--- a/Make.unix	2024-03-12 18:28:22.000000000 +0100
+++ b/Make.unix	2026-10-19 18:08:43.373584622 +0200
@@ -20,4 +20,5 @@ all: default
 
 libmachdep.a:
 	arch=`uname -m|sed 's/i.86/386/;s/Power Macintosh/power/; s/x86_64/amd64/; s/armv[567].*/arm/; s/aarch64/arm64/'`; \
+	[ -d posix-$$arch ] || arch=port; \
 	(cd posix-$$arch &&  make)
--- a/posix-port/Makefile	2024-03-12 18:28:22.000000000 +0100
+++ b/posix-port/Makefile	2026-10-19 18:08:43.373453383 +0200
@@ -4,6 +4,7 @@ LIB=../libmachdep.a
 
 OFILES=\
 	getcallerpc.$O\
+	tas.$O
 
 default: $(LIB)
 $(LIB): $(OFILES)
--- a/posix-port/tas.c	1970-01-01 01:00:00.000000000 +0100
+++ b/posix-port/tas.c	1970-01-01 01:00:00.000000000 +0100
@@ -0,0 +1,18 @@
+#include "u.h"
+#include "libc.h"
+
+int
+tas(int *x)
+{
+	int v;
+
+	v = __atomic_exchange_n(x, 1, __ATOMIC_ACQUIRE);
+	switch(v) {
+	case 0:
+	case 1:
+		return v;
+	default:
+		print("canlock: corrupted 0x%lux\n", v);
+		return 1;
+	}
+}