diff --git a/tcltk/magicdnull.c b/tcltk/magicdnull.c
index b75531b2..f33ce851 100644
--- a/tcltk/magicdnull.c
+++ b/tcltk/magicdnull.c
@@ -11,6 +11,8 @@
 
 #include <tcl.h>
 
+#include "tcltk/tcldir.h"
+
 /*----------------------------------------------------------------------*/
 /* Application initiation.  This is exactly like the AppInit routine	*/
 /* for "wish", minus the cruft, but with "tcl_rcFileName" set to	*/
@@ -29,7 +31,13 @@ magic_AppInit(interp)
     /* This is where we replace the home ".tclshrc" file with	*/
     /* magic's startup script.					*/
 
-    Tcl_SetVar(interp, "tcl_rcFileName", TCL_DIR "/magic.tcl", TCL_GLOBAL_ONLY);
+    {
+	char rc[PATH_MAX];
+	size_t rlen;
+	rlen = MagicTclDir(rc, sizeof(rc), "magic.tcl");
+	assert(rlen < sizeof(rc));
+	Tcl_SetVar(interp, "tcl_rcFileName", rc, TCL_GLOBAL_ONLY);
+    }
 
     /* Additional variable can be used to tell if magic is in batch mode */
     Tcl_SetVar(interp, "batch_mode", "true", TCL_GLOBAL_ONLY);
diff --git a/tcltk/magicexec.c b/tcltk/magicexec.c
index 73abbd3a..1a88646e 100644
--- a/tcltk/magicexec.c
+++ b/tcltk/magicexec.c
@@ -37,6 +37,8 @@
 #include <tk.h>
 #include <tcl.h>
 
+#include "tcltk/tcldir.h"
+
 /*----------------------------------------------------------------------*/
 /* Application initiation.  This is exactly like the AppInit routine	*/
 /* for "wish", minus the cruft, but with "tcl_rcFileName" set to	*/
@@ -59,7 +61,13 @@ magic_AppInit(interp)
     /* This is where we replace the home ".wishrc" file with	*/
     /* magic's startup script.					*/
 
-    Tcl_SetVar(interp, "tcl_rcFileName", TCL_DIR "/magic.tcl", TCL_GLOBAL_ONLY);
+    {
+	char rc[PATH_MAX];
+	size_t rlen;
+	rlen = MagicTclDir(rc, sizeof(rc), "magic.tcl");
+	assert(rlen < sizeof(rc));
+	Tcl_SetVar(interp, "tcl_rcFileName", rc, TCL_GLOBAL_ONLY);
+    }
     return TCL_OK;
 }
 
diff --git a/tcltk/tcldir.h b/tcltk/tcldir.h
new file mode 100644
index 00000000..0300c110
--- /dev/null
+++ b/tcltk/tcldir.h
@@ -0,0 +1,63 @@
+/*
+ * tcldir.h --
+ *
+ * Resolve magic's Tcl-runtime directory (where magic.tcl, tclmagic.so, and the
+ * Tcl auto_path packages live) at *run time* instead of hardwiring the
+ * compile-time install path.
+ *
+ * If the environment variable CAD_ROOT is set and non-empty, use
+ * $CAD_ROOT/magic/tcl -- so magic runs from a relocated install, or straight out
+ * of a build tree (via a staged CAD_ROOT), the same way the sys files already
+ * follow CAD_ROOT and the tcltk/magic.sh wrapper already computes
+ * TCL_MAG_DIR=${CAD_ROOT}/magic/tcl.  Otherwise fall back to the compile-time
+ * TCL_DIR (the default install location).
+ *
+ * Must be included in a translation unit compiled with -DTCL_DIR.
+ */
+
+#ifndef _MAGIC_TCLDIR_H
+#define _MAGIC_TCLDIR_H
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <assert.h>
+#include <limits.h>
+
+/* Platform path-length limit.  <limits.h> declares PATH_MAX when the system has
+ * a fixed limit (4096 on Linux); provide a safe fallback when it does not. */
+#ifndef PATH_MAX
+#define PATH_MAX 4096
+#endif
+
+/*
+ * Write magic's Tcl-runtime directory into buf (of the given size), with the
+ * optional suffix appended as a path component -- the '/' directory separator is
+ * supplied automatically, so pass "magic.tcl", not "/magic.tcl" (a NULL or empty
+ * suffix appends nothing).  A caller that wants "<dir>/magic.tcl" thus gets it in
+ * one call instead of a second snprintf.  Return the number of characters written
+ * -- snprintf semantics, as a size_t -- or 0 on a (very unlikely) snprintf
+ * encoding error.  The assert catches truncation: snprintf returns the length it
+ * *would* have written, so a return of >= size means the path did not fit.
+ * Callers should likewise assert the returned length < their buffer size.
+ */
+static inline size_t
+MagicTclDir(char *buf, size_t size, const char *suffix)
+{
+    const char *cad = getenv("CAD_ROOT");
+    const char *sep;
+    int n;
+
+    if (suffix == NULL) suffix = "";
+    sep = (*suffix != '\0') ? "/" : "";		/* imply the '/' separator */
+
+    if (cad != NULL && *cad != '\0')
+	n = snprintf(buf, size, "%s/magic/tcl%s%s", cad, sep, suffix);
+    else
+	n = snprintf(buf, size, "%s%s%s", TCL_DIR, sep, suffix);
+
+    if (n < 0) n = 0;			/* coerce encoding error to 0, assert below */
+    assert((size_t)n < size);
+    return (size_t)n;
+}
+
+#endif /* _MAGIC_TCLDIR_H */
diff --git a/tcltk/tclmagic.c b/tcltk/tclmagic.c
index 1c007ce1..d74978d2 100644
--- a/tcltk/tclmagic.c
+++ b/tcltk/tclmagic.c
@@ -30,6 +30,7 @@
 #endif
 
 #include "tcltk/tclmagic.h"
+#include "tcltk/tcldir.h"
 #include "utils/main.h"
 #include "utils/magic.h"
 #include "utils/geometry.h"
@@ -1528,7 +1529,15 @@ Tclmagic_Init(interp)
 
     /* Add the magic TCL directory to the Tcl library search path */
 
-    Tcl_Eval(interp, "lappend auto_path " TCL_DIR );
+    {
+	char dir[PATH_MAX], cmd[PATH_MAX];
+	size_t dlen, clen;
+	dlen = MagicTclDir(dir, sizeof(dir), NULL);
+	assert(dlen < sizeof(dir));
+	clen = snprintf(cmd, sizeof(cmd), "lappend auto_path {%s}", dir);
+	assert(clen < sizeof(cmd));
+	Tcl_Eval(interp, cmd);
+    }
 
     /* Get $CAD_ROOT from a Tcl variable, if it exists, and if not, then */
     /* set CAD_ROOT from the environment variable of the same name, if	 */