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How a C Program Works

This page explains the skeleton of every .c file you work with in these labs.

#include — importing tools

At the top of most .c files you will see lines like:

#include <stdio.h>
#include <wiringPi.h>

#include tells the compiler to pull in a header file — a list of function definitions from a library. Think of it like looking up a word in a dictionary before you use it: the compiler needs to know what printf or analogRead means before it can compile your code.

  • <stdio.h> provides printf and other input/output functions.
  • <wiringPi.h> provides pinMode, digitalWrite, analogRead, and the other GPIO functions.

#define — naming constants

#define PI 3.14159
#define MAX_SPEED 100

#define creates a named constant. Before compilation, the compiler replaces every use of PI in the code with 3.14159. This is exactly like a find-and-replace — it is not a variable and it does not occupy memory at runtime.

Using named constants instead of bare numbers makes code easier to read and easier to change. If you decide MAX_SPEED should be 200 instead of 100, you change one line instead of hunting through every place you wrote 100.

setup() and loop()

The motor lab files are structured around two functions:

void setup(void) {
    // runs once, right at the start
}

void loop(void) {
    // runs over and over, forever
}
  • setup() is called once when the program starts. Use it to configure pins and initialize anything that only needs to happen once.
  • loop() is called repeatedly in an endless loop. Every sensor read, motor command, and print statement you want to happen continuously goes here.

Where is main()?

In a standard C program, execution starts in main(). In these labs, main() is already written for you inside core.c. It calls setup() once and then calls loop() in an infinite loop. You do not need to write or edit core.c.

See also