From e2940f39b23f364a45832c94c94430fdf8a2c184 Mon Sep 17 00:00:00 2001
From: Baldur Karlsson <baldur@unity3d.com>
Date: Mon, 18 Apr 2016 12:06:17 -0400
Subject: [PATCH] Deal with quirks of MSVC's vsnprintf() implementation.

---
 mojoshader_common.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/mojoshader_common.c b/mojoshader_common.c
index a8b1cb8f..446d3897 100644
--- a/mojoshader_common.c
+++ b/mojoshader_common.c
@@ -557,9 +557,24 @@ int errorlist_add_va(ErrorList *list, const char *_fname,
     char scratch[128];
     va_list ap;
     va_copy(ap, va);
-    const int len = vsnprintf(scratch, sizeof (scratch), fmt, ap);
+    int len = vsnprintf(scratch, sizeof (scratch), fmt, ap);
     va_end(ap);
 
+    // on some versions of the windows C runtime, vsnprintf() returns -1
+    // if the buffer overflows instead of the length the string would have
+    // been as expected.
+    // In this case we make another copy of va and fetch the length only
+    // with another call to _vscprintf
+
+#ifdef _MSC_VER
+    if (len == -1)
+    {
+        va_copy(ap, va);
+        len = _vscprintf(fmt, ap);
+        va_end(ap);
+    }
+#endif
+
     char *failstr = (char *) list->m(len + 1, list->d);
     if (failstr == NULL)
     {