diff --git a/wasm-port/docs/porting-steps-standalone.md b/wasm-port/docs/porting-steps-standalone.md
index cd0ba33..8aa6e45 100644
--- a/wasm-port/docs/porting-steps-standalone.md
+++ b/wasm-port/docs/porting-steps-standalone.md
@@ -473,6 +473,12 @@ Current verified progress:
   file under `vendor/linuxcnc/` and the matching file under `../linuxcnc/`.
   `tools/source-manifest.txt` was also de-duplicated so the manifest is a
   single authoritative extraction list.
+- Direct source compile probes now also cover vendored `interp_base.cc` and
+  `gomath.c`. `interp_base.cc` uses a standalone `EMC2_HOME` compile-time
+  path boundary for LinuxCNC's dynamic interpreter lookup, and `gomath.c` is
+  compiled as C with `gcc` so its LinuxCNC C linkage is preserved. The
+  standalone `rtapi.h` shim was made C/C++ compatible for this C-source
+  boundary without changing vendored LinuxCNC files.
 
 ## Phase 4: Port INI Parsing Without Editing Upstream
 
diff --git a/wasm-port/runtime/core/shims/rtapi.h b/wasm-port/runtime/core/shims/rtapi.h
index 6feed6a..000c17f 100644
--- a/wasm-port/runtime/core/shims/rtapi.h
+++ b/wasm-port/runtime/core/shims/rtapi.h
@@ -1,9 +1,16 @@
 #pragma once
 
+#ifdef __cplusplus
 #include <cstdarg>
 #include <cstdio>
 #include <cstring>
 #include <cstddef>
+#else
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+#include <stddef.h>
+#endif
 
 #ifndef RTAPI_BEGIN_DECLS
 #ifdef __cplusplus
@@ -37,13 +44,13 @@ RTAPI_BEGIN_DECLS
 static inline int rtapi_snprintf(char *buf, unsigned long size, const char *fmt, ...) {
     va_list ap;
     va_start(ap, fmt);
-    int rc = vsnprintf(buf, static_cast<size_t>(size), fmt, ap);
+    int rc = vsnprintf(buf, (size_t)size, fmt, ap);
     va_end(ap);
     return rc;
 }
 
 static inline int rtapi_vsnprintf(char *buf, unsigned long size, const char *fmt, va_list ap) {
-    return vsnprintf(buf, static_cast<size_t>(size), fmt, ap);
+    return vsnprintf(buf, (size_t)size, fmt, ap);
 }
 
 static inline void rtapi_print(const char *fmt, ...) {
diff --git a/wasm-port/tests/native/verify_native_probes.sh b/wasm-port/tests/native/verify_native_probes.sh
index a1ca5bd..96ff56a 100755
--- a/wasm-port/tests/native/verify_native_probes.sh
+++ b/wasm-port/tests/native/verify_native_probes.sh
@@ -92,6 +92,8 @@ check_exitcode linuxcnc_interp_namedparams_source_probe
 check_exitcode linuxcnc_interp_write_source_probe
 check_exitcode linuxcnc_interp_o_word_source_probe
 check_exitcode linuxcnc_rs274ngc_pre_source_probe
+check_exitcode linuxcnc_interp_base_source_probe
+check_exitcode linuxcnc_gomath_source_probe
 
 NAMEDPARAM_STDOUT="$BUILD_DIR/linuxcnc_namedparam_harness.run.stdout.log"
 grep -Fq "init_named_parameters=0" "$NAMEDPARAM_STDOUT"
diff --git a/wasm-port/tools/build_native_probes.sh b/wasm-port/tools/build_native_probes.sh
index 1dac468..73fd6aa 100755
--- a/wasm-port/tools/build_native_probes.sh
+++ b/wasm-port/tools/build_native_probes.sh
@@ -10,6 +10,7 @@ INCLUDE_DIR="$ROOT_DIR/runtime/core/include"
 MINIMAL_GCODE_FIXTURE="$ROOT_DIR/tests/fixtures/gcode/minimal_linear.ngc"
 NAMEDPARAM_INI_FIXTURE="$ROOT_DIR/tests/fixtures/ini/namedparams.ini"
 CXX="${CXX:-g++}"
+CC="${CC:-gcc}"
 
 mkdir -p "$BUILD_DIR"
 
