diff --git a/amd64.c b/amd64.c
index 15468090..ab62bd92 100644
--- a/amd64.c
+++ b/amd64.c
@@ -120,6 +120,24 @@ cothread_t co_active() {
   return co_active_handle;
 }
 
+cothread_t co_derive(void* memory, unsigned int size, void (*entrypoint)(void)) {
+  cothread_t handle;
+  if(!co_swap) {
+    co_init();
+    co_swap = (void (*)(cothread_t, cothread_t))co_swap_function;
+  }
+  if(!co_active_handle) co_active_handle = &co_active_buffer;
+
+  if(handle = (cothread_t)memory) {
+    long long *p = (long long*)((char*)handle + size);  /* seek to top of stack */
+    *--p = (long long)crash;                            /* crash if entrypoint returns */
+    *--p = (long long)entrypoint;                       /* start of function */
+    *(long long*)handle = (long long)p;                 /* stack pointer */
+  }
+
+  return handle;
+}
+
 cothread_t co_create(unsigned int size, void (*entrypoint)(void)) {
   cothread_t handle;
   if(!co_swap) {
diff --git a/arm.c b/arm.c
index 313f1134..20c95310 100644
--- a/arm.c
+++ b/arm.c
@@ -40,8 +40,25 @@ cothread_t co_active() {
   return co_active_handle;
 }
 
+cothread_t co_derive(void* memory, unsigned int size, void (*entrypoint)(void)) {
+  unsigned long* handle;
+  if(!co_swap) {
+    co_init();
+    co_swap = (void (*)(cothread_t, cothread_t))co_swap_function;
+  }
+  if(!co_active_handle) co_active_handle = &co_active_buffer;
+
+  if(handle = (unsigned long*)memory) {
+    unsigned long* p = (unsigned long*)((unsigned char*)handle + size);
+    handle[8] = (unsigned long)p;
+    handle[9] = (unsigned long)entrypoint;
+  }
+
+  return handle;
+}
+
 cothread_t co_create(unsigned int size, void (*entrypoint)(void)) {
-  unsigned long* handle = 0;
+  unsigned long* handle;
   if(!co_swap) {
     co_init();
     co_swap = (void (*)(cothread_t, cothread_t))co_swap_function;
diff --git a/doc/usage.html b/doc/usage.html
index 38576dc5..994072f9 100644
--- a/doc/usage.html
+++ b/doc/usage.html
@@ -62,6 +62,14 @@ Return handle to current cothread. Always returns a valid handle, even when
 called from the main program thread.
 <hr/>
 
+<code>cothread_t co_derive(void* memory, unsigned int heapsize, void (*coentry)(void));</code><br/><br/>
+Initializes new cothread.</br>
+This function is identical to co_create, only it attempts to use the provided
+memory instead of allocating new memory on the heap. Please note that certain
+implementations (currently only Windows Fibers) cannot be created using existing
+memory, and as such, this function will fail.
+<hr/>
+
 <code>cothread_t co_create(unsigned int heapsize, void (*coentry)(void));</code><br/><br/>
 Create new cothread.<br/>
 Heapsize is the amount of memory allocated for the cothread stack, specified
diff --git a/fiber.c b/fiber.c
index 50af4a72..bdf4dd4e 100644
--- a/fiber.c
+++ b/fiber.c
@@ -24,6 +24,11 @@ cothread_t co_active() {
   return co_active_;
 }
 
+cothread_t co_derive(void* memory, unsigned int heapsize, void (*coentry)(void)) {
+  //Windows fibers do not allow users to supply their own memory
+  return (cothread_t)0;
+}
+
 cothread_t co_create(unsigned int heapsize, void (*coentry)(void)) {
   if(!co_active_) {
     ConvertThreadToFiber(0);
diff --git a/libco.h b/libco.h
index c8452154..b7f60852 100644
--- a/libco.h
+++ b/libco.h
@@ -1,5 +1,5 @@
 /*
-  libco v18.02 (2017-11-06)
+  libco v19 (2019-02-18)
   author: byuu
   license: ISC
 */
@@ -14,6 +14,7 @@ extern "C" {
 typedef void* cothread_t;
 
 cothread_t co_active();
+cothread_t co_derive(void*, unsigned int, void (*)(void));
 cothread_t co_create(unsigned int, void (*)(void));
 void co_delete(cothread_t);
 void co_switch(cothread_t);
diff --git a/ppc.c b/ppc.c
index 6d79b44f..f071fab2 100644
--- a/ppc.c
+++ b/ppc.c
@@ -258,6 +258,70 @@ static const uint32_t libco_ppc_code[1024] = {
   #define CO_SWAP_ASM(x, y) ((void (*)(cothread_t, cothread_t))(uintptr_t)libco_ppc_code)(x, y)
 #endif
 
