LinuxCNC · grandixximo · Sep 11, 2026 · Sep 11, 2026
diff --git a/docs/src/gcode/g-code.adoc b/docs/src/gcode/g-code.adoc
@@ -85,6 +85,7 @@ as the 'L number', and so on for any other letter.
 |<<gcode:g43,G43>>                 |Use Tool Length Offset from Tool Table
 |<<gcode:g43.1,G43.1>>             |Dynamic Tool Length Offset
 |<<gcode:g43.2,G43.2>>             |Apply additional Tool Length Offset
+|<<gcode:g43.4,G43.4>>             |Tool Length Offset on Primary Kinematics
 |<<gcode:g49,G49>>                 |Cancel Tool Length Offset
 |<<gcode:g52,G52>>                 |Local Coordinate System Offset
 |<<gcode:g53,G53>>                 |Move in Machine Coordinates
@@ -1717,11 +1718,59 @@ It is an error if:
 
 NOTE: G43.2 does not write to the tool table.
 
+[[gcode:g43.4]]
+== G43.4 Tool Length Offset on Primary Kinematics(((G43.4 Tool Length Offset on Primary Kinematics)))
+
+[source,ngc]
+----
+G43.4 <H->
+----
+
+* 'H' - tool number (optional)
+
+'G43.4' is 'G43' together with a switch to the kinematics the module
+declares its working transform, so the program runs with tool length
+compensation in the module's working kinematics. The switch happens
+first and the offset applies after it, as if the two had been written
+on consecutive lines. 'G49' is the matching cancel: it clears the
+offset and switches back to identity kinematics, as long as the offset
+in effect is still 'G43.4''s and no 'G12.1' or 'G13.1' has selected a
+kinematics since.
+
+The H word, the offset itself and the parameters it lands in are
+'G43''s. Which kinematics is the working one is declared by the module
+(KINSTYPE_PRIMARY, see the <<cha:switchable-kinematics,Switchable
+Kinematics>> chapter), the number is not the answer; a module that
+declares none rejects 'G43.4' at read time.
+
+The switch is a queue synchronisation point like
+'<<gcode:g12.1-g13.1,G12.1>>': when 'G43.4' or 'G49' changes the
+kinematics, the interpreter waits for queued motion to finish first, so
+both stop any blending in progress. Already on the target kinematics
+they do not switch and blend normally. A plain 'G43', 'G43.1' or
+'G43.2' never switches, whatever kinematics is selected, and the 'G49'
+that cancels one of them does not switch either.
+
+On a machine without switchable kinematics there is nothing to switch:
+'G43.4' is a plain 'G43' and 'G49' a plain cancel.
+
+It is an error if:
+
+* the kinematics module is switchable but declares no primary
+  kinematics, or
+* any of the 'G43' error conditions holds.
+
 [[gcode:g49]]
 == G49 Cancel Tool Length Compensation(((G49 Cancel Tool Length Offset)))
 
 * 'G49' - cancels tool length compensation
 
+'G49' also switches a switchable kinematics module back to its identity
+kinematics when it cancels a 'G43.4', undoing the switch that made. It
+leaves a kinematics selected by 'G12.1' or 'G13.1' alone, as it does
+after a plain 'G43', and a module that declares no identity kinematics
+gets the plain cancel.
+
 It is OK to program using the same offset already in use. It is also
 OK to program using no tool length offset if none is currently being
 used.

diff --git a/docs/src/gcode/overview.adoc b/docs/src/gcode/overview.adoc
@@ -973,7 +973,7 @@ The modal groups are shown in the following Table.
 |Feed Rate Mode (Group 5)               | G93, G94, G95
 |Units (Group 6)                        | G20, G21
 |Cutter Diameter Compensation (Group 7) | G40, G41, G42, G41.1, G42.1
-|Tool Length Offset (Group 8)           | G43, G43.1, G49
+|Tool Length Offset (Group 8)           | G43, G43.1, G43.2, G43.4, G49
 |Canned Cycles Return Mode (Group 10)   | G98, G99
 |Coordinate System (Group 12)           | G54, G55, G56, G57, G58, G59, G59.1, G59.2, G59.3
 |Control Mode (Group 13)                | G61, G61.1, G64

diff --git a/docs/src/motion/switchkins.adoc b/docs/src/motion/switchkins.adoc
@@ -218,8 +218,12 @@ A module that declares no identity kinstype refuses 'G13.1' with an
 error and can still be driven by number with 'G12.1'; see Code Notes
 for how a module declares its types.
 
