Add a basic sample to show how native runtime invokes wasm apps in WAMR and how wasm apps invoke native functions. (#207)

* Add printingAdd print time for wamrc, fix posix mmap bug time for wamrc, fixed a posix mmap bug.

Change-Id: Ib6517b8a69cf022a1a6a74efa1f98155aec143bc

* Add a basic sample to show how native runtime invokes wasm app in WAMR, and how wasm app invokes native functions.

Change-Id: I700ae413ad5e9ea04540d5187952305e1ee92d73

samples/basic/src/native_impl.c

@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+#include "bh_platform.h"
+#include "wasm_export.h"
+#include "math.h"
+
+// The first parameter is not exec_env because it is invoked by native funtions
+void reverse(char * str, int len)
+{
+    int i = 0, j = len - 1, temp;
+    while (i < j) {
+        temp = str[i];
+        str[i] = str[j];
+        str[j] = temp;
+        i++;
+        j--;
+    }
+}
+
+// The first parameter exec_env must be defined using type wasm_exec_env_t
+// which is the calling convention for exporting native API by WAMR.
+//
+// Converts a given integer x to string str[].
+// digit is the number of digits required in the output.
+// If digit is more than the number of digits in x,
+// then 0s are added at the beginning.
+int intToStr(wasm_exec_env_t exec_env, int x, char* str, int str_len, int digit)
+{
+    int i = 0;
+
+    printf ("calling into native function: %s\n", __FUNCTION__);
+
+    while (x) {
+        // native is responsible for checking the str_len overflow
+        if (i >= str_len) {
+            return -1;
+        }
+        str[i++] = (x % 10) + '0';
+        x = x / 10;
+    }
+
+    // If number of digits required is more, then
+    // add 0s at the beginning
+    while (i < digit) {
+        if (i >= str_len) {
+            return -1;
+        }
+        str[i++] = '0';
+    }
+
+    reverse(str, i);
+
+    if (i >= str_len)
+        return -1;
+    str[i] = '\0';
+    return i;
+}
+
+int get_pow(wasm_exec_env_t exec_env, int x, int y) {
+    printf ("calling into native function: %s\n", __FUNCTION__);
+    return (int)pow(x, y);
+}