libc: common: simple implementation for localtime() & localtime_r()

This implementation simply wraps around the gmtime() &
gmtime_r() functions, the results are always expressed as UTC.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>

doc/services/portability/posix/option_groups/index.rst

@@ -911,7 +911,7 @@ _POSIX_THREAD_SAFE_FUNCTIONS
     getpwnam_r(),
     getpwuid_r(),
     gmtime_r(), yes
-    localtime_r(),
+    localtime_r(), yes (UTC timezone only)
     putc_unlocked(),
     putchar_unlocked(),
     rand_r(), yes

lib/libc/common/CMakeLists.txt

@@ -7,6 +7,7 @@ zephyr_library_property(ALLOW_EMPTY TRUE)
 zephyr_library_sources_ifdef(CONFIG_COMMON_LIBC_ABORT source/stdlib/abort.c)
 zephyr_library_sources_ifdef(CONFIG_COMMON_LIBC_ASCTIME source/time/asctime.c)
 zephyr_library_sources_ifdef(CONFIG_COMMON_LIBC_GMTIME_R source/time/gmtime_r.c)
+zephyr_library_sources_ifdef(CONFIG_COMMON_LIBC_LOCALTIME_R_UTC source/time/localtime_r_utc.c)
 zephyr_library_sources_ifdef(CONFIG_COMMON_LIBC_TIME source/time/time.c)
 zephyr_library_sources_ifdef(CONFIG_COMMON_LIBC_MALLOC source/stdlib/malloc.c)
 zephyr_library_sources_ifdef(CONFIG_COMMON_LIBC_STRNLEN source/string/strnlen.c)

lib/libc/common/Kconfig

@@ -23,6 +23,14 @@ config COMMON_LIBC_GMTIME_R
 	help
 	  common implementation of gmtime_r().
 
+config COMMON_LIBC_LOCALTIME_R_UTC
+	bool
+	select COMMON_LIBC_GMTIME_R
+	help
+	  Simple implementation of localtime() & localtime_r().
+	  This option just wraps around the gmtime(), the result is always expressed as
+	  Coordinated Universal Time (UTC).
+
 config COMMON_LIBC_TIME
 	bool
 	help

lib/libc/common/source/time/localtime_r_utc.c

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2024 Meta Platforms
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <time.h>
+
+struct tm *localtime_r(const time_t *timer, struct tm *result)
+{
+	return gmtime_r(timer, result);
+}
+
+struct tm *localtime(const time_t *timer)
+{
+	return gmtime(timer);
+}

lib/libc/minimal/Kconfig

@@ -40,6 +40,7 @@ config MINIMAL_LIBC_TIME
 	select COMMON_LIBC_TIME if POSIX_TIMERS
 	select COMMON_LIBC_GMTIME_R
 	select COMMON_LIBC_ASCTIME
+	select COMMON_LIBC_LOCALTIME_R_UTC
 	default y
 	help
 	  Enable time() and gmtime_r() for the minimal libc.

lib/libc/minimal/include/time.h

@@ -52,6 +52,8 @@ struct tm *gmtime(const time_t *timep);
 struct tm *gmtime_r(const time_t *ZRESTRICT timep,
 		    struct tm *ZRESTRICT result);
 char *asctime(const struct tm *timeptr);
+struct tm *localtime(const time_t *timer);
+struct tm *localtime_r(const time_t *ZRESTRICT timer, struct tm *ZRESTRICT result);
 
 #if defined(CONFIG_COMMON_LIBC_ASCTIME_R) || defined(__DOXYGEN__)
 char *asctime_r(const struct tm *ZRESTRICT tp, char *ZRESTRICT buf);

lib/posix/options/Kconfig.pthread

@@ -157,6 +157,8 @@ config POSIX_THREAD_PRIO_PROTECT
 config POSIX_THREAD_SAFE_FUNCTIONS
 	bool "POSIX thread-safe functions"
 	select COMMON_LIBC_ASCTIME_R
+	select COMMON_LIBC_GMTIME_R
+	select COMMON_LIBC_LOCALTIME_R_UTC
 	help
 	  Select 'y' here to enable POSIX thread-safe functions including asctime_r(), ctime_r(),
 	  flockfile(), ftrylockfile(), funlockfile(), getc_unlocked(), getchar_unlocked(),

tests/lib/c_lib/common/src/main.c

@@ -1134,6 +1134,28 @@ ZTEST(libc_common, test_time_asctime)
 	}
 }
 
+/**
+  * @brief Test time function
+ *
+ * @see localtime(), localtime_r().
+ */
+ZTEST(libc_common, test_time_localtime)
+{
+	time_t tests1 = 0;
+	time_t tests2 = -5;
+	time_t tests3 = (time_t) -214748364800;
+	time_t tests4 = 951868800;
+
+	struct tm tp;
+
+	zassert_not_null(localtime(&tests1), "localtime failed");
+	zassert_not_null(localtime(&tests2), "localtime failed");
+
+	tp.tm_wday = -5;
+	zassert_not_null(localtime_r(&tests3, &tp), "localtime_r failed");
+	zassert_not_null(localtime_r(&tests4, &tp), "localtime_r failed");
+}
+
 /**
  *
  * @brief Test rand function