-See the G-code documentation for 'G12.1' and 'G13.1' for the full
-description.
+For tool length work there are spellings that name the kinematics by
+what it is rather than by number: 'G43.4' applies the tool length
+offset and switches to the kinstype the module declares its working
+transform, and the 'G49' that cancels it switches back to identity.
+See the G-code documentation for 'G43.4' and 'G49', and for 'G12.1'
+and 'G13.1', for the full description.
 
 === M-code commands
 
@@ -491,26 +495,31 @@ which kinstype is at fault.
 Each kinstype gets its own 'kinstype.is-N' pin, so a module
 providing the usual three keeps the pin names it always had.
 
-A module should also declare what each kinstype IS, again from within
+A module also declares what each kinstype IS, with flags from
+kinematics.h:
+
+. *KINSTYPE_IDENTITY*  no transform: the joints are the world
+. *KINSTYPE_PRIMARY*   the module's working transform
+
+A kinstype registered with switchkinsRegisterOps() carries its flag in
+the ops table itself, as the 'identity' or 'primary' field; a kinstype
+registered the older way gets it from a call, again from within
 switchkinsSetup():
 
 ----
 int switchkinsDeclare(int ktype, int flags);
 ----
 
-with flags from kinematics.h:
-
-. *KINSTYPE_IDENTITY*  no transform: the joints are the world
-. *KINSTYPE_PRIMARY*   the module's working transform
-
 G-code reads these declarations: 'G13.1' cancels to the kinstype
-declared KINSTYPE_IDENTITY, whatever its number, so a module whose
-identity kinematics is not kinstype 0 still gets a working 'G13.1'.
-At most one kinstype may be declared identity, and declaring a
-kinstype the module does not provide fails the module load.  A module
-that declares nothing keeps working exactly as before for 'G12.1 P-',
-but 'G13.1' is an error, since the number of the identity kinematics
-is then a guess.
+declared KINSTYPE_IDENTITY, and 'G43.4' switches to the kinstype
+declared KINSTYPE_PRIMARY, whatever their numbers, so a module whose
+kinematics are not in the conventional order still gets working
+spellings.  At most one kinstype may be declared identity and at most
+one primary, and declaring a kinstype the module does not provide
+fails the module load.  A module that declares nothing keeps working
+exactly as before for 'G12.1 P-' and 'G49', but 'G13.1' and 'G43.4'
+are an error, since the numbers of the identity and primary kinematics
+are then a guess.
 
 After calling switchkinsSetup(), rtapi_app_main() checks the
 supplied parameters, creates a HAL component, and then invokes

diff --git a/src/emc/kinematics/kinematics.h b/src/emc/kinematics/kinematics.h
@@ -18,6 +18,10 @@
 #include "emcpos.h" /* EmcPose */
 #include "rtapi_bool.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*
   The type of kinematics used.
 
