undefined reference to `app_main'

OS : macOS 10.11.6 PlatformIO: 1.7.1(IDE), 3.2.1(Core)

use espidf-hello-world example, then come error below(I can use Arduino example normally):

Indexing .pioenvs/esp32dev/libtcpip_adapter.a
Linking .pioenvs/esp32dev/firmware.elf
.pioenvs/esp32dev/libesp32.a(cpu_start.o):(.literal.main_task+0x0): undefined reference to `app_main'
.pioenvs/esp32dev/libesp32.a(cpu_start.o): In function `main_task':
/Users/linjinhui/.platformio/packages/framework-espidf/components/esp32/cpu_start.c:169: undefined reference to `app_main'
collect2: error: ld returned 1 exit status
*** [.pioenvs/esp32dev/firmware.elf] Error 1

========================== [ERROR] Took 15.54 seconds ==========================

[mintisan]

should add extern "C" in front of app_main

/* Hello World Example
   This example code is in the Public Domain (or CC0 licensed, at your option.)
   Unless required by applicable law or agreed to in writing, this
   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
   CONDITIONS OF ANY KIND, either express or implied.
 */
#include <stdio.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "esp_system.h"
#include "nvs_flash.h"

void hello_task(void *pvParameter)
{
        printf("Hello world!\n");
        for (int i = 10; i >= 0; i--) {
                printf("Restarting in %d seconds...\n", i);
                vTaskDelay(1000 / portTICK_RATE_MS);
        }
        printf("Restarting now.\n");
        fflush(stdout);
        system_restart();
}

extern "C" void app_main()
{
        nvs_flash_init();
        system_init();
        xTaskCreate(&hello_task, "hello_task", 2048, NULL, 5, NULL);
}

[derasd]

I had this error when I switched from compiling with platformio (vscode) to compiling with mingw32. The problem was that I had the code in the ./src folder instead of the ./main folder.