libc/time: This commit adds the difftime() function. Since the function returns a double, I have isolated it in a CONFIG_LIBC_DIFFTIME option (It also depends on the toolchain-dependent CONFIG_HAVE_DOUBLE so is not available on tiny platforms).

include/time.h

@@ -214,6 +214,10 @@ FAR char *ctime_r(FAR const time_t *timep, FAR char *buf);
 
 time_t time(FAR time_t *timep);
 
+#if defined(CONFIG_LIBC_DIFFTIME)
+double difftime(time_t time1, time_t time0);
+#endif
+
 int timer_create(clockid_t clockid, FAR struct sigevent *evp,
                  FAR timer_t *timerid);
 int timer_delete(timer_t timerid);

libc/Kconfig

@@ -350,6 +350,13 @@ config TIME_EXTENDED
 
 		Note: tm_isdst is always 0
 
+config LIBC_DIFFTIME
+	bool "Add support for difftime()"
+	default "n"
+	---help---
+		Add POSIX support for computing the difference between two time_t
+		values.
+
 config LIB_SENDFILE_BUFSIZE
 	int "sendfile() buffer size"
 	default 512

libc/time/Make.defs

@@ -49,6 +49,10 @@ CSRCS += lib_ctimer.c
 endif
 endif
 
+ifdef CONFIG_LIBC_DIFFTIME
+CSRCS += lib_difftime.c
+endif
+
 # Add the time directory to the build
 
 DEPPATH += --dep-path time

libc/time/lib_difftime.c

@@ -0,0 +1,65 @@
+/****************************************************************************
+ * libc/time/lib_difftime.c
+ *
+ *   Copyright (C) 2016 Gregory Nutt. All rights reserved.
+ *   Author: Sebastien Lorquet <sebastien@lorquet.fr>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name NuttX nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+#include <nuttx/compiler.h>
+
+#include <time.h>
+
+#ifdef CONFIG_HAVE_DOUBLE
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Function:  difftime
+ *
+ * Description:
+ *   The difftime() function returns the number of seconds elapsed
+ *   between time time1 and time time0, represented as a double.
+ *
+ ****************************************************************************/
+
+double difftime(time_t time1, time_t time0)
+{
+  return (double)time1 - (double)time0;
+}
+
+#endif /* CONFIG_HAVE_DOUBLE */