@@ -389,7 +393,6 @@ extern int userkKinematicsInverse(const struct EmcPose * world,
                                   double *joint,
                                   const KINEMATICS_INVERSE_FLAGS * iflags,
                                   KINEMATICS_FORWARD_FLAGS * fflags);
-#endif
 //*********************************************************************
 // xyzac,xyzbc;
 extern int trtKinematicsSetup(const int   comp_id,
@@ -434,3 +437,8 @@ extern int xyzbcKinematicsWorkFrame(const double *joints,
                                    const KINEMATICS_FORWARD_FLAGS *fflags);
 
 //*********************************************************************
+#ifdef __cplusplus
+}
+#endif
+
+#endif // __LINUXCNC_KINEMATICS_H
diff --git a/src/emc/rs274ngc/interp_array.cc b/src/emc/rs274ngc/interp_array.cc
@@ -91,7 +91,7 @@ const int Interp::gees[] = {
 /* 360 */  -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
 /* 380 */  -1,-1, 1, 1, 1, 1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
 /* 400 */   7,-1,-1,-1,-1,-1,-1,-1,-1,-1, 7, 7,-1,-1,-1,-1,-1,-1,-1,-1,
-/* 420 */   7, 7,-1,-1,-1,-1,-1,-1,-1,-1, 8, 8, 8,-1,-1,-1,-1,-1,-1,-1,
+/* 420 */   7, 7,-1,-1,-1,-1,-1,-1,-1,-1, 8, 8, 8,-1, 8,-1,-1,-1,-1,-1,
 /* 440 */  -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
 /* 460 */  -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
 /* 480 */  -1,-1,-1,-1,-1,-1,-1,-1,-1,-1, 8,-1,-1,-1,-1,-1,-1,-1,-1,-1,

diff --git a/src/emc/rs274ngc/interp_base.hh b/src/emc/rs274ngc/interp_base.hh
@@ -63,6 +63,10 @@ public:
     virtual void print_state_tag(StateTag const &tag) = 0;
     virtual void set_loglevel(int level) = 0;
     virtual void set_loop_on_main_m99(bool state) = 0;
+    // true while the startup code runs at task init, when the motion
+    // queue cannot drain yet: a kinematics switch there queues without
+    // the drain-and-assert wait, which could never complete
+    virtual void set_in_startup_code(bool) {};
     virtual FILE* get_stdout() { return stdout; };
 };
 

diff --git a/src/emc/rs274ngc/interp_check.cc b/src/emc/rs274ngc/interp_check.cc
@@ -286,8 +286,8 @@ int Interp::check_other_codes(block_pointer block)       //!< pointer to a block
   }
 
   if (block->h_flag) {
-    CHKS((block->g_modes[GM_TOOL_LENGTH_OFFSET] != G_43 && motion != G_76 && block->g_modes[GM_TOOL_LENGTH_OFFSET] != G_43_2),
-      _("H word with no G43 or G76 to use it"));
+    CHKS((block->g_modes[GM_TOOL_LENGTH_OFFSET] != G_43 && motion != G_76 && block->g_modes[GM_TOOL_LENGTH_OFFSET] != G_43_2 && block->g_modes[GM_TOOL_LENGTH_OFFSET] != G_43_4),
+      _("H word with no G43, G43.4 or G76 to use it"));
   }
 
   if (block->i_flag) {    /* could still be useless if yz_plane arc */

diff --git a/src/emc/rs274ngc/interp_convert.cc b/src/emc/rs274ngc/interp_convert.cc
@@ -34,7 +34,7 @@
 #include "interp_internal.hh"
 #include "interp_queue.hh"
 #include "interp_parameter_def.hh"
-#include "kinematics.h"          // KINSTYPE_IDENTITY, SWITCHKINS_MAX_TYPES
+#include <kinematics.h>          // KINSTYPE_IDENTITY, SWITCHKINS_MAX_TYPES
 
