diff --git a/src/libexpr-tests/main.cc b/src/libexpr-tests/main.cc
index 66afc2272..52cca53c4 100644
--- a/src/libexpr-tests/main.cc
+++ b/src/libexpr-tests/main.cc
@@ -14,7 +14,7 @@ int main (int argc, char **argv) {
     // Disable build hook. We won't be testing remote builds in these unit tests. If we do, fix the above build hook.
     settings.buildHook = {};
 
-    #if __linux__ // should match the conditional around sandboxBuildDir declaration.
+    #ifdef __linux__ // should match the conditional around sandboxBuildDir declaration.
 
     // When building and testing nix within the host's Nix sandbox, our store dir will be located in the host's sandboxBuildDir, e.g.:
     // Host
diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index b82fbb09c..6de458823 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -13,7 +13,7 @@
 #include <aws/core/client/ClientConfiguration.h>
 #endif
 
-#if __linux__
+#ifdef __linux__
 # include "nix/util/namespaces.hh"
 #endif
 
@@ -622,7 +622,7 @@ struct curlFileTransfer : public FileTransfer
         });
         #endif
 
-        #if __linux__
+        #ifdef __linux__
         try {
             tryUnshareFilesystem();
         } catch (nix::Error & e) {
diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index 5e56be7f3..db91e5213 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -335,7 +335,7 @@ static std::string quoteRegexChars(const std::string & raw)
     return std::regex_replace(raw, specialRegex, R"(\$&)");
 }
 
-#if __linux__
+#ifdef __linux__
 static void readFileRoots(const std::filesystem::path & path, UncheckedRoots & roots)
 {
     try {
@@ -427,7 +427,7 @@ void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
     }
 #endif
 
-#if __linux__
+#ifdef __linux__
     readFileRoots("/proc/sys/kernel/modprobe", unchecked);
     readFileRoots("/proc/sys/kernel/fbsplash", unchecked);
     readFileRoots("/proc/sys/kernel/poweroff_cmd", unchecked);
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 6b93e34bb..1df0a846e 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -181,11 +181,11 @@ StringSet Settings::getDefaultSystemFeatures()
        actually require anything special on the machines. */
     StringSet features{"nixos-test", "benchmark", "big-parallel"};
 
-    #if __linux__
+    #ifdef __linux__
     features.insert("uid-range");
     #endif
 
-    #if __linux__
+    #ifdef __linux__
     if (access("/dev/kvm", R_OK | W_OK) == 0)
         features.insert("kvm");
     #endif
@@ -205,7 +205,7 @@ StringSet Settings::getDefaultExtraPlatforms()
     if (std::string{NIX_LOCAL_SYSTEM} == "x86_64-linux" && !isWSL1())
         extraPlatforms.insert("i686-linux");
 
-#if __linux__
+#ifdef __linux__
     StringSet levels = computeLevels();
     for (auto iter = levels.begin(); iter != levels.end(); ++iter)
         extraPlatforms.insert(*iter + "-linux");
@@ -224,7 +224,7 @@ StringSet Settings::getDefaultExtraPlatforms()
 
 bool Settings::isWSL1()
 {
-#if __linux__
+#ifdef __linux__
     struct utsname utsbuf;
     uname(&utsbuf);
     // WSL1 uses -Microsoft suffix
diff --git a/src/libstore/include/nix/store/globals.hh b/src/libstore/include/nix/store/globals.hh
index 38757bcd4..82211d8dc 100644
--- a/src/libstore/include/nix/store/globals.hh
+++ b/src/libstore/include/nix/store/globals.hh
@@ -34,7 +34,7 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
 };
 
 const uint32_t maxIdsPerBuild =
-    #if __linux__
+    #ifdef __linux__
     1 << 16
     #else
     1
@@ -467,7 +467,7 @@ public:
         )", {}, true, Xp::AutoAllocateUids};
 
     Setting<uint32_t> startId{this,
-        #if __linux__
+        #ifdef __linux__
         0x34000000,
         #else
         56930,
@@ -476,7 +476,7 @@ public:
         "The first UID and GID to use for dynamic ID allocation."};
 
     Setting<uint32_t> uidCount{this,
-        #if __linux__
+        #ifdef __linux__
         maxIdsPerBuild * 128,
         #else
         128,
@@ -484,7 +484,7 @@ public:
         "id-count",
         "The number of UIDs/GIDs to use for dynamic ID allocation."};
 
-    #if __linux__
+    #ifdef __linux__
     Setting<bool> useCgroups{
         this, false, "use-cgroups",
         R"(
@@ -596,7 +596,7 @@ public:
 
     Setting<SandboxMode> sandboxMode{
         this,
-        #if __linux__
+        #ifdef __linux__
           smEnabled
         #else
           smDisabled
@@ -671,7 +671,7 @@ public:
         )"};
 #endif
 
