P4.2: declare fcntl as varargs in debug-repl FFI

ober

009614be61a6e11bb7b9b120fe0d80a2a3933812

diff --git a/src/jcode/core/debug-repl.ss b/src/jcode/core/debug-repl.ss
index 9ac3ef3..c59f981 100644
--- a/src/jcode/core/debug-repl.ss
+++ b/src/jcode/core/debug-repl.ss
@@ -49,7 +49,13 @@
 (def c-setsockopt  (foreign-procedure "setsockopt" (int int int void* int) int))
 (def c-htons       (foreign-procedure "htons" (unsigned-short) unsigned-short))
 (def c-getsockname (foreign-procedure "getsockname" (int void* void*) int))
-(def c-fcntl       (foreign-procedure "fcntl" (int int int) int))
+;; P4.2: fcntl is varargs (int int, ...). On macOS arm64 fixed-args use
+;; one register file and varargs another — declaring a fixed (int int int)
+;; binding lands the third arg in the wrong slot and the kernel sees junk
+;; (F_SETFL silently rejects, set-nonblocking! becomes a no-op, then the
+;; accept thread blocks every other green thread). __varargs_after marks
+;; the variadic boundary so Chez uses the right calling convention.
+(def c-fcntl       (foreign-procedure (__varargs_after 2) "fcntl" (int int int) int))
 (def c-dup         (foreign-procedure "dup" (int) int))
 
 (def c-errno-location