diff --git a/examples/thread.c b/examples/thread.c
index 4b4073922..106bb32fe 100644
--- a/examples/thread.c
+++ b/examples/thread.c
@@ -9,19 +9,34 @@
 #endif
 #include "libc/stdio/stdio.h"
 #include "libc/thread/create.h"
+#include "libc/thread/self.h"
+#include "libc/thread/detach.h"
+#include "libc/thread/join.h"
 #include "libc/time/time.h"
 
 int worker(void* arg) {
+  cthread_t self = cthread_self();
+  int tid = self->tid;
+  sleep(1);
+  //sleep(10000);
+  //printf("[%p] %d\n", self, tid);
   (void)arg;
-  return 0;
+  return 4;
 }
 
 int main() {
   cthread_t thread;
   int rc = cthread_create(&thread, NULL, &worker, NULL);
   if (rc == 0) {
-    printf("thread created: %p\n", thread);
-    sleep(1000);
+    //printf("thread created: %p\n", thread);
+    sleep(1);
+#if 1
+    cthread_join(thread, &rc);
+#else
+    rc = cthread_detach(thread);
+    sleep(2);
+#endif
+    //printf("thread joined: %p -> %d\n", thread, rc);
   } else {
     printf("ERROR: thread could not be started: %d\n", rc);
   }
diff --git a/libc/thread/create.c b/libc/thread/create.c
index 3f016defc..2f1c57093 100644
--- a/libc/thread/create.c
+++ b/libc/thread/create.c
@@ -19,7 +19,7 @@
 #include "libc/thread/create.h"
 #include "libc/linux/clone.h"
 #include "libc/runtime/runtime.h"
-#include "libc/linux/mmap.h"
+#include "libc/sysv/consts/nr.h"
 #include "libc/sysv/consts/clone.h"
 #include "libc/sysv/consts/map.h"
 #include "libc/sysv/consts/prot.h"
@@ -28,7 +28,7 @@
 
 static cthread_t _thread_allocate(const cthread_attr_t* attr) {
   size_t stacksize = attr->stacksize;
-  size_t guardsize = 0;//attr->guardsize;
+  size_t guardsize = attr->guardsize;
   // FIXME: properly count TLS size
   size_t tlssize = 0;
   
@@ -59,7 +59,7 @@ static cthread_t _thread_allocate(const cthread_attr_t* attr) {
   td->tls.bottom = tls_bottom;
   td->alloc.top = alloc_top;
   td->alloc.bottom = alloc_bottom;
-  td->tid = 0;
+  td->state = (attr->mode & CTHREAD_CREATE_DETACHED) ? cthread_detached : cthread_started;
   
   return td;
 }
@@ -68,41 +68,41 @@ int cthread_create(cthread_t*restrict p, const cthread_attr_t*restrict attr, int
   extern wontreturn void _thread_run(int(*func)(void*), void* arg);
   
   cthread_attr_t default_attr;
-  if (!attr) cthread_attr_init(&default_attr);
+  cthread_attr_init(&default_attr);
   cthread_t td = _thread_allocate(attr ? attr : &default_attr);
-  if (!attr) cthread_attr_destroy(&default_attr);
+  cthread_attr_destroy(&default_attr);
   if (!td) return errno;
   
   *p = td;
   
-  uintptr_t stack = (uintptr_t)(td->stack.top);
+  register cthread_t td_ asm("r8") = td;
+  register int* ptid_ asm("rdx") = &td->tid;
+  register int* ctid_ asm("r10") = &td->tid;
+  register int(*func_)(void*) asm("r12") = func;
+  register void* arg_ asm("r13") = arg;
   
