diff --git a/ipc/chromium/src/base/basictypes.h b/ipc/chromium/src/base/basictypes.h
index 8cd84bee59ed..f35616ab25d3 100644
--- a/ipc/chromium/src/base/basictypes.h
+++ b/ipc/chromium/src/base/basictypes.h
@@ -36,7 +36,7 @@ const int64_t kint64min = ((int64_t)GG_LONGLONG(0x8000000000000000));
 const int64_t kint64max = ((int64_t)GG_LONGLONG(0x7FFFFFFFFFFFFFFF));
 
 // Platform- and hardware-dependent printf specifiers
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  define PRId64L "I64d"
 #  define PRIu64L "I64u"
 #  define PRIx64L "I64x"
diff --git a/ipc/chromium/src/base/command_line.cc b/ipc/chromium/src/base/command_line.cc
index 3118dcbe7b6a..6ba225a46a4d 100644
--- a/ipc/chromium/src/base/command_line.cc
+++ b/ipc/chromium/src/base/command_line.cc
@@ -27,7 +27,7 @@ CommandLine* CommandLine::current_process_commandline_ = NULL;
 const wchar_t* const kSwitchPrefixes[] = {L"--", L"-", L"/"};
 const wchar_t kSwitchTerminator[] = L"--";
 const wchar_t kSwitchValueSeparator[] = L"=";
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 // Unixes don't use slash as a switch.
 const char* const kSwitchPrefixes[] = {"--", "-"};
 const char kSwitchTerminator[] = "--";
@@ -97,7 +97,7 @@ CommandLine::CommandLine(const std::wstring& program) {
     command_line_string_ = L'"' + program + L'"';
   }
 }
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 CommandLine::CommandLine(int argc, const char* const* argv) {
   for (int i = 0; i < argc; ++i) argv_.push_back(argv[i]);
   InitFromArgv();
@@ -178,7 +178,7 @@ void CommandLine::Init(int argc, const char* const* argv) {
 #if defined(OS_WIN)
   current_process_commandline_ = new CommandLine;
   current_process_commandline_->ParseFromString(::GetCommandLineW());
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   current_process_commandline_ = new CommandLine(argc, argv);
 #endif
 }
@@ -342,7 +342,7 @@ void CommandLine::PrependWrapper(const std::wstring& wrapper) {
   command_line_string_ = wrapper + L" " + command_line_string_;
 }
 
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 void CommandLine::AppendSwitch(const std::wstring& switch_string) {
   std::string ascii_switch = WideToASCII(switch_string);
   argv_.push_back(kSwitchPrefixes[0] + ascii_switch);
diff --git a/ipc/chromium/src/base/command_line.h b/ipc/chromium/src/base/command_line.h
index d89824d8492e..89c941fbc5dc 100644
--- a/ipc/chromium/src/base/command_line.h
+++ b/ipc/chromium/src/base/command_line.h
@@ -36,7 +36,7 @@ class CommandLine {
   // Creates a parsed version of the given command-line string.
   // The program name is assumed to be the first item in the string.
   void ParseFromString(const std::wstring& command_line);
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   // Initialize from an argv vector (or directly from main()'s argv).
   CommandLine(int argc, const char* const* argv);
   explicit CommandLine(const std::vector<std::string>& argv);
@@ -87,7 +87,7 @@ class CommandLine {
   const std::wstring& command_line_string() const {
     return command_line_string_;
   }
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   // Returns the original command line string as a vector of strings.
   const std::vector<std::string>& argv() const { return argv_; }
 #endif
@@ -158,7 +158,7 @@ class CommandLine {
   // The type of native command line arguments.
   typedef std::wstring StringType;
 
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   // The argv array, with the program name in argv_[0].
   std::vector<std::string> argv_;
 
diff --git a/ipc/chromium/src/base/condition_variable.h b/ipc/chromium/src/base/condition_variable.h
index 864479ec2834..baf54dd2539c 100644
--- a/ipc/chromium/src/base/condition_variable.h
+++ b/ipc/chromium/src/base/condition_variable.h
@@ -71,7 +71,7 @@
 #include "base/lock.h"
 #include "build/build_config.h"
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <pthread.h>
 #endif
 
@@ -106,7 +106,7 @@ class ConditionVariable {
 #if defined(OS_WIN)
   CONDITION_VARIABLE cv_;
   SRWLOCK* const srwlock_;
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   pthread_cond_t condition_;
   pthread_mutex_t* user_mutex_;
 #endif
diff --git a/ipc/chromium/src/base/eintr_wrapper.h b/ipc/chromium/src/base/eintr_wrapper.h
index c4a8b1a0adf5..a2fc816c6e96 100644
--- a/ipc/chromium/src/base/eintr_wrapper.h
+++ b/ipc/chromium/src/base/eintr_wrapper.h
@@ -18,7 +18,7 @@
 
