The C Programming Language

Video of this week’s lecture - https://web.microsoftstream.com/video/0011fda8-4667-4e60-b89c-b5d0c7810477

Introduction

C is a general purpose, procedural programming language. It was developed in the early 1970s by Dennis Ritchie at Bell Labs. It was designed to map efficiently to machine instructions to make fast running applications.

C is a powerful language that gives access to low level functionality that is often forbidden or restricted in other programming languages. Direct access to memory allows the programmer to access anything - devices, drives, memory, registers, for good or bad.

The downside to this level of access is that you have to be very aware of what you are doing and how you are accessing the memory to prevent disaster!

C is the basis on which many other languages are based, including C++, C# and Objective C.

C should look a lot like Java, but unlike Java C is not an object-oriented language. You cannot create classes in C. Methods in C are called functions.

Hello World

We’ll start the traditional way with Hello World.

Here’s Hello World in Java.

class Main {
  public static void main(String[] args) {
    System.out.println("Hello world!");
  }
}

The C version is a little different but should look very familiar.

#include <stdio.h>

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

Breaking the code down.

#include <stdio.h>

int main(void){

}

printf("Hello World\n");

return 0;

Compiling And Executing C Code

In replit.com you can simply press

If you have Visual Studio Code setup with CodeRunner you can right click on the c file and select run from the menu. (When we get round to creating projects with multiple files you will need to right click on the c file with the main() function in it).

Alternatively, the way to compile C code from the Linux terminal is

• Click on the terminal window
• Enter gcc main.c
• Enter ./a.out

gcc main.c - main.c is the name of our C file. gcc is the command for the GNU C Compiler. gcc main.c compiles the main.c file to the standard output file a.out.

If you wish you can specify the output executable filename with gcc main.c -o HelloWorld. The -o flag allows you to specify the filename.

./a.out executes the file a.out.

You can see which files exits in the folder with the ls command.

The ls Command

The ls command lists all the files in the current folder that are not hidden.
By using the -l flag you can see the full details of each unhidden file.

ls -l
total 16
-rwxr-xr-x 1 runner runner 8304 Sep 20 10:02 main
-rw-r--r-- 1 runner runner   77 Sep 20 08:25 main.c

The first part of this, -rwxr-xr-x, shows the accessibility of the file. The first character indicates the if the entry is a folder or not (d for folder, - for anything else i.e. a file). The rest of the string is made up of three sets of three letters, indicating the permissions of types of users.

• The first three characters indicate the owners permissions
• The middle three characters indicate the group permissions for the group owning the file
• The last three characters indicate the premissions for others, i.e. everyone else The three characters in each set indicate whether or not that user can do something with the file. A - indicates the user cannot do it, a letter indicates they can.
• The first character indicates if the user-type can read the file. If it is r then the user-type can read the file, they can see its contents but not change them.
• The second character is w if the user-type can write the file, if they can edit it. It is possible to have write but not read access.
• The third character is x if the user-type can execute the file.

The second column of the listing shows how many link to the file.

The third column indicates the owner of the file.

The fourth column indicates the group owner of the file.

The fifth column indicates the size of the file in bytes. If the entry is a directory the size is always 4096.

The sixth column indicates the last time and date the file was modified.

The seventh column indicates the name of the file or directory.

The ls command can also use the -A flag to show hidden files. Flags can be combined to show multiple items.

ls -la
total 20
drwxr-xr-x 1 runner runner   20 Sep 20 10:05 .
drwxr-xr-x 1 runner runner 4096 Sep 20 10:05 ..
-rwxr-xr-x 1 runner runner 8304 Sep 20 10:02 main
-rw-r--r-- 1 runner runner   77 Sep 20 08:25 main.c

This shows all the information for all the files, including any hidden ones.

The entry . is the current folder. The .. entry is the folder above the current one.

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