* exceptions.cc: Add stdlib.h include for alloca declaration.

* poll.cc: Ditto.
* termios.cc: Ditto.
* syscalls.cc (_write): Only allow zero length when fd is valid.

winsup/cygwin/syscalls.cc

@@ -355,12 +355,6 @@ _read (int fd, void *ptr, size_t len)
 extern "C" ssize_t
 _write (int fd, const void *ptr, size_t len)
 {
-  if (len == 0)
-    return 0;
-
-  if (__check_invalid_read_ptr_errno (ptr, len))
-    return -1;
-
   int res = -1;
 
   sigframe thisframe (mainthread);
@@ -368,6 +362,16 @@ _write (int fd, const void *ptr, size_t len)
   if (cfd < 0)
     goto done;
 
+  /* No further action required for len == 0 */
+  if (len == 0)
+    {
+      res = 0;
+      goto done;
+    }
+
+  if (len && __check_invalid_read_ptr_errno (ptr, len))
+    goto done;
+
   /* Could block, so let user know we at least got here.  */
   if (fd == 1 || fd == 2)
     paranoid_printf ("write (%d, %p, %d)", fd, ptr, len);