$OpenBSD: patch-src_gmic_cpp,v 1.3 2021/09/06 14:54:05 espie Exp $

pthread_self will do in a pinch
Index: src/gmic.cpp
--- src/gmic.cpp.orig
+++ src/gmic.cpp
@@ -2022,7 +2022,7 @@ const CImgList<T>& _gmic_display(CImgDisplay &disp, co
 #include "gmic.h"
 using namespace cimg_library;
 
-#include "gmic_stdlib.h"
+#include "stdlib_defs.h"
 
 // Define convenience macros, variables and functions.
 //----------------------------------------------------
@@ -2445,6 +2445,8 @@ CImgList<void*> gmic::list_p_is_abort = CImgList<void*
 bool *gmic::abort_ptr(bool *const p_is_abort) {
 #if defined(__MACOSX__) || defined(__APPLE__)
   void* tid = (void*)(cimg_ulong)getpid();
+#elif defined(__OpenBSD__)
+  void* tid = (void*)pthread_self();
 #elif cimg_OS==1
   void* tid = (void*)(cimg_ulong)syscall(SYS_gettid);
 #elif cimg_OS==2
@@ -2783,6 +2785,8 @@ gmic::~gmic() {
   cimg::mutex(21);
 #if defined(__MACOSX__) || defined(__APPLE__)
   void* tid = (void*)(cimg_ulong)getpid();
+#elif defined(__OpenBSD__)
+  void* tid = (void*)pthread_self();
 #elif cimg_OS==1
   void* tid = (void*)(cimg_ulong)syscall(SYS_gettid);
 #elif cimg_OS==2
