esp32-alarm-clock/main/time_conversion.c

/*
 * SPDX-FileCopyrightText: 2023 Helmut Pozimski <helmut@pozimski.eu>
 *
 * SPDX-License-Identifier: GPL-2.0-only
 */

#include <freertos/FreeRTOS.h>
#include <freertos/semphr.h>

#include <time.h>
#include <string.h>

#include "configuration.h"
 
static SemaphoreHandle_t mutex = NULL;
static char* tz = NULL;

static void initialize_tz() {
	unsetenv("TZ");
	setenv("TZ", LOCAL_TIMEZONE, 1);
	tz = getenv("TZ");
}

static void set_tz_gmt() {
	strncpy(tz, "GMT0", 5);
   tzset();
}

static void set_tz_local() {
   strncpy(tz, LOCAL_TIMEZONE, strlen(LOCAL_TIMEZONE) +1);
   tzset();
}

struct tm* convert_to_local(struct tm *timeinfo) {
	if (mutex == NULL) {
		mutex = xSemaphoreCreateMutex();
	}
	xSemaphoreTake(mutex, portMAX_DELAY);
	
	if (tz == NULL) {
		initialize_tz();
	}
	
   set_tz_gmt();
   time_t epoch_time = mktime(timeinfo);
	set_tz_local();
   struct tm* result = localtime(&epoch_time);
	set_tz_gmt();
	
	xSemaphoreGive(mutex);
	return result;
}