Please consider the following formatting changes to #12596

Framework/Foundation/CMakeLists.txt

@@ -41,6 +41,9 @@ add_executable(o2-test-framework-Signpost
 add_executable(o2-test-framework-ThreadSanitizer
                test/test_ThreadSanitizer.cxx)
 
+add_executable(o2-log
+               src/o2Log.cxx)
+
 target_link_libraries(o2-test-framework-Signpost PRIVATE O2::FrameworkFoundation)
 target_link_libraries(o2-test-framework-ThreadSanitizer
                       PRIVATE O2::FrameworkFoundation Threads::Threads)

Framework/Foundation/src/o2Log.cxx

@@ -0,0 +1,123 @@
+// Copyright 2019-2020 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+#ifdef __APPLE__
+#import <dlfcn.h>
+#include <mach-o/dyld.h>
+#include <mach/host_info.h>
+#include <mach/mach.h>
+#include <mach/mach_host.h>
+#include <sys/mman.h>
+#include <sys/sysctl.h>
+#else
+#include <sys/ptrace.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#endif
+
+#include <cassert>
+#include <cerrno>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <getopt.h>
+
+#ifdef __APPLE__
+int xnu_write(int pid, void* addr, unsigned char* data, size_t dsize)
+{
+  assert(dsize != 0);
+  assert(addr != nullptr);
+  assert(data != nullptr);
+
+  auto* ptxt = (unsigned char*)malloc(dsize);
+
+  assert(ptxt != nullptr);
+  memcpy(ptxt, data, dsize);
+
+  mach_port_t task;
+  mach_msg_type_number_t dataCunt = dsize;
+
+  kern_return_t kret = task_for_pid(mach_task_self(), pid, &task);
+  if (kret != KERN_SUCCESS) {
+    printf("task_for_pid failed: %s. Are you root?", mach_error_string(kret));
+    return 0;
+  }
+
+  vm_protect(task, (vm_address_t)addr, (vm_size_t)dsize, 0,
+             VM_PROT_READ | VM_PROT_WRITE | VM_PROT_ALL);
+
+  kret = vm_write(task, (vm_address_t)addr, (pointer_t)ptxt, dataCunt);
+
+  return kret;
+}
+#elif __linux__
+int xnu_write(int pid, void* addr, unsigned char* data, size_t dsize)
+{
+  if ((ptrace(PTRACE_ATTACH, pid, NULL, NULL)) < 0) {
+    perror("ptrace(ATTACH)");
+    exit(1);
+  }
+  int waitStat = 0;
+  int waitRes = waitpid(pid, &waitStat, WUNTRACED);
+  if (waitRes != pid || !WIFSTOPPED(waitStat)) {
+    perror("....:");
+    printf("Something went wrong...\n");
+    exit(1);
+  }
+
+  if ((ptrace(PTRACE_POKEDATA, pid, addr, data))) {
+    perror("pokedata");
+  }
+
+  if ((ptrace(PTRACE_DETACH, pid, NULL, NULL) < 0)) {
+    perror("ptrace(DETACH)");
+    exit(1);
+  }
+  return 0;
+}
+#endif
+
+// Writes a 4-byte value to the specified address in the target process
+// If the address in question points to private_o2_log_<signpost>::stacktrace
+// This will have the side effect of enabling the signpost.
+int main(int argc, char** argv)
+{
+  // Use getopt_long to parse command line arguments
+  // -p pid
+  // -a address
+  // -s stacktrace level (default 1, 0 to disable)
+  static struct option long_options[] = {
+    {"pid", required_argument, nullptr, 'p'},
+    {"address", required_argument, nullptr, 'a'},
+    {"stacktrace", required_argument, nullptr, 's'},
+    {nullptr, 0, nullptr, 0}};
+  int opt;
+  pid_t pid;
+  int stacktrace = 1;
+  void* addr;
+  while ((opt = getopt_long(argc, argv, "p:a:", long_options, nullptr)) != -1) {
+    switch (opt) {
+      case 'p':
+        pid = atoi(optarg);
+        break;
+      case 'a':
+        addr = (void*)strtoul(optarg, nullptr, 16);
+        break;
+      case 's':
+        stacktrace = strtol(optarg, nullptr, 10);
+        break;
+      default:
+        printf("Usage: %s -p pid -a address [-s level]\n", argv[0]);
+        exit(EXIT_FAILURE);
+    }
+  }
+  xnu_write(pid, addr, (unsigned char*)&stacktrace, 4);
+  return 0;
+}