-#if __linux__
+#ifdef __linux__
     Setting<std::string> sandboxShmSize{
         this, "50%", "sandbox-dev-shm-size",
         R"(
@@ -1066,7 +1066,7 @@ public:
         // Don't document the machine-specific default value
         false};
 
-#if __linux__
+#ifdef __linux__
     Setting<bool> filterSyscalls{
         this, true, "filter-syscalls",
         R"(
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index b26fdeed8..fff0b35bf 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -38,7 +38,7 @@
 # include <grp.h>
 #endif
 
-#if __linux__
+#ifdef __linux__
 # include <sched.h>
 # include <sys/statvfs.h>
 # include <sys/mount.h>
@@ -571,7 +571,7 @@ void LocalStore::upgradeDBSchema(State & state)
    bind mount.  So make the Nix store writable for this process. */
 void LocalStore::makeStoreWritable()
 {
-#if __linux__
+#ifdef __linux__
     if (!isRootUser()) return;
     /* Check if /nix/store is on a read-only mount. */
     struct statvfs stat;
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 6721a6f2d..9e606d0ab 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -1328,7 +1328,7 @@ ref<Store> openStore(StoreReference && storeURI)
                 return std::make_shared<LocalStore>(params);
             else if (pathExists(settings.nixDaemonSocketFile))
                 return std::make_shared<UDSRemoteStore>(params);
-            #if __linux__
+            #ifdef __linux__
             else if (!pathExists(stateDir)
                 && params.empty()
                 && !isRootUser()
diff --git a/src/libstore/unix/build/local-derivation-goal.cc b/src/libstore/unix/build/local-derivation-goal.cc
index 915b8b1d8..185c98bb6 100644
--- a/src/libstore/unix/build/local-derivation-goal.cc
+++ b/src/libstore/unix/build/local-derivation-goal.cc
@@ -42,7 +42,7 @@
 #endif
 
 /* Includes required for chroot support. */
-#if __linux__
+#ifdef __linux__
 # include "nix/store/fchmodat2-compat.hh"
 # include <sys/ioctl.h>
 # include <net/if.h>
@@ -418,7 +418,7 @@ LocalDerivationGoal::~LocalDerivationGoal()
 
 inline bool LocalDerivationGoal::needsHashRewrite()
 {
-#if __linux__
+#ifdef __linux__
     return !useChroot;
 #else
     /* Darwin requires hash rewriting even when sandboxing is enabled. */
@@ -459,7 +459,7 @@ void LocalDerivationGoal::killChild()
 void LocalDerivationGoal::killSandbox(bool getStats)
 {
     if (cgroup) {
-        #if __linux__
+        #ifdef __linux__
         auto stats = destroyCgroup(*cgroup);
         if (getStats) {
             buildResult.cpuUser = stats.cpuUser;
@@ -513,14 +513,14 @@ Goal::Co LocalDerivationGoal::tryLocalBuild()
 
     auto & localStore = getLocalStore();
     if (localStore.storeDir != localStore.realStoreDir.get()) {
-        #if __linux__
+        #ifdef __linux__
             useChroot = true;
         #else
             throw Error("building using a diverted store is not supported on this platform");
         #endif
     }
 
-    #if __linux__
+    #ifdef __linux__
     if (useChroot) {
         if (!mountAndPidNamespacesSupported()) {
             if (!settings.sandboxFallback)
@@ -747,7 +747,7 @@ bool LocalDerivationGoal::cleanupDecideWhetherDiskFull()
 }
 
 
-#if __linux__
+#ifdef __linux__
 static void doBind(const Path & source, const Path & target, bool optional = false) {
     debug("bind mounting '%1%' to '%2%'", source, target);
 
@@ -820,12 +820,12 @@ static void handleChildException(bool sendException)
 void LocalDerivationGoal::startBuilder()
 {
     if ((buildUser && buildUser->getUIDCount() != 1)
-        #if __linux__
+        #ifdef __linux__
         || settings.useCgroups
         #endif
         )
     {
-        #if __linux__
+        #ifdef __linux__
         experimentalFeatureSettings.require(Xp::Cgroups);
 
         /* If we're running from the daemon, then this will return the
@@ -1071,7 +1071,7 @@ void LocalDerivationGoal::startBuilder()
             pathsInChroot[i] = {i, true};
         }
 
-#if __linux__
+#ifdef __linux__
         /* Create a temporary directory in which we set up the chroot
            environment using bind-mounts.  We put it in the Nix store
            so that the build outputs can be moved efficiently from the
@@ -1285,7 +1285,7 @@ void LocalDerivationGoal::startBuilder()
 
     /* Fork a child to build the package. */
 
-#if __linux__
+#ifdef __linux__
     if (useChroot) {
         /* Set up private namespaces for the build:
 
