Crash on OpenBSD (amd64)

[markand]

Hi,

First of all, I love minicoro and was able to play with it on various systems with many architectures. I tried on my favorite of choice, OpenBSD (amd64 for now) and unfortunately as secure as it is I guess the current assembly code is not compatible with.

Trying a very minimal example ends with:

[a.out]25923/502807 sp=3fbcd3a2388 inside 7b6448f83000-7b6449382fff: not MAP_STACK
Segmentation fault (core dumped)

The core dump does not provide any useful information though:

(lldb) bt
* thread #1, stop reason = signal SIGSEGV
  * frame #0: 0x00000fddd963b909 ld.so`_dl_bind(object=0x00000fde64344800, index=<unavailable>) at rtld_machine.c:352:13
    frame #1: 0x00000fddd9643421 ld.so`_dl_bind_start at ldasm.S:107
    frame #2: 0x00000fdb9d46ba98 a.out`entrypoint(self=0x00000fde7e04e000) at test.c:9:2
    frame #3: 0x00000fdb9d46bb78 a.out`_mco_main(co=0x00000fde7e04e000) at minicoro.h:622:3

I don't really know how to make context switch in assembly so I'll not be able to help. However, I've seen various others doing less performant alternatives based on setjmp/longjmp as last resort. Not sure if that could make for asymmetric coroutines but I'll have a look as temporary fix if possible

[markand]

Okay, it was finally was easier than I thought. On OpenBSD mmap must be used and MAP_STACK provided as argument. I'm thinking of a diff like:

--- a/minicoro.h  Wed Jan  7 13:18:48 2026
+++ b/minicoro.h       Wed Jan  7 13:16:19 2026
@@ -483,6 +483,14 @@
   #include <windows.h>
 #endif

+#if defined(__OpenBSD__)
+  #define MCO_USE_VMEM_ALLOCATOR
+#else
+  #if !defined(MAP_STACK)
+    #define MAP_STACK 0
+  #endif
+#endif
+
 #ifndef MCO_NO_DEFAULT_ALLOCATOR
   #if defined(MCO_USE_VMEM_ALLOCATOR) && defined(_WIN32)
     static void* mco_alloc(size_t size, void* allocator_data) {
@@ -500,7 +508,7 @@
     #include <sys/mman.h>
     static void* mco_alloc(size_t size, void* allocator_data) {
       _MCO_UNUSED(allocator_data);
-      void *ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+      void *ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_STACK | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
       return ptr != MAP_FAILED ? ptr : NULL;
     }
     static void mco_dealloc(void* ptr, size_t size, void* allocator_data) {