What is C? Your first program

Your phone, a washing machine, a car's engine control unit and the router that brings the internet into your home all have programs running inside them. A surprising number of those programs are written in a language that is more than fifty years old — C. C is fast, compact and very close to the hardware. While learning it, you will see how a computer really works with memory.

A short history

C was created in 1972 by Dennis Ritchie at Bell Labs in the USA. He needed a language for writing the Unix operating system: assembly language was tied too closely to one particular computer, while the high-level languages of the time were too slow. C was the answer — a language almost as fast as assembly, yet one that could be moved to other computers.

1. 1972
   C is born

   Dennis Ritchie creates C at Bell Labs as a successor to the B language.

2. 1973
   Unix is rewritten in C

   The kernel of the Unix operating system is rewritten in C.

3. 1978
   The K&R book

   Brian Kernighan and Dennis Ritchie publish the book The C Programming Language.

4. 1989
   The first standard: C89

   The American National Standards Institute (ANSI) approves the first official C standard.

5. 1999
   The C99 standard

   Adds // comments, the long long type and declaring variables inside a for loop.

6. 2011
   The C11 standard

   A modern standard with support for multithreaded programs. C17 and C23 followed later.

Where is C used today?

• Operating systems: the kernels of Linux and Windows are largely written in C.
• Embedded systems: microcontrollers in washing machines, cars, medical devices and routers — places where memory is small and speed matters.
• Tools and other languages: the standard Python interpreter (CPython), the SQLite database and the Git version control system are written in C.
• A foundation for other languages: C++, Java, C# and JavaScript took their curly braces, semicolons and operators from C. That is why these languages will look familiar after C.

Your first program

Here is the classic first program. Save it in a file called hello.c:

#include <stdio.h>

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

Hello, World!

• #include <stdio.h> brings in the header file of the standard input/output library, where the printf function is declared.
• int main(void) is the main function: every C program starts running here. int means the function returns an integer, and void means it takes no parameters.
• The curly braces { } enclose the function's body. Every statement ends with a semicolon (;).
• printf prints text on the screen, and \n moves the cursor to a new line.
• return 0; tells the operating system that the program finished successfully.

From code to program: compiling

A computer cannot run the text in hello.c directly. C is a compiled language: first a compiler translates all the source code into machine code and saves it as an executable file. The most popular C compilers are GCC, Clang and Microsoft's MSVC.

1. 1
   Write the code

   Create a file with the .c extension in any text editor or IDE (for example VS Code) and type the code.

2. 2
   Compile it

   Type gcc hello.c -o hello in a terminal. The compiler checks the code: if there are no mistakes, it creates an executable file called hello; if there are, it shows the line number of each error.

3. 3
   Run it

   Type ./hello on Linux and macOS, or hello.exe on Windows. Every time you change the code, you have to compile it again.

gcc hello.c -o hello
./hello

Hello, World!

printf and format specifiers

printf can do more than print fixed text. Inside the text you place format specifiers — codes that start with % — and each of them is replaced by the next value after the comma. Don't worry about the variables yet — we will study them in detail in the next lesson.

#include <stdio.h>

int main(void) {
    int age = 14;
    double height = 1.62;
    char grade = 'A';
    printf("Name: %s\n", "Leyla");
    printf("Age: %d\n", age);
    printf("Height: %f m\n", height);
    printf("Height: %.2f m\n", height);
    printf("Grade: %c\n", grade);
    printf("Progress: 75%%\n");
    return 0;
}

Name: Leyla
Age: 14
Height: 1.620000 m
Height: 1.62 m
Grade: A
Progress: 75%

Notice that %f always prints six digits after the point, while %.2f rounds the number to two. To print the % sign itself, you write %%.

Some characters cannot be typed inside the quotes directly. They start with a backslash and are called escape sequences: \n — a new line, \t — a tab, \" — a double quote, \\ — the backslash itself.

#include <stdio.h>

int main(void) {
    printf("Line 1\nLine 2\n");
    printf("She said \"Hi!\"\n");
    printf("Folder: C:\\code\n");
    printf("Sum: %d\n", 2 + 3);
    return 0;
}

Line 1
Line 2
She said "Hi!"
Folder: C:\code
Sum: 5