libc: add nx_strdup() & nx_strndup() support

Signed-off-by: ligd <liguiding1@xiaomi.com>

libs/libc/stdio/lib_vasprintf.c

@@ -34,6 +34,101 @@
  * Public Functions
  ****************************************************************************/
 
+/****************************************************************************
+ * Name: nx_vasprintf
+ *
+ * Description:
+ *   This function is similar to vsprintf, except that it dynamically
+ *   allocates a string (as with kmm_malloc) to hold the output, instead of
+ *   putting the output in a buffer you allocate in advance.  The ptr
+ *   argument should be the address of a char * object, and a successful
+ *   call to vasprintf stores a pointer to the newly allocated string at that
+ *   location.
+ *
+ * Returned Value:
+ *   The returned value is the number of characters allocated for the buffer,
+ *   or less than zero if an error occurred. Usually this means that the
+ *   buffer could not be allocated.
+ *
+ ****************************************************************************/
+
+int nx_vasprintf(FAR char **ptr, FAR const IPTR char *fmt, va_list ap)
+{
+  struct lib_outstream_s nulloutstream;
+  struct lib_memoutstream_s memoutstream;
+
+  /* On some architectures, va_list is really a pointer to a structure on
+   * the stack. And the va_arg builtin will modify that instance of va_list.
+   * Since vasprintf traverse the parameters in the va_list twice, the
+   * va_list will be altered in this first cases and the second usage will
+   * fail. This is a known issue with x86_64.
+   */
+
+#ifdef va_copy
+  va_list ap2;
+#endif
+  FAR char *buf;
+  int nbytes;
+
+  DEBUGASSERT(ptr != NULL && fmt != NULL);
+
+#ifdef va_copy
+  va_copy(ap2, ap);
+#endif
+
+  /* First, use a nullstream to get the size of the buffer.  The number
+   * of bytes returned may or may not include the null terminator.
+   */
+
+  lib_nulloutstream(&nulloutstream);
+  lib_vsprintf(&nulloutstream, fmt, ap);
+
+  /* Then allocate a buffer to hold that number of characters, adding one
+   * for the null terminator.
+   */
+
+  buf = kmm_malloc(nulloutstream.nput + 1);
+  if (buf == NULL)
+    {
+#ifdef va_copy
+      va_end(ap2);
+#endif
+      return ERROR;
+    }
+
+  /* Initialize a memory stream to write into the allocated buffer.  The
+   * memory stream will reserve one byte at the end of the buffer for the
+   * null terminator and will not report this in the number of output bytes.
+   */
+
+  lib_memoutstream(&memoutstream, buf, nulloutstream.nput + 1);
+
+  /* Then let lib_vsprintf do it's real thing */
+
+#ifdef va_copy
+  nbytes = lib_vsprintf(&memoutstream.common, fmt, ap2);
+  va_end(ap2);
+#else
+  nbytes = lib_vsprintf(&memoutstream.common, fmt, ap);
+#endif
+
+  /* Return a pointer to the string to the caller.  NOTE: the memstream put()
+   * method has already added the NUL terminator to the end of the string
+   * (not included in the nput count).
+   */
+
+  DEBUGASSERT(nbytes < 0 || nbytes == nulloutstream.nput);
+
+  if (nbytes < 0)
+    {
+      kmm_free(buf);
+      return ERROR;
+    }
+
+  *ptr = buf;
+  return nbytes;
+}
+
 /****************************************************************************
  * Name: vasprintf
  *
@@ -52,6 +147,7 @@
  *
  ****************************************************************************/
 
+#undef vasprintf
 int vasprintf(FAR char **ptr, FAR const IPTR char *fmt, va_list ap)
 {
   struct lib_outstream_s nulloutstream;