Index: libcxx/include/__support/ibm/xlocale.h
===================================================================
--- libcxx/include/__support/ibm/xlocale.h
+++ libcxx/include/__support/ibm/xlocale.h
@@ -10,6 +10,7 @@
 #ifndef _LIBCPP_SUPPORT_IBM_XLOCALE_H
 #define _LIBCPP_SUPPORT_IBM_XLOCALE_H
 
+#include <stdarg.h>
 #include <__support/ibm/locale_mgmt_aix.h>
 
 #include "cstdlib"
@@ -251,21 +252,22 @@
 }
 
 static inline
-int vasprintf(char **strp, const char *fmt, va_list ap)
-{
-  const size_t buff_size = 256;
-  int str_size;
-  if ((*strp = (char *)malloc(buff_size)) == NULL)
-  {
-    return -1;
-  }
-  if ((str_size = vsnprintf(*strp, buff_size, fmt,  ap)) >= buff_size)
-  {
-    if ((*strp = (char *)realloc(*strp, str_size + 1)) == NULL)
-    {
-      return -1;
+int vasprintf(char **strp, const char *fmt, va_list ap) {
+  int str_size = -1;
+  va_list ap2;
+  va_copy(ap2, ap);
+  int count = vsnprintf(0, 0, fmt, ap2);
+  va_end(ap2);
+  if (count < 0) return -1;
+  const size_t buf_size = count + 1;
+  char *const buf = (char *)malloc(buf_size);
+  if (buf) {
+    *strp = buf;
+    str_size = vsnprintf(*strp, buf_size, fmt, ap);
+    if (str_size < 0 || (size_t)str_size >= buf_size) {
+      free(buf);
+      str_size = -1;
     }
-    str_size = vsnprintf(*strp, str_size + 1, fmt,  ap);
   }
   return str_size;
 }