 #include "build/build_config.h"
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 
 #  include <errno.h>
 
@@ -58,6 +58,6 @@
 #  define IGNORE_EINTR(x) (x)
 #  define HANDLE_RV_EINTR(x) (x)
 
-#endif  // OS_POSIX
+#endif  // XP_UNIX
 
 #endif  // !BASE_EINTR_WRAPPER_H_
diff --git a/ipc/chromium/src/base/file_path.cc b/ipc/chromium/src/base/file_path.cc
index 6afb7d9b863a..63a03bd41ccd 100644
--- a/ipc/chromium/src/base/file_path.cc
+++ b/ipc/chromium/src/base/file_path.cc
@@ -246,14 +246,14 @@ FilePath FilePath::AppendASCII(const std::string& component) const {
   DCHECK(IsStringASCII(component));
 #if defined(OS_WIN)
   return Append(ASCIIToWide(component));
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   return Append(component);
 #endif
 }
 
 bool FilePath::IsAbsolute() const { return IsPathAbsolute(path_); }
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 // See file_path.h for a discussion of the encoding of paths on POSIX
 // platforms.  These *Hack() functions are not quite correct, but they're
 // only temporary while we fix the remainder of the code.
diff --git a/ipc/chromium/src/base/file_path.h b/ipc/chromium/src/base/file_path.h
index 6fe4bbdd2963..4c9ee2250be4 100644
--- a/ipc/chromium/src/base/file_path.h
+++ b/ipc/chromium/src/base/file_path.h
@@ -85,7 +85,7 @@
 // pathnames on different platforms.
 class FilePath {
  public:
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
   // On most platforms, native pathnames are char arrays, and the encoding
   // may or may not be specified.  On Mac OS X, native pathnames are encoded
   // in UTF-8.
@@ -239,7 +239,7 @@ class FilePath {
 };
 
 // Macros for string literal initialization of FilePath::CharType[].
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  define FILE_PATH_LITERAL(x) x
 #elif defined(OS_WIN)
 #  define FILE_PATH_LITERAL(x) L##x
diff --git a/ipc/chromium/src/base/lock.h b/ipc/chromium/src/base/lock.h
index 17fe0dad6acd..e2d356ecc522 100644
--- a/ipc/chromium/src/base/lock.h
+++ b/ipc/chromium/src/base/lock.h
@@ -33,7 +33,7 @@ class Lock {
   // Whether Lock mitigates priority inversion when used from different thread
   // priorities.
   static bool HandlesMultipleThreadPriorities() {
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
     // POSIX mitigates priority inversion by setting the priority of a thread
     // holding a Lock to the maximum priority of any other thread waiting on it.
     return base::internal::LockImpl::PriorityInheritanceAvailable();
@@ -47,7 +47,7 @@ class Lock {
 #endif
   }
 
-#if defined(OS_POSIX) || defined(OS_WIN)
+#if defined(XP_UNIX) || defined(OS_WIN)
   // Both Windows and POSIX implementations of ConditionVariable need to be
   // able to see our lock and tweak our debugging counters, as they release and
   // acquire locks inside of their condition variable APIs.
diff --git a/ipc/chromium/src/base/lock_impl.h b/ipc/chromium/src/base/lock_impl.h
index 30df64c9b0ae..96fe67d292a6 100644
--- a/ipc/chromium/src/base/lock_impl.h
+++ b/ipc/chromium/src/base/lock_impl.h
@@ -12,7 +12,7 @@
 
 #if defined(OS_WIN)
 #  include <windows.h>
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 #  include <pthread.h>
 #endif
 
@@ -26,7 +26,7 @@ class LockImpl {
  public:
 #if defined(OS_WIN)
   using NativeHandle = SRWLOCK;
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   using NativeHandle = pthread_mutex_t;
 #endif
 
