diff --git a/ffi/libc.lua b/ffi/libc.lua
index e22c5c5..d473315 100644
--- a/ffi/libc.lua
+++ b/ffi/libc.lua
@@ -1,18 +1,33 @@
--- ffi/libc.lua — shared libc bindings: errno, signal, write, read, chdir.
+-- ffi/libc.lua — shared libc bindings: chdir, errno, strerror.
+-- Phase 0: just enough to make `cd` interception work in executor.lua.
+-- See docs/PHASE0.md §7.
 
 local ffi = require("ffi")
 
 ffi.cdef[[
 int   chdir(const char *path);
-int   errno;
+int  *__errno_location(void);
 char *strerror(int errnum);
 ]]
 
+local C = ffi.C
+
 local M = {}
 
 -- Apply chdir per PHASE0.md §7 (intercepts `cd` so wd persists across popen).
+-- Returns: true on success; false, errmsg on failure.
 function M.chdir(path)
-    error("libc.chdir: not implemented (Phase 0 pending)")
+    local rc = C.chdir(path)
+    if rc == 0 then return true end
+    return false, ffi.string(C.strerror(C.__errno_location()[0]))
+end
+
+function M.errno()
+    return C.__errno_location()[0]
+end
+
+function M.strerror(en)
+    return ffi.string(C.strerror(en))
 end
 
 return M