicative of an issue: they should always be fixed. An important thing to note is that when multiple issues are listed, it is important to fix them in the order they are reported, because mistakes down the list may result from the ones up the list.

Comments

Comments are annotations in the source file that will not be processed by the compiler. It is important for a programmer to comment code a bit to explain what it does: for other people to understand his/her code, but also for himself/herself to dive back into complex pieces of code written a long time ago. There are two comments style supported in C:

/* style 1 */
// style 2

Everything that is between the /* and */ delimiters is commented. One can also use the // to comment the rest of the line.