@@ -49,7 +49,7 @@ class LockImpl {
   // unnecessary.
   NativeHandle* native_handle() { return &native_handle_; }
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
   // Whether this lock will attempt to use priority inheritance.
   static bool PriorityInheritanceAvailable();
 #endif
diff --git a/ipc/chromium/src/base/message_loop.cc b/ipc/chromium/src/base/message_loop.cc
index e7178a6792a7..e915f89a0887 100644
--- a/ipc/chromium/src/base/message_loop.cc
+++ b/ipc/chromium/src/base/message_loop.cc
@@ -23,7 +23,7 @@
 #if defined(OS_MACOSX)
 #  include "base/message_pump_mac.h"
 #endif
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include "base/message_pump_libevent.h"
 #endif
 #if defined(OS_LINUX) || defined(OS_BSD)
@@ -275,7 +275,7 @@ MessageLoop::MessageLoop(Type type, nsISerialEventTarget* aEventTarget)
     DCHECK(type_ == TYPE_UI);
     pump_ = new base::MessagePumpForUI();
   }
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   if (type_ == TYPE_UI) {
 #  if defined(OS_MACOSX)
     pump_ = base::MessagePumpMac::Create();
@@ -287,7 +287,7 @@ MessageLoop::MessageLoop(Type type, nsISerialEventTarget* aEventTarget)
   } else {
     pump_ = new base::MessagePumpDefault();
   }
-#endif    // OS_POSIX
+#endif    // XP_UNIX
 
   // We want GetCurrentSerialEventTarget() to return the real nsThread if it
   // will be used to dispatch tasks. However, under all other cases; we'll want
@@ -705,7 +705,7 @@ bool MessageLoopForIO::WaitForIOCompletion(DWORD timeout, IOHandler* filter) {
   return pump_io()->WaitForIOCompletion(timeout, filter);
 }
 
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 
 bool MessageLoopForIO::WatchFileDescriptor(int fd, bool persistent, Mode mode,
                                            FileDescriptorWatcher* controller,
diff --git a/ipc/chromium/src/base/message_loop.h b/ipc/chromium/src/base/message_loop.h
index b6e4938ee01f..a8a1430d6f93 100644
--- a/ipc/chromium/src/base/message_loop.h
+++ b/ipc/chromium/src/base/message_loop.h
@@ -22,7 +22,7 @@
 // We need this to declare base::MessagePumpWin::Dispatcher, which we should
 // really just eliminate.
 #  include "base/message_pump_win.h"
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 #  include "base/message_pump_libevent.h"
 #endif
 
@@ -329,7 +329,7 @@ class MessageLoop : public base::MessagePump::Delegate {
   base::MessagePumpWin* pump_win() {
     return static_cast<base::MessagePumpWin*>(pump_.get());
   }
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   base::MessagePumpLibevent* pump_libevent() {
     return static_cast<base::MessagePumpLibevent*>(pump_.get());
   }
@@ -527,7 +527,7 @@ class MessageLoopForIO : public MessageLoop {
     return static_cast<base::MessagePumpForIO*>(pump_.get());
   }
 
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   typedef base::MessagePumpLibevent::Watcher Watcher;
   typedef base::MessagePumpLibevent::FileDescriptorWatcher
       FileDescriptorWatcher;
@@ -547,7 +547,7 @@ class MessageLoopForIO : public MessageLoop {
   typedef base::MessagePumpLibevent::SignalWatcher SignalWatcher;
   bool CatchSignal(int sig, SignalEvent* sigevent, SignalWatcher* delegate);
 
-#endif  // defined(OS_POSIX)
+#endif  // defined(XP_UNIX)
 };
 
 // Do not add any member variables to MessageLoopForIO!  This is important b/c
diff --git a/ipc/chromium/src/base/message_pump_libevent.cc b/ipc/chromium/src/base/message_pump_libevent.cc
index c1f945bd4d48..004001fddd20 100644
--- a/ipc/chromium/src/base/message_pump_libevent.cc
+++ b/ipc/chromium/src/base/message_pump_libevent.cc
@@ -8,7 +8,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
-#if defined(ANDROID) || defined(OS_POSIX)
+#if defined(ANDROID) || defined(XP_UNIX)
 #  include <unistd.h>
 #endif
 
