Fix for MinGW, although not complete -- doesn't have _vscprintf().

src/libCom/osi/os/WIN32/osdStdio.c

@@ -17,13 +17,27 @@
 int epicsShareAPI epicsVsnprintf(char *str, size_t len,
     const char *fmt, va_list ap)
 {
-    int needed = _vscprintf(fmt, ap);
     int retval = _vsnprintf(str, len, fmt, ap);
 
+#ifdef _MSC_VER
+    int needed = _vscprintf(fmt, ap);
+
     if ((int) len < needed + 1) {
         str[len - 1] = 0;
         return needed;
     }
+#else
+    /* Unfortunately MINGW doesn't provide _vscprintf and their
+     * _vsnprintf follows MS' broken return value semantics.
+     */
+    if (retval == -1) {
+        if (len)
+            str[len - 1] = 0;
+        return len;
+    } else if (retval == (int) len) {
+        str[--retval] = 0;
+    }
+#endif
 
     return retval;
 }