pp_readline,close,getc: explain NULL stack arg

add code comments to explain why these functions can sometimes be called
with a NULL pointer on the stack.

(feature introduced by v5.15.2-112-g30901a8)

1 files changed, 4 insertions, 0 deletions

diff --git a/pp_sys.c b/pp_sys.c
index c8e7a9f531..e3aee18694 100644
--- a/pp_sys.c
+++ b/pp_sys.c
@@ -664,6 +664,8 @@ PP(pp_open)
 PP(pp_close)
 {
     dSP;
+    /* pp_coreargs pushes a NULL to indicate no args passed to
+     * CORE::close() */
     GV * const gv =
 	MAXARG == 0 || (!TOPs && !POPs) ? PL_defoutgv : MUTABLE_GV(POPs);
 
@@ -1369,6 +1371,8 @@ PP(pp_select)
 PP(pp_getc)
 {
     dSP; dTARGET;
+    /* pp_coreargs pushes a NULL to indicate no args passed to
+     * CORE::getc() */
     GV * const gv =
 	MAXARG==0 || (!TOPs && !POPs) ? PL_stdingv : MUTABLE_GV(POPs);
     IO *const io = GvIO(gv);