diff --git a/ipc/chromium/src/base/platform_file.h b/ipc/chromium/src/base/platform_file.h
index c76d6f29c5ab..e6535b8ed3ab 100644
--- a/ipc/chromium/src/base/platform_file.h
+++ b/ipc/chromium/src/base/platform_file.h
@@ -19,7 +19,7 @@ namespace base {
 #if defined(OS_WIN)
 typedef HANDLE PlatformFile;
 const PlatformFile kInvalidPlatformFileValue = INVALID_HANDLE_VALUE;
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 typedef int PlatformFile;
 const PlatformFile kInvalidPlatformFileValue = -1;
 #endif
diff --git a/ipc/chromium/src/base/platform_thread.h b/ipc/chromium/src/base/platform_thread.h
index 0b7c461aeb3a..942b17f9f80d 100644
--- a/ipc/chromium/src/base/platform_thread.h
+++ b/ipc/chromium/src/base/platform_thread.h
@@ -21,7 +21,7 @@
 #  include <windows.h>
 typedef DWORD PlatformThreadId;
 typedef void* PlatformThreadHandle;  // HANDLE
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 #  include <pthread.h>
 typedef pthread_t PlatformThreadHandle;
 #  if defined(OS_LINUX) || defined(OS_OPENBSD) || defined(OS_SOLARIS) || \
diff --git a/ipc/chromium/src/base/process.h b/ipc/chromium/src/base/process.h
index 222aaefff18a..bdfd65264044 100644
--- a/ipc/chromium/src/base/process.h
+++ b/ipc/chromium/src/base/process.h
@@ -34,7 +34,7 @@ const ProcessHandle kInvalidProcessHandle = INVALID_HANDLE_VALUE;
 // property, however even if that were to change, it seems safe to rely on this
 // particular value never being used.
 const ProcessId kInvalidProcessId = kuint32max;
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 // On POSIX, our ProcessHandle will just be the PID.
 typedef pid_t ProcessHandle;
 typedef pid_t ProcessId;
diff --git a/ipc/chromium/src/base/process_util.h b/ipc/chromium/src/base/process_util.h
index af1c595c1433..103ec1a1e8bc 100644
--- a/ipc/chromium/src/base/process_util.h
+++ b/ipc/chromium/src/base/process_util.h
@@ -105,7 +105,7 @@ void CloseProcessHandle(ProcessHandle process);
 // Win XP SP1 as well.
 ProcessId GetProcId(ProcessHandle process);
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 // Close all file descriptors, except for std{in,out,err} and those
 // for which the given function returns true.  Only call this function
 // in a child process where you know that there aren't any other
@@ -141,7 +141,7 @@ struct LaunchOptions {
 
   std::vector<HANDLE> handles_to_inherit;
 #endif
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
   environment_map env_map;
 
   // If non-null, specifies the entire environment to use for the
@@ -204,7 +204,7 @@ Result<Ok, LaunchError> LaunchApp(const std::wstring& cmdline,
                                   const LaunchOptions& options,
                                   ProcessHandle* process_handle);
 
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 // Runs the application specified in argv[0] with the command line argv.
 //
 // The pid will be stored in process_handle if that pointer is
@@ -280,7 +280,7 @@ Result<Ok, LaunchError> LaunchApp(const CommandLine& cl, const LaunchOptions&,
 // Returns true if this is successful, false otherwise.
 bool KillProcess(ProcessHandle process, int exit_code);
 
-#ifdef OS_POSIX
+#ifdef XP_UNIX
 // Returns whether the given process has exited.  If it returns true,
 // the process status has been consumed and `IsProcessDead` should not
 // be called again on the same process (like `waitpid`).
diff --git a/ipc/chromium/src/base/shared_memory.h b/ipc/chromium/src/base/shared_memory.h
index ec3f9fc25972..b93fb76305e1 100644
--- a/ipc/chromium/src/base/shared_memory.h
+++ b/ipc/chromium/src/base/shared_memory.h
@@ -9,7 +9,7 @@
 
 #include "build/build_config.h"
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <sys/types.h>
 #  include <semaphore.h>
 #endif
@@ -149,7 +149,7 @@ class SharedMemory {
   // something there in the meantime.
   static void* FindFreeAddressSpace(size_t size);
 
