diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index e36aba6..146d266 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -16,7 +16,7 @@ jobs:
       - name: Install
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: beta
+          toolchain: 1.66.0
           profile: minimal
           override: true
       - name: Build
@@ -48,7 +48,7 @@ jobs:
       - name: Install
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: beta
+          toolchain: stable
           target: ${{ matrix.target }}
           profile: minimal
           override: true
diff --git a/Cargo.toml b/Cargo.toml
index 49e11cb..c904ec1 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
-[project]
+[package]
 name = "probe"
-version = "0.3.0"
+version = "0.4.0"
 authors = ["Josh Stone <cuviper@gmail.com>"]
 description = "Static instrumentation probes"
 documentation = "https://docs.rs/probe/"
@@ -8,6 +8,7 @@ homepage = "https://github.com/cuviper/rust-libprobe"
 repository = "https://github.com/cuviper/rust-libprobe"
 license = "Apache-2.0 OR MIT"
 edition = "2021"
+rust-version = "1.66"
 exclude = ["/.github/**"]
 
 [lib]
diff --git a/README.md b/README.md
index 2cffcfc..d8cb9c2 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 # libprobe: Static probes for Rust
 
 [![probe crate](https://img.shields.io/crates/v/probe.svg)](https://crates.io/crates/probe)
-![minimum rustc 1.59](https://img.shields.io/badge/rustc-1.59+-red.svg)
+![minimum rustc 1.66](https://img.shields.io/badge/rustc-1.66+-red.svg)
 [![probe documentation](https://docs.rs/probe/badge.svg)](https://docs.rs/probe)
 [![build status](https://github.com/cuviper/rust-libprobe/workflows/CI/badge.svg)](https://github.com/cuviper/rust-libprobe/actions)
 
@@ -20,7 +20,7 @@ The recommended way to use it is to add a line into your Cargo.toml such as:
 
 ```toml
 [dependencies]
-probe = "0.3"
+probe = "0.4"
 ```
 
 Then `use probe::probe;` in your code and insert macro calls wherever you want
diff --git a/examples/semaphore.rs b/examples/semaphore.rs
new file mode 100644
index 0000000..b53c60b
--- /dev/null
+++ b/examples/semaphore.rs
@@ -0,0 +1,16 @@
+use probe::probe;
+
+fn main() {
+    let mut iter = 0;
+    loop {
+        iter += 1;
+        probe!(foo, iter, {
+            // This delay is an exaggeration of the overhead of a probe argument, but it's only
+            // incurred while something is attached to the probe, thanks to the semaphore.
+            std::thread::sleep(std::time::Duration::from_secs(1));
+            iter
+        });
+    }
+}
+
+// bcc/tools/trace.py -p $(pidof semaphore) 'u::foo:iter "iter = %d", arg1'
diff --git a/src/platform/default.rs b/src/platform/default.rs
index 21aeed8..39d4f04 100644
--- a/src/platform/default.rs
+++ b/src/platform/default.rs
@@ -1,5 +1,10 @@
 #[doc(hidden)]
 #[macro_export]
 macro_rules! platform_probe(
-    ($provider:ident, $name:ident, $($arg:expr,)*) => ()
+    ($provider:ident, $name:ident, $($arg:expr,)*) => ({
+        // Expand the arguments so they don't cause unused warnings.
+        if false {
+            let _ = ($($arg,)*);
+        }
+    })
 );
diff --git a/src/platform/systemtap.rs b/src/platform/systemtap.rs
index e05ba6f..82413b8 100644
--- a/src/platform/systemtap.rs
+++ b/src/platform/systemtap.rs
@@ -101,7 +101,9 @@ macro_rules! sdt_asm(
 #[macro_export]
 macro_rules! _sdt_asm(
     ($size:literal, options ($($opt:ident),*), $provider:ident, $name:ident, $($argstr:literal, $arg:expr,)*) => (
-        ::core::arch::asm!(concat!(r#"
+        static mut SEMAPHORE: u16 = 0;
+        if ::core::ptr::read_volatile(&SEMAPHORE) != 0 {
+            ::core::arch::asm!(concat!(r#"
 990:    nop
         .pushsection .note.stapsdt,"?","note"
         .balign 4
@@ -110,7 +112,7 @@ macro_rules! _sdt_asm(
 992:    .balign 4
 993:    ."#, $size, r#"byte 990b
         ."#, $size, r#"byte _.stapsdt.base
-        ."#, $size, r#"byte 0 // FIXME set semaphore address
+        ."#, $size, r#"byte {}
         .asciz ""#, stringify!($provider), r#""
         .asciz ""#, stringify!($name), r#""
         .asciz ""#, $($argstr,)* r#""
@@ -123,10 +125,10 @@ macro_rules! _sdt_asm(
 _.stapsdt.base: .space 1
         .size _.stapsdt.base, 1
         .popsection
-.endif
-"#
-            ),
-            $(in(reg) (($arg) as isize) ,)*
-            options(readonly, nostack, preserves_flags, $($opt),*),
-        )
+.endif"#),
+                sym SEMAPHORE,
+                $(in(reg) (($arg) as isize) ,)*
+                options(readonly, nostack, preserves_flags, $($opt),*),
+            );
+        }
     ));