-  stack -= sizeof(void*); *(void**)stack = func;
-  stack -= sizeof(void*); *(void**)stack = arg;
-  
-  long flags = CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | CLONE_PARENT | CLONE_THREAD | /*CLONE_IO |*/ CLONE_SETTLS;
-  
-  // It is not necessary to check the return of the syscall here as the return address for the thread is setup to to `_thread_run`.
-  // In case of success: the parent callee returns immediately to the caller forwarding the success to the callee
-  //                     the child return immediately to the entry point of `thread_spawn`
-  // In case of error: the parent callee returns immediately to the caller forwarding the error
-  //                   the child is never created (and so cannot returned in the wrong place)
-  int rc = LinuxClone(flags, (void*)stack, NULL, NULL, (void*)td);
-  if (!rc) {
-    // child
-    asm volatile(
-      "xor %rbp,%rbp\n\t"
-      /* pop arguments */
-      "pop %rdi\n\t"
-      "pop %rax\n\t"
-      /* call function */
-      "call *%rax\n\t"
-      /* thread exit */
-      "mov %rax, %rdi\n\t"
-      "jmp cthread_exit"
-    );
-    unreachable;
+  long flags = CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | CLONE_PARENT | CLONE_THREAD | /*CLONE_IO |*/ CLONE_SETTLS | CLONE_PARENT_SETTID | CLONE_CHILD_CLEARTID;
+  int rc;
+  // asm ensures the (empty) stack of the child thread is not used
+  asm volatile(
+    "syscall\n\t" // clone
+    "test\t%0, %0\n\t" // if not child
+    "jne\t.L.cthread_create.%=\n\t" // jump to `parent` label
+    "xor\t%%rbp, %%rbp\n\t" // reset stack frame pointer
+    "mov\t%2, %%rdi\n\t"
+    "call\t*%1\n\t" // call `func(arg)`
+    "mov\t%%rax, %%rdi\n\t"
+    "jmp\tcthread_exit\n" // exit thread
+    ".L.cthread_create.%=:"
+    : "=a"(rc)
+    : "r"(func_), "r"(arg_), "0"(__NR_clone), "D"(flags), "S"(td->stack.top), "r"(ptid_), "r"(ctid_), "r"(td_)
+    : "rcx", "r11", "cc", "memory"
+  );
+  if (__builtin_expect(rc < 0, 0)) {
+    // `clone` has failed. The thread must be deallocated.
+    size_t size = (intptr_t)(td->alloc.top) - (intptr_t)(td->alloc.bottom);
+    munmap(td->alloc.bottom, size);
+    return -rc;
   }
-  if (rc < 0) return rc;
   return 0;
 }
diff --git a/libc/thread/create.h b/libc/thread/create.h
index 35007ba92..b6e77b950 100644
--- a/libc/thread/create.h
+++ b/libc/thread/create.h
@@ -1,15 +1,14 @@
 #ifndef COSMOPOLITAN_LIBC_THREAD_CREATE_H_
 #define COSMOPOLITAN_LIBC_THREAD_CREATE_H_
+#include "libc/thread/attr.h"
+#include "libc/thread/descriptor.h"
 #if !(__ASSEMBLER__ + __LINKER__ + 0)
 COSMOPOLITAN_C_START_
 
 /**
- * @fileoverview thread
+ * @fileoverview Create a cosmopolitan thread
  */
 
-#include "libc/thread/attr.h"
-#include "libc/thread/descriptor.h"
-
 int cthread_create(cthread_t*restrict, const cthread_attr_t*restrict, int (*)(void*), void*restrict);
 
 
diff --git a/libc/thread/descriptor.h b/libc/thread/descriptor.h
index 7d2ee0661..793590ffd 100644
--- a/libc/thread/descriptor.h
+++ b/libc/thread/descriptor.h
@@ -7,12 +7,23 @@ COSMOPOLITAN_C_START_
  * @fileoverview thread types
  */
 
+enum cthread_state {
+  cthread_started = 0,
+  cthread_joining = 1,
+  cthread_finished = 2,
+  cthread_detached = 4,
+};
+
+
 struct cthread_descriptor_t {
-    struct cthread_descriptor_t* self; // mandatory for TLS
-    struct {
-      void *top, *bottom;
-    } stack, tls, alloc;
-    int tid;
+  struct cthread_descriptor_t* self; // mandatory for TLS
+  struct {
+    void *top, *bottom;
+  } stack, tls, alloc;
+  int state;
+  int tid;
+  int rc;
+  void* pthread_ret_ptr;
 };
 
 typedef struct cthread_descriptor_t* cthread_t;
diff --git a/libc/thread/detach.c b/libc/thread/detach.c
new file mode 100644
index 000000000..a6defc262
--- /dev/null
+++ b/libc/thread/detach.c
@@ -0,0 +1,31 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 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 "libc/thread/detach.h"
+#include "libc/thread/descriptor.h"
+#include "libc/runtime/runtime.h"
+
+int cthread_detach(cthread_t td) {
+  int state;
+  asm volatile("lock xadd\t%1, %0" : "+m"(td->state), "=r"(state) : "1"(cthread_detached) : "cc");
+  if ((state & cthread_finished)) {
+    size_t size = (intptr_t)(td->alloc.top) - (intptr_t)(td->alloc.bottom);
+    munmap(td->alloc.bottom, size);
+  }
+  return 0;
+}
diff --git a/libc/thread/detach.h b/libc/thread/detach.h
new file mode 100644
index 000000000..4185792f5
--- /dev/null
+++ b/libc/thread/detach.h
@@ -0,0 +1,16 @@
+#ifndef COSMOPOLITAN_LIBC_THREAD_DETACH_H_
+#define COSMOPOLITAN_LIBC_THREAD_DETACH_H_
+#include "libc/thread/descriptor.h"
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+/**
+ * @fileoverview detach a thread
+ */
+
+int cthread_detach(cthread_t);
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_THREAD_DETACH_H_ */
+ 
diff --git a/libc/thread/exit.c b/libc/thread/exit.c
index b7e7a598d..f12260744 100644
--- a/libc/thread/exit.c
+++ b/libc/thread/exit.c
@@ -17,30 +17,28 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/thread/exit.h"
+#include "libc/thread/self.h"
 #include "libc/thread/descriptor.h"
-#include "libc/linux/munmap.h"
+#include "libc/sysv/consts/nr.h"
 