-#ifdef OS_POSIX
+#ifdef XP_UNIX
   // If named POSIX shm is being used, append the prefix (including
   // the leading '/') that would be used by a process with the given
   // pid to the given string and return true.  If not, return false.
@@ -165,7 +165,7 @@ class SharedMemory {
   // Unmapping shared memory requires the mapped size on Unix but not
   // Windows; this encapsulates that difference.
   struct MappingDeleter {
-#ifdef OS_POSIX
+#ifdef XP_UNIX
     // A default-constructed deleter must be used only with nullptr
     // (to allow default-constructing UniqueMapping).  A deleter with
     // a size must be used at most once.
@@ -184,7 +184,7 @@ class SharedMemory {
   // If true indicates this came from an external source so needs extra checks
   // before being mapped.
   bool external_section_ = false;
-#elif defined(OS_POSIX) && !defined(ANDROID)
+#elif defined(XP_UNIX) && !defined(ANDROID)
   mozilla::UniqueFileHandle frozen_file_;
   bool is_memfd_ = false;
 #endif
diff --git a/ipc/chromium/src/base/string_util.cc b/ipc/chromium/src/base/string_util.cc
index 6217721f8fd0..067843f5eaec 100644
--- a/ipc/chromium/src/base/string_util.cc
+++ b/ipc/chromium/src/base/string_util.cc
@@ -140,7 +140,7 @@ class StringToInt64Traits {
                                         string_type::value_type** endptr) {
 #ifdef OS_WIN
     return _strtoi64(str, endptr, kBase);
-#else  // assume OS_POSIX
+#else  // assume XP_UNIX
     return strtoll(str, endptr, kBase);
 #endif
   }
@@ -158,7 +158,7 @@ class String16ToInt64Traits {
                                         string_type::value_type** endptr) {
 #ifdef OS_WIN
     return _wcstoi64(str, endptr, kBase);
-#else  // assume OS_POSIX
+#else  // assume XP_UNIX
     std::string ascii_string = UTF16ToASCII(string16(str));
     char* ascii_end = NULL;
     value_type ret = strtoll(ascii_string.c_str(), &ascii_end, kBase);
diff --git a/ipc/chromium/src/base/string_util.h b/ipc/chromium/src/base/string_util.h
index 066a3fb678e5..d312b24a092b 100644
--- a/ipc/chromium/src/base/string_util.h
+++ b/ipc/chromium/src/base/string_util.h
@@ -104,7 +104,7 @@ bool IsWprintfFormatPortable(const wchar_t* format);
 
 #if defined(OS_WIN)
 #  include "base/string_util_win.h"
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 #  include "base/string_util_posix.h"
 #else
 #  error Define string operations appropriately for your platform
diff --git a/ipc/chromium/src/base/thread_local.h b/ipc/chromium/src/base/thread_local.h
index 15401f3c945e..3715b1ecc481 100644
--- a/ipc/chromium/src/base/thread_local.h
+++ b/ipc/chromium/src/base/thread_local.h
@@ -54,7 +54,7 @@
 
 #if defined(OS_WIN)
 #  include <windows.h>
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 #  include <pthread.h>
 #endif
 
@@ -64,7 +64,7 @@ namespace base {
 struct ThreadLocalPlatform {
 #if defined(OS_WIN)
   typedef DWORD SlotType;
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   typedef pthread_key_t SlotType;
 #endif
 
diff --git a/ipc/chromium/src/base/thread_local_storage.h b/ipc/chromium/src/base/thread_local_storage.h
index cf488a2d8de6..4c39074823d8 100644
--- a/ipc/chromium/src/base/thread_local_storage.h
+++ b/ipc/chromium/src/base/thread_local_storage.h
@@ -9,7 +9,7 @@
 
 #include "base/basictypes.h"
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <pthread.h>
 #endif
 
@@ -58,7 +58,7 @@ class ThreadLocalStorage {
     bool initialized_;
 #if defined(OS_WIN)
     int slot_;
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
     pthread_key_t key_;
 #endif
 
diff --git a/ipc/chromium/src/base/time_posix.cc b/ipc/chromium/src/base/time_posix.cc
index c07181ff54c0..133f9c3e215f 100644
--- a/ipc/chromium/src/base/time_posix.cc
+++ b/ipc/chromium/src/base/time_posix.cc
@@ -15,7 +15,7 @@
 #else
 #  include <time.h>
 #endif
-#if defined(ANDROID) || defined(OS_POSIX)
+#if defined(ANDROID) || defined(XP_UNIX)
 #  include <unistd.h>
 #endif
 
@@ -176,7 +176,7 @@ TimeTicks TimeTicks::Now() {
   // With numer and denom = 1 (the expected case), the 64-bit absolute time
   // reported in nanoseconds is enough to last nearly 585 years.
 
-#elif defined(OS_OPENBSD) || defined(OS_POSIX) &&                   \
+#elif defined(OS_OPENBSD) || defined(XP_UNIX) &&                   \
                                  defined(_POSIX_MONOTONIC_CLOCK) && \
                                  _POSIX_MONOTONIC_CLOCK >= 0
 
