Write the core of an ESP-IDF button-to-LED program, with the LED on GPIO5 and an active-low button on GPIO4. Which GPIO calls does it need?

Configure GPIO5 as output and GPIO4 as input with the pull-up, then loop: read the button with gpio_get_level, set the LED with gpio_set_level, sleep 10 ms with vTaskDelay.

#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "driver/gpio.h"

#define LED_GPIO    GPIO_NUM_5
#define BUTTON_GPIO GPIO_NUM_4

void app_main(void)
{
    gpio_reset_pin(LED_GPIO);
    gpio_set_direction(LED_GPIO, GPIO_MODE_OUTPUT);

    gpio_reset_pin(BUTTON_GPIO);
    gpio_set_direction(BUTTON_GPIO, GPIO_MODE_INPUT);
    gpio_set_pull_mode(BUTTON_GPIO, GPIO_PULLUP_ONLY);

    while (1) {
        int pressed = (gpio_get_level(BUTTON_GPIO) == 0);  /* active low */
        gpio_set_level(LED_GPIO, pressed);
        vTaskDelay(pdMS_TO_TICKS(10));
    }
}

What each call does:

Here the LED follows the button: on while held. app_main itself runs as a FreeRTOS task, so the loop can live there. In a larger program it would be its own task created with xTaskCreate. The same settings can also be made in one call with gpio_config() and a gpio_config_t struct, which is what Espressif's examples use.

Go deeper:

• ESP-IDF — GPIO driver API — every function above, plus gpio_config() and the interrupt API.
• ESP-IDF example — generic_gpio — Espressif's input/output example with a GPIO interrupt and a queue.