libc: common: implement ctime() & ctime_r()

Add implementation & test for `ctime()` & `ctime_r()`.
Since this function is equivalent to
`asctime(localtime(clock))`, it inherits the limitation of
`localtime()` as well, which only supports UTC results
currently.

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

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

@@ -900,7 +900,7 @@ _POSIX_THREAD_SAFE_FUNCTIONS
     :widths: 50,10
 
     asctime_r(), yes
-    ctime_r(),
+    ctime_r(), yes (UTC timezone only)
     flockfile(),
     ftrylockfile(),
     funlockfile(),

lib/libc/common/CMakeLists.txt

@@ -8,6 +8,7 @@ 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_CTIME source/time/ctime.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

@@ -18,6 +18,19 @@ config COMMON_LIBC_ASCTIME_R
 	help
 	  common implementation of asctime_r().
 
+config COMMON_LIBC_CTIME
+	bool
+	select COMMON_LIBC_LOCALTIME_R_UTC
+	help
+	  common implementation of ctime().
+
+config COMMON_LIBC_CTIME_R
+	bool "Thread-safe version of ctime()"
+	default y if POSIX_THREAD_SAFE_FUNCTIONS
+	select COMMON_LIBC_CTIME
+	help
+	  common implementation of ctime_r().
+
 config COMMON_LIBC_GMTIME_R
 	bool
 	help

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

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2024 Meta Platforms
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <time.h>
+
+/**
+ * `ctime()` is equivalent to `asctime(localtime(clock))`
+ * See: https://pubs.opengroup.org/onlinepubs/009695399/functions/ctime.html
+ */
+
+char *ctime(const time_t *clock)
+{
+	return asctime(localtime(clock));
+}
+
+#if defined(CONFIG_COMMON_LIBC_CTIME_R)
+char *ctime_r(const time_t *clock, char *buf)
+{
+	struct tm tmp;
+
+	return asctime_r(localtime_r(clock, &tmp), buf);
+}
+#endif /* CONFIG_COMMON_LIBC_CTIME_R */

lib/libc/minimal/Kconfig

@@ -41,6 +41,7 @@ config MINIMAL_LIBC_TIME
 	select COMMON_LIBC_GMTIME_R
 	select COMMON_LIBC_ASCTIME
 	select COMMON_LIBC_LOCALTIME_R_UTC
+	select COMMON_LIBC_CTIME
 	default y
 	help
 	  Enable time() and gmtime_r() for the minimal libc.

lib/libc/minimal/include/time.h

@@ -54,11 +54,16 @@ struct tm *gmtime_r(const time_t *ZRESTRICT timep,
 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);
+char *ctime(const time_t *clock);
 
 #if defined(CONFIG_COMMON_LIBC_ASCTIME_R) || defined(__DOXYGEN__)
 char *asctime_r(const struct tm *ZRESTRICT tp, char *ZRESTRICT buf);
 #endif /* CONFIG_COMMON_LIBC_ASCTIME_R */
 
+#if defined(CONFIG_COMMON_LIBC_CTIME_R) || defined(__DOXYGEN__)
+char *ctime_r(const time_t *clock, char *buf);
+#endif /* CONFIG_COMMON_LIBC_CTIME_R */
+
 time_t time(time_t *tloc);
 
 #ifdef __cplusplus

lib/posix/options/Kconfig.pthread

@@ -159,6 +159,7 @@ config POSIX_THREAD_SAFE_FUNCTIONS
 	select COMMON_LIBC_ASCTIME_R
 	select COMMON_LIBC_GMTIME_R
 	select COMMON_LIBC_LOCALTIME_R_UTC
+	select COMMON_LIBC_CTIME_R
 	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

@@ -1135,7 +1135,7 @@ ZTEST(libc_common, test_time_asctime)
 }
 
 /**
-  * @brief Test time function
+ * @brief Test time function
  *
  * @see localtime(), localtime_r().
  */
@@ -1156,6 +1156,23 @@ ZTEST(libc_common, test_time_localtime)
 	zassert_not_null(localtime_r(&tests4, &tp), "localtime_r failed");
 }
 
+/**
+ * @brief Test time function
+ *
+ * @see ctime(), ctime_r().
+ */
+ZTEST(libc_common, test_time_ctime)
+{
+	char buf[26] = {0};
+	time_t time = 1718260000;
+
+	zassert_not_null(ctime_r(&time, buf));
+	zassert_equal(strncmp("Thu Jun 13 06:26:40 2024\n", buf, sizeof(buf)), 0);
+
+	zassert_not_null(ctime(&time));
+	zassert_equal(strncmp("Thu Jun 13 06:26:40 2024\n", ctime(&time), sizeof(buf)), 0);
+}
+
 /**
  *
  * @brief Test rand function

tests/lib/c_lib/common/testcase.yaml

@@ -14,6 +14,7 @@ tests:
       - CONFIG_MINIMAL_LIBC_NON_REENTRANT_FUNCTIONS=y
       - CONFIG_MINIMAL_LIBC_RAND=y
       - CONFIG_COMMON_LIBC_ASCTIME_R=y
+      - CONFIG_COMMON_LIBC_CTIME_R=y
   libraries.libc.common.newlib:
     filter: CONFIG_NEWLIB_LIBC_SUPPORTED
     min_ram: 32