diff --git a/ipc/chromium/src/base/waitable_event.h b/ipc/chromium/src/base/waitable_event.h
index 40ed103b01c6..3cccb5dbe3a4 100644
--- a/ipc/chromium/src/base/waitable_event.h
+++ b/ipc/chromium/src/base/waitable_event.h
@@ -13,7 +13,7 @@
 #  include <windows.h>
 #endif
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <list>
 #  include <utility>
 #  include "base/condition_variable.h"
diff --git a/ipc/chromium/src/chrome/common/ipc_channel.h b/ipc/chromium/src/chrome/common/ipc_channel.h
index 3cd04ba3de7f..cb2f22af4a66 100644
--- a/ipc/chromium/src/chrome/common/ipc_channel.h
+++ b/ipc/chromium/src/chrome/common/ipc_channel.h
@@ -141,7 +141,7 @@ class Channel {
   // be out of date.
   bool IsClosed() const;
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
   // On POSIX an IPC::Channel wraps a socketpair(), this method returns the
   // FD # for the client end of the socket and the equivalent FD# to use for
   // mapping it into the Child process.
@@ -166,7 +166,7 @@ class Channel {
   // must be set as `MODE_SERVER`, and that side will be responsible for calling
   // `DuplicateHandle` to transfer the handle between processes.
   void StartAcceptingHandles(Mode mode);
-#endif  // defined(OS_POSIX)
+#endif  // defined(XP_UNIX)
 
   // On Windows: Generates a channel ID that, if passed to the client
   // as a shared secret, will validate the client's authenticity.
diff --git a/ipc/glue/EnvironmentMap.h b/ipc/glue/EnvironmentMap.h
index 5cd0ef8c5846..dedb6a7757da 100644
--- a/ipc/glue/EnvironmentMap.h
+++ b/ipc/glue/EnvironmentMap.h
@@ -36,7 +36,7 @@ typedef std::map<NativeEnvironmentString, NativeEnvironmentString>
 NativeEnvironmentString AlterEnvironment(const wchar_t* env,
                                          const EnvironmentMap& changes);
 
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
 
 typedef std::string NativeEnvironmentString;
 typedef std::map<NativeEnvironmentString, NativeEnvironmentString>
diff --git a/ipc/glue/GeckoChildProcessHost.cpp b/ipc/glue/GeckoChildProcessHost.cpp
index c308bd166504..47ea50dd56e6 100644
--- a/ipc/glue/GeckoChildProcessHost.cpp
+++ b/ipc/glue/GeckoChildProcessHost.cpp
@@ -282,7 +282,7 @@ class WindowsProcessLauncher : public BaseProcessLauncher {
 typedef WindowsProcessLauncher ProcessLauncher;
 #endif  // XP_WIN
 
-#ifdef OS_POSIX
+#ifdef XP_UNIX
 class PosixProcessLauncher : public BaseProcessLauncher {
  public:
   PosixProcessLauncher(GeckoChildProcessHost* aHost,
@@ -379,7 +379,7 @@ typedef LinuxProcessLauncher ProcessLauncher;
 #  elif
 #    error "Unknown platform"
 #  endif
-#endif  // OS_POSIX
+#endif  // XP_UNIX
 
 using base::ProcessHandle;
 using mozilla::ipc::BaseProcessLauncher;
@@ -557,7 +557,7 @@ mozilla::BinPathType BaseProcessLauncher::GetPathToBinary(
       MOZ_CRASH("GetModuleFileNameW failed (FIXME)");
     }
     exePath = FilePath::FromWStringHack(exePathBuf);
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
     exePath = FilePath(CommandLine::ForCurrentProcess()->argv()[0]);
 #else
 #  error Sorry; target OS not supported yet.
@@ -1192,7 +1192,7 @@ Result<Ok, LaunchError> LinuxProcessLauncher::DoSetup() {
 }
 #endif  // MOZ_WIDGET_GTK
 
-#ifdef OS_POSIX
+#ifdef XP_UNIX
 Result<Ok, LaunchError> PosixProcessLauncher::DoSetup() {
   Result<Ok, LaunchError> aError = BaseProcessLauncher::DoSetup();
   if (aError.isErr()) {
@@ -1348,7 +1348,7 @@ Result<Ok, LaunchError> PosixProcessLauncher::DoSetup() {
   mChildArgv.push_back(ChildProcessType());
   return Ok();
 }
-#endif  // OS_POSIX
+#endif  // XP_UNIX
 
 #if defined(MOZ_WIDGET_ANDROID)
 RefPtr<ProcessHandlePromise> AndroidProcessLauncher::DoLaunch() {
@@ -1357,7 +1357,7 @@ RefPtr<ProcessHandlePromise> AndroidProcessLauncher::DoLaunch() {
 }
 #endif  // MOZ_WIDGET_ANDROID
 
