The C Programming Language

What is C?

C is a statically typed, general-purpose programming language, typically used to program low level applications and libraries, operating systems, embedded systems and database systems among other things.

Why are we learning it?

Learning C will help us understand fundamental concepts such as memory and processing far better than a high level language like Javascript will.

The primary aim of learning C here isn't to necessarily write production grade code, but to have a firm grasp of how code works at the lowest levels. So, we will not really focus on unit testing in C etc. Instead our focus will be primarily on memory management and data structures.

How is it different from JavaScript?

C	Javascript
Statically typed	Dynamically Typed
Minimal/no type inference	Inferred types
Runs directly on the OS	Runs on an interpreter
Developer managed memory	Runtime managed memory
No automatic garbage collection	Automatic garbage collection

The biggest difference for us will be the fact that C is statically typed. What does this mean?

Consider

let a = 2;

Here, Javascript infers that a has an integer or a numerical value, automatically allocates the required space and then performs the initialisation.

However in C:

int a = 2;

Here we explicitly are requesting an interger's worth memory(typically 4 bytes on 64 bit systems). Though, this may just as well be written as:

float a = 2;
double b = 2;
char c = 2;

In the above example, each of the variables declared have a very specific amount of space and each declaration implies that in the future those variables will be treated a specific way.

For instance, the following is incorrect and will generate a compile warning:

int a = 2;
a = "abc";

Importantly, a will not be "abc" here but an unknown random value. Further, a compiler can be used to apply strict rules that prevent something like this from being compiled. Once a variable has been declared with a certain type, it remains of that type permanently.

Even functions have to be declared with a "signature" that specifies the type(s) we are passing in and returning.

int add(int x, int y) {
  return x + y;
}

Nearly all of what we will learn C for is related to this difference in types and memory management. While this is not a comprehensive list of differences, it should provide you with a brief overview.

---

Hello World

The code itself is quite straightforward

#include <stdio.h>

int main(void)
{
  printf("Hello World\n");
  return 0;
}

At this point, consider going to compiling and execution. The rest of this section describes the details of the above program which are unnecessary to get started with.

However, beneath the simplicity of this program, a lot is happening.

main is a function that takes no arguments and returns 0. In C, the main function is special and is always the entry point of execution. The return value of the main function is the status/exit code of the program.

printf is a function. But in order to use it, you have to include the library in which it is declared. The library happens to be stdio.h. While include seems similar to require in Javascript, it is actually quite different. Unlike require which is a function, #include is actually an instruction to the compiler and the compiler simply replaces the line #include <stdio.h> with the contents of stdio.h.

Compiling and execution

To simply compile a file named hello_world.c

gcc hello_world.c

This will generate a file called a.out. To execute what you've just compiled, type in ./a.out

To compile the file into a differently named executable:

gcc -o hello_world hello_world.c