+static uint32_t* co_derive_(void* memory, unsigned size, uintptr_t entry) {
+  (void)entry;
+
+  uint32_t* t = (uint32_t*)memory;
+
+  #if LIBCO_PPCDESC
+  if(t) {
+    memcpy(t, (void*)entry, sizeof(void*) * 3);  /* copy entry's descriptor */
+    *(const void**)t = libco_ppc_code;  /* set function pointer to swap routine */
+  }
+  #endif
+
+  return t;
+}
+
+cothread_t co_derive(void* memory, unsigned int size, void (*entry_)(void)) {
+  uintptr_t entry = (uintptr_t)entry_;
+  uint32_t* t = 0;
+
+  /* be sure main thread was successfully allocated */
+  if(co_active()) {
+    t = co_derive_(memory, size, entry);
+  }
+
+  if(t) {
+    uintptr_t sp;
+    int shift;
+
+    /* save current registers into new thread, so that any special ones will have proper values when thread is begun */
+    CO_SWAP_ASM(t, t);
+
+    #if LIBCO_PPCDESC
+    entry = (uintptr_t)*(void**)entry;  /* get real address */
+    #endif
+
+    /* put stack near end of block, and align */
+    sp = (uintptr_t)t + size - above_stack;
+    sp -= sp % stack_align;
+
+    /* on PPC32, we save and restore GPRs as 32 bits. for PPC64, we
+       save and restore them as 64 bits, regardless of the size the ABI
+       uses. so, we manually write pointers at the proper size. we always
+       save and restore at the same address, and since PPC is big-endian,
+       we must put the low byte first on PPC32. */
+
+    /* if uintptr_t is 32 bits, >>32 is undefined behavior,
+       so we do two shifts and don't have to care how many bits uintptr_t is. */
+    #if LIBCO_PPC64
+    shift = 16;
+    #else
+    shift = 0;
+    #endif
+
+    /* set up so entry will be called on next swap */
+    t[ 8] = (uint32_t)(entry >> shift >> shift);
+    t[ 9] = (uint32_t)entry;
+
+    t[10] = (uint32_t)(sp >> shift >> shift); 
+    t[11] = (uint32_t)sp;
+  }
+
+  return t;
+}
+
 static uint32_t* co_create_(unsigned size, uintptr_t entry) {
   (void)entry;
 
diff --git a/sjlj.c b/sjlj.c
index 1d0cb59a..d99572a3 100644
--- a/sjlj.c
+++ b/sjlj.c
@@ -38,10 +38,52 @@ cothread_t co_active() {
   return (cothread_t)co_running;
 }
 
+cothread_t co_derive(void* memory, unsigned int size, void (*coentry)(void)) {
+  if(!co_running) co_running = &co_primary;
+
+  cothread_struct* thread = (cothread_struct*)memory;
+  memory = (unsigned char*)memory + sizeof(cothread_struct);
+  size -= sizeof(cothread_struct);
+  if(thread) {
+    struct sigaction handler;
+    struct sigaction old_handler;
+
+    stack_t stack;
+    stack_t old_stack;
+
+    thread->coentry = thread->stack = 0;
+
+    stack.ss_flags = 0;
+    stack.ss_size = size;
+    thread->stack = stack.ss_sp = memory;
+    if(stack.ss_sp && !sigaltstack(&stack, &old_stack)) {
+      handler.sa_handler = springboard;
+      handler.sa_flags = SA_ONSTACK;
+      sigemptyset(&handler.sa_mask);
+      creating = thread;
+
+      if(!sigaction(SIGUSR1, &handler, &old_handler)) {
+        if(!raise(SIGUSR1)) {
+          thread->coentry = coentry;
+        }
+        sigaltstack(&old_stack, 0);
+        sigaction(SIGUSR1, &old_handler, 0);
+      }
+    }
+
+    if(thread->coentry != coentry) {
+      co_delete(thread);
+      thread = 0;
+    }
+  }
+
+  return (cothread_t)thread;
+}
+
 cothread_t co_create(unsigned int size, void (*coentry)(void)) {
   if(!co_running) co_running = &co_primary;
 
-  cothread_struct *thread = (cothread_struct*)malloc(sizeof(cothread_struct));
+  cothread_struct* thread = (cothread_struct*)malloc(sizeof(cothread_struct));
   if(thread) {
     struct sigaction handler;
     struct sigaction old_handler;
diff --git a/ucontext.c b/ucontext.c
index 9ba47c88..edf513d4 100644
--- a/ucontext.c
+++ b/ucontext.c
@@ -31,6 +31,23 @@ cothread_t co_active() {
   return (cothread_t)co_running;
 }
 
+cothread_t co_derive(void* memory, unsigned int heapsize, void (*coentry)(void)) {
+  if(!co_running) co_running = &co_primary;
+  ucontext_t* thread = (ucontext_t*)memory;
+  memory = (unsigned char*)memory + sizeof(ucontext_t);
+  heapsize -= sizeof(ucontext_t);
+  if(thread) {
+    if((!getcontext(thread) && !(thread->uc_stack.ss_sp = 0)) && (thread->uc_stack.ss_sp = memory)) {
+      thread->uc_link = co_running;
+      thread->uc_stack.ss_size = heapsize;
+      makecontext(thread, coentry, 0);
+    } else {
+      thread = 0;
+    }
+  }
+  return (cothread_t)thread;
+}
+
 cothread_t co_create(unsigned int heapsize, void (*coentry)(void)) {
   if(!co_running) co_running = &co_primary;
   ucontext_t* thread = (ucontext_t*)malloc(sizeof(ucontext_t));
diff --git a/x86.c b/x86.c
index b5c38216..d79306c6 100644
--- a/x86.c
+++ b/x86.c
@@ -74,6 +74,24 @@ cothread_t co_active() {
   return co_active_handle;
 }
 
+cothread_t co_derive(void* memory, unsigned int size, void (*entrypoint)(void)) {
+  cothread_t handle;
+  if(!co_swap) {
+    co_init();
+    co_swap = (void (fastcall*)(cothread_t, cothread_t))co_swap_function;
+  }
+  if(!co_active_handle) co_active_handle = &co_active_buffer;
+
+  if(handle = (cothread_t)memory) {
+    long *p = (long*)((char*)handle + size);  /* seek to top of stack */
+    *--p = (long)crash;                       /* crash if entrypoint returns */
+    *--p = (long)entrypoint;                  /* start of function */
+    *(long*)handle = (long)p;                 /* stack pointer */
+  }
+
+  return handle;
+}
+
 cothread_t co_create(unsigned int size, void (*entrypoint)(void)) {
   cothread_t handle;
   if(!co_swap) {