-#ifdef OS_POSIX
+#ifdef XP_UNIX
 RefPtr<ProcessHandlePromise> PosixProcessLauncher::DoLaunch() {
   ProcessHandle handle = 0;
   Result<Ok, LaunchError> aError =
@@ -1381,7 +1381,7 @@ Result<Ok, LaunchError> PosixProcessLauncher::DoFinishLaunch() {
 
   return Ok();
 }
-#endif  // OS_POSIX
+#endif  // XP_UNIX
 
 #ifdef XP_MACOSX
 Result<Ok, LaunchError> MacProcessLauncher::DoFinishLaunch() {
diff --git a/ipc/glue/IPCMessageUtils.h b/ipc/glue/IPCMessageUtils.h
index ab9b84e3ea83..416cf1eb5f8f 100644
--- a/ipc/glue/IPCMessageUtils.h
+++ b/ipc/glue/IPCMessageUtils.h
@@ -25,7 +25,7 @@ class PickleIterator;
 #  pragma warning(disable : 4800)
 #endif
 
-#if !defined(OS_POSIX)
+#if !defined(XP_UNIX)
 // This condition must be kept in sync with the one in
 // ipc_message_utils.h, but this dummy definition of
 // base::FileDescriptor acts as a static assert that we only get one
@@ -120,7 +120,7 @@ struct ParamTraits<uint8_t> {
   }
 };
 
-#if !defined(OS_POSIX)
+#if !defined(XP_UNIX)
 // See above re: keeping definitions in sync
 template <>
 struct ParamTraits<base::FileDescriptor> {
@@ -133,7 +133,7 @@ struct ParamTraits<base::FileDescriptor> {
     return false;
   }
 };
-#endif  // !defined(OS_POSIX)
+#endif  // !defined(XP_UNIX)
 
 template <>
 struct ParamTraits<mozilla::void_t> {
diff --git a/ipc/glue/ProtocolUtils.cpp b/ipc/glue/ProtocolUtils.cpp
index 51fb4d7e5690..b6c40407d16f 100644
--- a/ipc/glue/ProtocolUtils.cpp
+++ b/ipc/glue/ProtocolUtils.cpp
@@ -7,7 +7,7 @@
 #include "base/process_util.h"
 #include "base/task.h"
 
-#ifdef OS_POSIX
+#ifdef XP_UNIX
 #  include <errno.h>
 #endif
 #include <type_traits>
@@ -77,7 +77,7 @@ void AnnotateSystemError() {
   int64_t error = 0;
 #if defined(XP_WIN)
   error = ::GetLastError();
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
   error = errno;
 #endif
   if (error) {
diff --git a/ipc/ipdl/test/cxx/IPDLUnitTests.template.cpp b/ipc/ipdl/test/cxx/IPDLUnitTests.template.cpp
index 30eea428388e..875c3f9de1c5 100644
--- a/ipc/ipdl/test/cxx/IPDLUnitTests.template.cpp
+++ b/ipc/ipdl/test/cxx/IPDLUnitTests.template.cpp
@@ -54,7 +54,7 @@ const char* IPDLUnitTestName() {
 #if defined(OS_WIN)
     vector<wstring> args = CommandLine::ForCurrentProcess()->GetLooseValues();
     gIPDLUnitTestName = ::strdup(WideToUTF8(args[0]).c_str());
-#elif defined(OS_POSIX)
+#elif defined(XP_UNIX)
     vector<string> argv = CommandLine::ForCurrentProcess()->argv();
     gIPDLUnitTestName = ::moz_xstrdup(argv[1].c_str());
 #else
diff --git a/ipc/ipdl/test/cxx/TestDemon.cpp b/ipc/ipdl/test/cxx/TestDemon.cpp
index 811f6c33140a..396974bb567e 100644
--- a/ipc/ipdl/test/cxx/TestDemon.cpp
+++ b/ipc/ipdl/test/cxx/TestDemon.cpp
@@ -6,7 +6,7 @@
 #include <stdlib.h>
 
 #include "IPDLUnitTests.h"  // fail etc.
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <sys/time.h>
 #  include <unistd.h>
 #else
@@ -27,7 +27,7 @@ static int gStackHeight = 0;
 static bool gFlushStack = false;
 
 static int Choose(int count) {
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
   return random() % count;
 #else
   return rand() % count;
@@ -48,7 +48,7 @@ void TestDemonParent::Main() {
     QuitParent();
     return;
   }
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
   srandom(time(nullptr));
 #else
   srand(time(nullptr));
@@ -72,7 +72,7 @@ void TestDemonParent::ArtificialSleep() {
   if (Choose(2) == 0) {
     // Sleep for anywhere from 0 to 100 milliseconds.
     unsigned micros = Choose(100) * 1000;
-#  ifdef OS_POSIX
+#  ifdef XP_UNIX
     usleep(micros);
 #  else
     Sleep(micros / 1000);
@@ -218,7 +218,7 @@ TestDemonChild::TestDemonChild() : mIncoming(), mOutgoing() {
 TestDemonChild::~TestDemonChild() { MOZ_COUNT_DTOR(TestDemonChild); }
 