 #include "units.h"
 #define TOOL_INSIDE_ARC(side, turn) (((side)==CUTTER_COMP::LEFT&&(turn)>0)||((side)==CUTTER_COMP::RIGHT&&(turn)<0))
@@ -4136,12 +4136,13 @@ int Interp::convert_m(block_pointer block,       //!< pointer to a block of RS27
 
   if (FEATURE(RETAIN_G43)) {
 
-      if ((settings->active_g_codes[9] == G_43) && ONCE(STEP_RETAIN_G43)) {
+      if (((settings->active_g_codes[9] == G_43) ||
+           (settings->active_g_codes[9] == G_43_4)) && ONCE(STEP_RETAIN_G43)) {
         if(settings->selected_pocket > 0) {
             struct block_struct g43;
             init_block(&g43);
-            block->g_modes[gees[G_43]] = G_43;
-            CHP(convert_tool_length_offset(G_43, &g43, settings));
+            block->g_modes[gees[settings->active_g_codes[9]]] = settings->active_g_codes[9];
+            CHP(convert_tool_length_offset(settings->active_g_codes[9], &g43, settings));
         } else {
             struct block_struct g49;
             init_block(&g49);
@@ -4451,8 +4452,10 @@ int Interp::convert_modal_0(int code,    						//!< G-code, must be from group 0
     // will be queued: ask every time.  The exception is an
     // ON_ABORT_COMMAND routine, run by one execute() call that cannot
     // service INTERP_EXECUTE_FINISH and would drop the rest of the
-    // routine; the abort has just flushed the queue anyway.
-    if (!settings->in_abort_command) {
+    // routine; the abort has just flushed the queue anyway.  The startup
+    // code is the other exception: it runs before the main loop can
+    // service the wait, and no motion exists yet to protect.
+    if (!settings->in_abort_command && !settings->in_startup_code) {
       settings->kinsSwitch_flag = true;
     }
     CHP(convert_kins_switch(code, block, settings));
@@ -6478,6 +6481,45 @@ int Interp::convert_tool_change(setup_pointer settings)  //!< pointer to machine
 
 /****************************************************************************/
 
+// the kinematics module declares what each type is (KINSTYPE_* flags);
+// where the flags say nothing at all there is no kinematics attached
+// (sai, preview) and the codes fall back to type 0, as before
+static int kins_type_info_available()
+{
+  int k;
+
+  for (k = 0; k < SWITCHKINS_MAX_TYPES; k++) {
+    if (GET_EXTERNAL_KINS_TYPE_FLAGS(k) >= 0) return 1;
+  }
+  return 0;
+}
+
+// the type carrying a KINSTYPE_ flag, or -1 when the module declares none;
+// -1 for a type is "no information", and it matches every flag, so it must
+// be excluded before the bit test
+static int flagged_kins_type(int flag)
+{
+  int k, f;
+
+  for (k = 0; k < SWITCHKINS_MAX_TYPES; k++) {
+    f = GET_EXTERNAL_KINS_TYPE_FLAGS(k);
+    if (f >= 0 && (f & flag)) { return k; }
+  }
+  return -1;
+}
+
+// a kinematics switch as G12.1/G13.1 do, with the drain wait and its two
+// exceptions; already on the type there is nothing to do
+static void switch_kins_type(int kins_type, setup_pointer settings)
+{
+  if (settings->kins_type == kins_type) { return; }
+  if (!settings->in_abort_command && !settings->in_startup_code) {
+    settings->kinsSwitch_flag = true;
+  }
+  SELECT_KINS_TYPE(kins_type);
+  settings->kins_type = kins_type;
+}
+
 /*! convert_tool_length_offset
 
 Returned Value: int
@@ -6518,9 +6560,21 @@ int Interp::convert_tool_length_offset(int g_code,       //!< g_code being execu
 
