Make gmtime() configurable at compile-time

include/mbedtls/platform_util.h

@@ -25,7 +25,18 @@
 #ifndef MBEDTLS_PLATFORM_UTIL_H
 #define MBEDTLS_PLATFORM_UTIL_H
 
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
+#include "mbedtls/platform_time.h"
+
 #include <stddef.h>
+#if defined(MBEDTLS_HAVE_TIME_DATE)
+#include <time.h>
+#endif /* MBEDTLS_HAVE_TIME_DATE */
 
 #ifdef __cplusplus
 extern "C" {
@@ -55,6 +66,38 @@ extern "C" {
  */
 void mbedtls_platform_zeroize( void *buf, size_t len );
 
+#if defined(MBEDTLS_HAVE_TIME_DATE)
+/**
+ * \brief       Thread safe implementation of gmtime()
+ *
+ *              The function is an abstraction that when called behaves similar
+ *              to the gmtime() function from the C standard, but is thread
+ *              safe.
+ *
+ *              Mbed TLS will try to identify the underlying platform and
+ *              configure an appropriate underlying implementation (e.g.
+ *              gmtime_r() for POSIX and gmtime_s() for Windows). If this is
+ *              not possible, then gmtime() will be used. In this case, calls
+ *              from the library to gmtime() will be guarded by the mutex
+ *              mbedtls_threading_gmtime_mutex if MBEDTLS_THREADING_C is
+ *              enabled. It is recommended that calls from outside the library
+ *              are also guarded by this mutex.
+ *
+ *              If MBEDTLS_PLATFORM_GMTIME_ALT is defined, then Mbed TLS will
+ *              unconditionally use the alternative implementation for
+ *              mbedtls_platform_gmtime() supplied by the user at compile time
+ *
+ * \param tt    Pointer to an object containing time (in seconds) since the
+ *              Epoc to be converted
+ * \param tm    Pointer to an object where the results will be stored
+ *
+ * \return      Pointer to an object of type struct tm on success, otherwise
+ *              NULL
+ */
+struct tm *mbedtls_platform_gmtime( const mbedtls_time_t *tt,
+                                    struct tm *tm_buf );
+#endif /* MBEDTLS_HAVE_TIME_DATE */
+
 #ifdef __cplusplus
 }
 #endif