 mozilla::ipc::IPCResult TestDemonChild::RecvStart() {
-#ifdef OS_POSIX
+#ifdef XP_UNIX
   srandom(time(nullptr));
 #else
   srand(time(nullptr));
@@ -235,7 +235,7 @@ void TestDemonChild::ArtificialSleep() {
   if (Choose(2) == 0) {
     // Sleep for anywhere from 0 to 100 milliseconds.
     unsigned micros = Choose(100) * 1000;
-#  ifdef OS_POSIX
+#  ifdef XP_UNIX
     usleep(micros);
 #  else
     Sleep(micros / 1000);
diff --git a/ipc/ipdl/test/cxx/TestHighestPrio.cpp b/ipc/ipdl/test/cxx/TestHighestPrio.cpp
index e6715fba8eb5..499c352a5a4d 100644
--- a/ipc/ipdl/test/cxx/TestHighestPrio.cpp
+++ b/ipc/ipdl/test/cxx/TestHighestPrio.cpp
@@ -8,7 +8,7 @@
 #include "TestHighestPrio.h"
 
 #include "IPDLUnitTests.h"  // fail etc.
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <unistd.h>
 #else
 #  include <windows.h>
diff --git a/ipc/ipdl/test/cxx/TestRPC.cpp b/ipc/ipdl/test/cxx/TestRPC.cpp
index 404de01aa852..cf7a1b6c364e 100644
--- a/ipc/ipdl/test/cxx/TestRPC.cpp
+++ b/ipc/ipdl/test/cxx/TestRPC.cpp
@@ -1,7 +1,7 @@
 #include "TestRPC.h"
 
 #include "IPDLUnitTests.h"  // fail etc.
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <unistd.h>
 #else
 #  include <windows.h>
diff --git a/ipc/ipdl/test/cxx/TestSyncWakeup.cpp b/ipc/ipdl/test/cxx/TestSyncWakeup.cpp
index 4c99aa1bab43..90875b702b6f 100644
--- a/ipc/ipdl/test/cxx/TestSyncWakeup.cpp
+++ b/ipc/ipdl/test/cxx/TestSyncWakeup.cpp
@@ -1,4 +1,4 @@
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <unistd.h>  // sleep()
 #endif
 
@@ -35,7 +35,7 @@ mozilla::ipc::IPCResult TestSyncWakeupParent::RecvSync1() {
     // XXX ugh ... need to ensure that the async message and sync
     // reply come in "far enough" apart that this test doesn't pass on
     // accident
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
   // NB: can't use PR_Sleep (i.e. Sleep() on windows) because it's
   // only spec'd to block the current thread, not the current
   // process.  We need the IO thread to sleep as well.
@@ -49,7 +49,7 @@ mozilla::ipc::IPCResult TestSyncWakeupParent::RecvSync1() {
 mozilla::ipc::IPCResult TestSyncWakeupParent::RecvSync2() {
   if (!SendNote2()) fail("sending Note2()");
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
   // see above
   sleep(5);
   puts(" (sleeping for 5 seconds. sorry!)");
diff --git a/ipc/ipdl/test/cxx/TestUrgency.cpp b/ipc/ipdl/test/cxx/TestUrgency.cpp
index 5845f8210d35..840b9d71e939 100644
--- a/ipc/ipdl/test/cxx/TestUrgency.cpp
+++ b/ipc/ipdl/test/cxx/TestUrgency.cpp
@@ -1,7 +1,7 @@
 #include "TestUrgency.h"
 
 #include "IPDLUnitTests.h"  // fail etc.
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <unistd.h>
 #else
 #  include <windows.h>
@@ -10,7 +10,7 @@
 namespace mozilla {
 namespace _ipdltest {
 
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 static void Sleep(int ms) { sleep(ms / 1000); }
 #endif
 
diff --git a/ipc/ipdl/test/cxx/TestUrgentHangs.cpp b/ipc/ipdl/test/cxx/TestUrgentHangs.cpp
index e6139e38c5dd..96227dd79174 100644
--- a/ipc/ipdl/test/cxx/TestUrgentHangs.cpp
+++ b/ipc/ipdl/test/cxx/TestUrgentHangs.cpp
@@ -5,7 +5,7 @@
 
 #include "IPDLUnitTests.h"  // fail etc.
 #include "prthread.h"
-#if defined(OS_POSIX)
+#if defined(XP_UNIX)
 #  include <unistd.h>
 #else
 #  include <windows.h>