-static wontreturn void _self_exit(void* p, size_t s, int r) {
-  // asm is necessary as the stack does not exist between the unmap and the actual exit
+wontreturn void cthread_exit(int rc) {
+  cthread_t td = cthread_self();
+  td->rc = rc;
+  size_t size = (intptr_t)(td->alloc.top) - (intptr_t)(td->alloc.bottom);
+  
+  int state;
   asm volatile(
-    "mov $11, %%rax\n\t"
-    "syscall\n\t"
-    "mov %%rbx, %%rdi\n\t"
-    "mov $60, %%rax\n\t"
-    "syscall"
-    :
-    : "D"(p), "S"(s), "b"(r)
-    : "memory"
+    "lock xadd\t%1, %0\n\t" // mark thread as finished
+    "test\t%2, %b1\n\t" // test if thread was detached
+    "jz .L.cthread_exit.%=\n\t" // skip unmap if not detached
+    "syscall\n" // unmap thread
+    ".L.cthread_exit.%=:\n\t"
+    "mov\t%%rbx, %%rdi\n\t" //rc
+    "mov\t$60, %%rax\n\t"
+    "syscall" // thread exit
+    : "+m"(td->state), "=&r"(state)
+    : "I"(cthread_detached), "1"(cthread_finished), "a"(__NR_munmap), "b"(rc), "D"(td->alloc.bottom), "S"(size)
+    : "rcx", "r11", "cc", "memory"
   );
   unreachable;
 }
-
-wontreturn void cthread_exit(int rc) {
-  // TODO: wait joiners
-  
-  cthread_t td;
-  asm("mov %%fs:0, %0" : "=r"(td));
-  size_t size = (intptr_t)(td->alloc.top) - (intptr_t)(td->alloc.bottom);
-  
-  _self_exit(td->alloc.top, size, rc);
-}
diff --git a/libc/thread/exit.h b/libc/thread/exit.h
index ad4844ddd..679b472c2 100644
--- a/libc/thread/exit.h
+++ b/libc/thread/exit.h
@@ -4,7 +4,7 @@
 COSMOPOLITAN_C_START_
 
 /**
- * @fileoverview internal function called after a thread exits
+ * @fileoverview exit the current thread
  */
 
 wontreturn void cthread_exit(int);
diff --git a/libc/thread/join.c b/libc/thread/join.c
new file mode 100644
index 000000000..bf21ee9a9
--- /dev/null
+++ b/libc/thread/join.c
@@ -0,0 +1,49 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 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 "libc/thread/join.h"
+#include "libc/thread/descriptor.h"
+#include "libc/runtime/runtime.h"
+#include "libc/sysv/consts/nr.h"
+#include "libc/sysv/consts/futex.h"
+
+int cthread_join(cthread_t td, int* rc) {
+  int tid = td->tid; // tid must be loaded before lock xadd
+  // otherwise, tid could be set to 0 even though `state` is not finished
+  
+  // mark thread as joining
+  int state;
+  asm volatile("lock xadd\t%1, %0" : "+m"(td->state), "=r"(state) : "1"(cthread_joining) : "cc");
+  
+  if (!(state & cthread_finished)) {
+    int flags = FUTEX_WAIT; // PRIVATE makes it hang
+    register struct timespec* timeout asm("r10") = NULL;
+    asm volatile (
+      "syscall"
+      :
+      : "a"(__NR_futex), "D"(&td->tid), "S"(flags), "d"(tid), "r"(timeout)
+      : "rcx", "r11", "cc", "memory"
+    );
+  }
+  
+  *rc = td->rc;
+  
+  size_t size = (intptr_t)(td->alloc.top) - (intptr_t)(td->alloc.bottom);
+  munmap(td->alloc.bottom, size);
+  return 0;
+}
diff --git a/libc/thread/join.h b/libc/thread/join.h
new file mode 100644
index 000000000..4676ad7d2
--- /dev/null
+++ b/libc/thread/join.h
@@ -0,0 +1,16 @@
+#ifndef COSMOPOLITAN_LIBC_THREAD_JOIN_H_
+#define COSMOPOLITAN_LIBC_THREAD_JOIN_H_
+#include "libc/thread/descriptor.h"
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+/**
+ * @fileoverview join a thread
+ */
+
+int cthread_join(cthread_t, int*);
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_THREAD_JOIN_H_ */
+ 
\ No newline at end of file
diff --git a/libc/thread/self.c b/libc/thread/self.c
new file mode 100644
index 000000000..d3b3098d7
--- /dev/null
+++ b/libc/thread/self.c
@@ -0,0 +1,21 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 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 "libc/thread/self.h"
+
+extern inline cthread_t cthread_self(void);
diff --git a/libc/thread/self.h b/libc/thread/self.h
new file mode 100644
index 000000000..4ab15498c
--- /dev/null
+++ b/libc/thread/self.h
@@ -0,0 +1,20 @@
+#ifndef COSMOPOLITAN_LIBC_THREAD_SELF_H_
+#define COSMOPOLITAN_LIBC_THREAD_SELF_H_
+#include "libc/thread/descriptor.h"
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+/**
+ * @fileoverview get the thread descriptor of the current thread
+ */
+
+inline cthread_t cthread_self(void) {
+  cthread_t self;
+  asm ("mov %%fs:0, %0" : "=r"(self));
+  return self;
+}
+
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_THREAD_SELF_H_ */