diff --git a/ctl/shared_ptr.cc b/ctl/shared_ptr.cc
new file mode 100644
index 000000000..ae8e4e75d
--- /dev/null
+++ b/ctl/shared_ptr.cc
@@ -0,0 +1,97 @@
+// -*- mode:c++; indent-tabs-mode:nil; c-basic-offset:4; coding:utf-8 -*-
+// vi: set et ft=cpp ts=4 sts=4 sw=4 fenc=utf-8 :vi
+//
+// Copyright 2024 Justine Alexandra Roberts Tunney
+//
+// Permission to use, copy, modify, and/or distribute this software for
+// any purpose with or without fee is hereby granted, provided that the
+// above copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include "shared_ptr.h"
+
+#include "libc/intrin/atomic.h"
+
+namespace {
+
+inline void
+incref(_Atomic(size_t)* r)
+{
+    size_t r2 = atomic_fetch_add_explicit(r, 1, memory_order_relaxed);
+    if (r2 > ((size_t)-1) >> 1)
+        __builtin_trap();
+}
+
+inline int
+decref(_Atomic(size_t)* r)
+{
+    if (!atomic_fetch_sub_explicit(r, 1, memory_order_release)) {
+        atomic_thread_fence(memory_order_acquire);
+        return 1;
+    }
+    return 0;
+}
+
+inline size_t
+getref(const _Atomic(size_t)* r)
+{
+    return atomic_load_explicit(r, memory_order_relaxed);
+}
+
+} // namespace
+
+namespace ctl {
+
+namespace __ {
+
+void
+shared_control::add_shared() noexcept
+{
+    incref(&shared);
+}
+
+void
+shared_control::release_shared() noexcept
+{
+    if (decref(&shared)) {
+        on_zero_shared();
+        release_weak();
+    }
+}
+
+void
+shared_control::add_weak() noexcept
+{
+    incref(&weak);
+}
+
+void
+shared_control::release_weak() noexcept
+{
+    if (decref(&weak))
+        on_zero_weak();
+}
+
+size_t
+shared_control::use_count() const noexcept
+{
+    return 1 + getref(&shared);
+}
+
+size_t
+shared_control::weak_count() const noexcept
+{
+    return getref(&weak);
+}
+
+} // namespace __
+
+} // namespace ctl
diff --git a/ctl/shared_ptr.h b/ctl/shared_ptr.h
new file mode 100644
index 000000000..619c3d7a6
--- /dev/null
+++ b/ctl/shared_ptr.h
@@ -0,0 +1,107 @@
+// -*-mode:c++;indent-tabs-mode:nil;c-basic-offset:4;tab-width:8;coding:utf-8-*-
+// vi: set et ft=cpp ts=4 sts=4 sw=4 fenc=utf-8 :vi
+#ifndef COSMOPOLITAN_CTL_SHARED_PTR_H_
+#define COSMOPOLITAN_CTL_SHARED_PTR_H_
+#include "unique_ptr.h"
+
+namespace ctl {
+
+// TODO(mrdomino): move
+struct exception
+{
+    virtual const char* what() const noexcept
+    {
+        return "exception";
+    }
+};
+
+namespace __ {
+
+struct shared_control
+{
+    _Atomic(size_t) shared;
+    _Atomic(size_t) weak;
+
+    constexpr shared_control() noexcept : shared(0), weak(0)
+    {
+    }
+    virtual ~shared_control()
+    {
+    }
+    void add_shared() noexcept;
+    void release_shared() noexcept;
+    void add_weak() noexcept;
+    void release_weak() noexcept;
+    size_t use_count() const noexcept;
+    size_t weak_count() const noexcept;
+
+  private:
+    virtual void on_zero_shared() noexcept = 0;
+    virtual void on_zero_weak() noexcept = 0;
+};
+
+template<typename T>
+struct shared_pointer : shared_control
+{
+    T* p;
+
+    static shared_pointer* make(T* p)
+    {
+        return new shared_pointer(p);
+    }
+
+  private:
+    shared_pointer(T* p) noexcept : p(p)
+    {
+    }
+
+    void on_zero_shared() noexcept override
+    {
+        delete p;
+    }
+
+    void on_zero_weak() noexcept override
+    {
+        delete this;
+    }
+};
+
+} // namespace __
+
+struct bad_weak_ptr : ctl::exception
+{
+    const char* what() const noexcept override
+    {
+        return "bad_weak_ptr";
+    }
+};
+
+template<typename T>
+class shared_ptr
+{
+  public:
+    constexpr shared_ptr(nullptr_t = nullptr) noexcept
+      : p(nullptr), ctl(nullptr)
+    {
+    }
+    shared_ptr(T* const p2)
+    {
+        auto hold = ctl::unique_ptr(p2);
+        ctl = __::shared_pointer<T>::make(p2);
+        p = hold.release();
+    }
+
+    ~shared_ptr()
+    {
+        if (ctl)
+            ctl->release_shared();
+    }
+
+  private:
+    T* p;
+    __::shared_control* ctl;
+};
+
+} // namespace ctl
+
+#endif // COSMOPOLITAN_CTL_SHARED_PTR_H_
diff --git a/test/ctl/shared_ptr_test.cc b/test/ctl/shared_ptr_test.cc
new file mode 100644
index 000000000..c00eab891
--- /dev/null
+++ b/test/ctl/shared_ptr_test.cc
@@ -0,0 +1,35 @@
+// -*- mode:c++; indent-tabs-mode:nil; c-basic-offset:4; coding:utf-8 -*-
+// vi: set et ft=cpp ts=4 sts=4 sw=4 fenc=utf-8 :vi
+//
+// Copyright 2024 Justine Alexandra Roberts Tunney
+//
+// Permission to use, copy, modify, and/or distribute this software for
+// any purpose with or without fee is hereby granted, provided that the
+// above copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include "ctl/shared_ptr.h"
+
+template<typename T>
+using Ptr = ctl::shared_ptr<T>;
+
+int
+main()
+{
+    {
+        Ptr<int> x;
+    }
+
+    {
+        Ptr<int> x(new int());
+    }
+    return 0;
+}