   CHKS((settings->cutter_comp_side != CUTTER_COMP::OFF),
        (_("Cannot change tool offset with cutter radius compensation on")));
+  if (g_code == G_43_4) {
+    int primary = flagged_kins_type(KINSTYPE_PRIMARY);
+    // G43.4 is G43 on the module's working transform: switch first, then
+    // apply the offset, as if the switch line had run and drained.  With
+    // no kinematics attached there is nothing to switch to.
+    CHKS(primary < 0 && kins_type_info_available(), NCE_NO_PRIMARY_KINEMATICS_TYPE);
+    if (primary >= 0) { switch_kins_type(primary, settings); }
+    settings->kins_by_g43_4 = true;
+  } else if (g_code != G_49) {
+    // the offset in effect is no longer G43.4's, so G49 has no switch to undo
+    settings->kins_by_g43_4 = false;
+  }
   if (g_code == G_49) {
     idx = 0;
-  } else if (g_code == G_43) {
+  } else if (g_code == G_43 || g_code == G_43_4) {
       logDebug("convert_tool_length_offset h_flag=%d h_number=%d toolchange_flag=%d current_pocket=%d\n",
 	      block->h_flag,block->h_number,settings->toolchange_flag,settings->current_pocket);
     if(block->h_flag) {
@@ -6600,7 +6654,7 @@ int Interp::convert_tool_length_offset(int g_code,       //!< g_code being execu
         if(block->w_flag) tool_offset.w += block->w_number;
     }
   } else {
-    ERS("BUG: Code not G43, G43.1, G43.2, or G49");
+    ERS("BUG: Code not G43, G43.1, G43.2, G43.4, or G49");
   }
   USE_TOOL_LENGTH_OFFSET(tool_offset);
 
@@ -6641,6 +6695,16 @@ int Interp::convert_tool_length_offset(int g_code,       //!< g_code being execu
   settings->parameters[5088] = PROGRAM_TO_USER_LEN(tool_offset.v);
   settings->parameters[5089] = PROGRAM_TO_USER_LEN(tool_offset.w);
 
+  if (g_code == G_49 && settings->kins_by_g43_4) {
+    // G49 undoes what G43.4 did: after the cancel it drops the machine
+    // to identity kinematics, as if G13.1 had run on the next line.  A
+    // kinematics the program selected itself is left alone, and a
+    // module that declares no identity type keeps the plain cancel.
+    int identity = flagged_kins_type(KINSTYPE_IDENTITY);
+    if (identity >= 0) { switch_kins_type(identity, settings); }
+    settings->kins_by_g43_4 = false;
+  }
+
   return INTERP_OK;
 }
 
@@ -6698,36 +6762,16 @@ so no motion is ever planned across a change of kinematics.
 
 */
 
-// the kinematics module declares what each type is (KINSTYPE_* flags);
-// where the flags say nothing at all there is no kinematics attached
-// (sai, preview) and the codes fall back to type 0, as before
-static int kins_type_info_available()
-{
-  int k;
-
-  for (k = 0; k < SWITCHKINS_MAX_TYPES; k++) {
-    if (GET_EXTERNAL_KINS_TYPE_FLAGS(k) >= 0) return 1;
-  }
-  return 0;
-}
-
 int Interp::convert_kins_switch(int code,                //!< G_12_1 or G_13_1
                                 block_pointer block,     //!< pointer to a block of RS274 instructions
                                 setup_pointer settings)  //!< pointer to machine settings
 {
   int kins_type;
 
   if (code == G_13_1) {
-    int k;
-
     // G13.1 cancels to identity kinematics; which type that is, the
     // module declares, the number is not the answer
-    for (k = 0, kins_type = -1; k < SWITCHKINS_MAX_TYPES; k++) {
-      if (GET_EXTERNAL_KINS_TYPE_FLAGS(k) & KINSTYPE_IDENTITY) {
-        kins_type = k;
-        break;
-      }
-    }
+    kins_type = flagged_kins_type(KINSTYPE_IDENTITY);
     if (kins_type < 0) {
       CHKS(kins_type_info_available(), NCE_NO_IDENTITY_KINEMATICS_TYPE);
       kins_type = 0; // no kinematics attached: standalone interpreter
@@ -6744,6 +6788,8 @@ int Interp::convert_kins_switch(int code,                //!< G_12_1 or G_13_1
 
   SELECT_KINS_TYPE(kins_type);
   settings->kins_type = kins_type;
+  // the program has taken the kinematics over from G43.4
+  settings->kins_by_g43_4 = false;
   return INTERP_OK;
 }