@@ -45,6 +46,11 @@ MINIMAL_FLAGS=(
   -DLINUXCNC_STANDALONE_USE_RS274_PRE_STATE
 )
 
+INTERP_BASE_FLAGS=(
+  "${MINIMAL_FLAGS[@]}"
+  -DEMC2_HOME=\"/standalone\"
+)
+
 MINIMAL_LINK_FLAGS=(
   -Wl,--gc-sections
 )
@@ -54,6 +60,24 @@ TP_FLAGS=(
   -fpermissive
 )
 
+C_FLAGS=(
+  -std=gnu11
+  -O2
+  -D_GNU_SOURCE
+  -DM_PI=3.14159265358979323846
+  -I"$WRAP_DIR"
+  -I"$SHIM_DIR"
+  -I"$INCLUDE_DIR"
+  -I"$VENDOR_DIR/src"
+  -I"$VENDOR_DIR/src/rtapi"
+  -I"$VENDOR_DIR/src/emc"
+  -I"$VENDOR_DIR/src/emc/nml_intf"
+  -I"$VENDOR_DIR/src/emc/motion"
+  -I"$VENDOR_DIR/src/emc/tp"
+  -I"$VENDOR_DIR/src/emc/kinematics"
+  -I"$VENDOR_DIR/src/libnml/posemath"
+)
+
 write_command_file() {
   local file="$1"
   shift
@@ -129,6 +153,32 @@ compile_object() {
   return 1
 }
 
+compile_c_object() {
+  local target="$1"
+  local source="$2"
+  local obj="$3"
+  shift 3
+
+  local stdout_log="$BUILD_DIR/$target.stdout.log"
+  local stderr_log="$BUILD_DIR/$target.stderr.log"
+  local depfile="$obj.d"
+  local cmdfile="$obj.cmd"
+  local cmd=("$CC" "$@" -MMD -MP -MF "$depfile" -c "$source" -o "$obj")
+
+  mkdir -p "$(dirname "$obj")"
+  if ! object_needs_rebuild "$obj" "$depfile" "$cmdfile" "${cmd[@]}"; then
+    printf 'up to date: %s\n' "$source" >> "$stdout_log"
+    return 0
+  fi
+
+  printf 'compile-c: %s\n' "$source" >> "$stdout_log"
+  if "${cmd[@]}" >> "$stdout_log" 2>> "$stderr_log"; then
+    write_command_file "$cmdfile" "${cmd[@]}"
+    return 0
+  fi
+  return 1
+}
+
 link_needs_rebuild() {
   local output="$1"
   local cmdfile="$2"
@@ -247,6 +297,27 @@ build_object_target() {
   return "$status"
 }
 
+build_c_object_target() {
+  local target="$1"
+  local output="$2"
+  local source="$3"
+  local flags_name="$4"
+
+  local stdout_log="$BUILD_DIR/$target.stdout.log"
+  local stderr_log="$BUILD_DIR/$target.stderr.log"
+  : > "$stdout_log"
+  : > "$stderr_log"
+
+  local -n flags="$flags_name"
+  local status=0
+  if ! compile_c_object "$target" "$source" "$output" "${flags[@]}"; then
+    status=1
+  fi
+
+  echo "$status" > "$BUILD_DIR/$target.exitcode"
+  return "$status"
+}
+
 NO_LINK_FLAGS=()
 
 INI_PROBE_SOURCES=(
@@ -629,4 +700,16 @@ build_object_target \
   "$VENDOR_DIR/src/emc/rs274ngc/rs274ngc_pre.cc" \
   MINIMAL_FLAGS
 
+build_object_target \
+  linuxcnc_interp_base_source_probe \
+  "$BUILD_DIR/linuxcnc_interp_base_source_probe.o" \
+  "$VENDOR_DIR/src/emc/rs274ngc/interp_base.cc" \
+  INTERP_BASE_FLAGS
+
+build_c_object_target \
+  linuxcnc_gomath_source_probe \
+  "$BUILD_DIR/linuxcnc_gomath_source_probe.o" \
+  "$VENDOR_DIR/src/libnml/posemath/gomath.c" \
+  C_FLAGS
+
 echo "native probes complete"