Functions & Scope

1#include <stdio.h>
2
3// Step 1: DEFINE the function (write the recipe)
4// "int" = return type (what comes out)
5// "a" and "b" = parameters (what goes in)
6int add(int a, int b) {
7    int result = a + b;    // do the calculation
8    return result;         // send the answer back
9}
10
11// Step 2: CALL the function (use the recipe)
12int main(void) {
13    int sum = add(3, 4);    // call add with 3 and 4 → sum becomes 7
14    printf("3 + 4 = %d\n", sum);   // prints: 3 + 4 = 7
15
16    // You can call it multiple times with different inputs
17    int another = add(10, 20);
18    printf("10 + 20 = %d\n", another);  // prints: 10 + 20 = 30
19
20    return 0;
21}

1void greet(const char *name) {     // void = "nothing comes out"
2    printf("Hello, %s!\n", name);  // just print, no return
3}
4
5// Call it:
6greet("Rajath");    // prints: Hello, Rajath!
7greet("ESP32");     // prints: Hello, ESP32!

1// 1. Function declaration (prototype) — common in header (.h) files
2int add(int a, int b);
3
4// 2. void return — function does something but returns nothing
5void blink_led(int gpio, int delay_ms) {
6    gpio_set_level(gpio, 1);
7    vTaskDelay(pdMS_TO_TICKS(delay_ms));
8    gpio_set_level(gpio, 0);
9}
10
11// 3. static — limits the function's scope to THIS file only (.c file)
12static void helper_function(void) {
13    // Only code inside the exact same file can call this.
14    // It hides the function from the rest of the project.
15}
16
17// 4. const parameters — promises not to modify the input
18void print_label(const char *label) {
19    printf("%s\n", label);
20    // label[0] = 'X';  // compiler error — can't modify const
21}

1gpio_set_level(2, 1);           // Turn on LED on GPIO 2
2gpio_set_level(2, 0);           // Turn off LED on GPIO 2
3vTaskDelay(pdMS_TO_TICKS(500)); // Wait 500ms