@@ -1482,7 +1482,7 @@ void LocalDerivationGoal::initTmpDir()
 {
     /* In a sandbox, for determinism, always use the same temporary
        directory. */
-#if __linux__
+#ifdef __linux__
     tmpDirInSandbox = useChroot ? settings.sandboxBuildDir : tmpDir;
 #else
     tmpDirInSandbox = tmpDir;
@@ -1728,7 +1728,7 @@ void LocalDerivationGoal::addDependency(const StorePath & path)
 
         debug("materialising '%s' in the sandbox", worker.store.printStorePath(path));
 
-        #if __linux__
+        #ifdef __linux__
 
             Path source = worker.store.Store::toRealPath(path);
             Path target = chrootRootDir + worker.store.printStorePath(path);
@@ -1778,7 +1778,7 @@ void LocalDerivationGoal::chownToBuilder(const Path & path)
 
 void setupSeccomp()
 {
-#if __linux__
+#ifdef __linux__
     if (!settings.filterSyscalls) return;
 #if HAVE_SECCOMP
     scmp_filter_ctx ctx;
@@ -1898,7 +1898,7 @@ void LocalDerivationGoal::runChild()
            } catch (SystemError &) { }
         }
 
-#if __linux__
+#ifdef __linux__
         if (useChroot) {
 
             userNamespaceSync.writeSide = -1;
@@ -2132,7 +2132,7 @@ void LocalDerivationGoal::runChild()
         /* Close all other file descriptors. */
         unix::closeExtraFDs();
 
-#if __linux__
+#ifdef __linux__
         linux::setPersonality(drv->platform);
 #endif
 
diff --git a/src/libstore/unix/user-lock.cc b/src/libstore/unix/user-lock.cc
index 770b00e2d..eb0bac887 100644
--- a/src/libstore/unix/user-lock.cc
+++ b/src/libstore/unix/user-lock.cc
@@ -10,7 +10,7 @@
 
 namespace nix {
 
-#if __linux__
+#ifdef __linux__
 
 static std::vector<gid_t> get_group_list(const char *username, gid_t group_id)
 {
@@ -94,7 +94,7 @@ struct SimpleUserLock : UserLock
                 if (lock->uid == getuid() || lock->uid == geteuid())
                     throw Error("the Nix user should not be a member of '%s'", settings.buildUsersGroup);
 
-                #if __linux__
+                #ifdef __linux__
                 /* Get the list of supplementary groups of this user. This is
                  * usually either empty or contains a group such as "kvm". */
 
@@ -193,7 +193,7 @@ std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useUserNamespace)
 
 bool useBuildUsers()
 {
-    #if __linux__
+    #ifdef __linux__
     static bool b = (settings.buildUsersGroup != "" || settings.autoAllocateUids) && isRootUser();
     return b;
     #elif __APPLE__
diff --git a/src/libutil/current-process.cc b/src/libutil/current-process.cc
index 4103c0515..8aef47146 100644
--- a/src/libutil/current-process.cc
+++ b/src/libutil/current-process.cc
@@ -13,7 +13,7 @@
 # include <mach-o/dyld.h>
 #endif
 
-#if __linux__
+#ifdef __linux__
 # include <mutex>
 # include "nix/util/cgroup.hh"
 # include "nix/util/namespaces.hh"
@@ -23,7 +23,7 @@ namespace nix {
 
 unsigned int getMaxCPU()
 {
-    #if __linux__
+    #ifdef __linux__
     try {
         auto cgroupFS = getCgroupFS();
         if (!cgroupFS) return 0;
@@ -82,7 +82,7 @@ void restoreProcessContext(bool restoreMounts)
     unix::restoreSignals();
     #endif
     if (restoreMounts) {
-        #if __linux__
+        #ifdef __linux__
         restoreMountNamespace();
         #endif
     }
@@ -106,7 +106,7 @@ std::optional<Path> getSelfExe()
 {
     static auto cached = []() -> std::optional<Path>
     {
-        #if __linux__ || __GNU__
+        #if defined(__linux__) || defined(__GNU__)
         return readLink("/proc/self/exe");
         #elif __APPLE__
         char buf[1024];
diff --git a/src/libutil/file-descriptor.cc b/src/libutil/file-descriptor.cc
index 4fc0f06cd..9e0827442 100644
--- a/src/libutil/file-descriptor.cc
+++ b/src/libutil/file-descriptor.cc
@@ -113,7 +113,7 @@ void AutoCloseFD::fsync() const
 
 void AutoCloseFD::startFsync() const
 {
-#if __linux__
+#ifdef __linux__
         if (fd != -1) {
             /* Ignore failure, since fsync must be run later anyway. This is just a performance optimization. */
             ::sync_file_range(fd, 0, 0, SYNC_FILE_RANGE_WRITE);
diff --git a/src/libutil/unix/file-descriptor.cc b/src/libutil/unix/file-descriptor.cc
index 6ce307252..73ee49982 100644
--- a/src/libutil/unix/file-descriptor.cc
+++ b/src/libutil/unix/file-descriptor.cc
@@ -163,7 +163,7 @@ void Pipe::create()
 
 //////////////////////////////////////////////////////////////////////
 
-#if __linux__ || __FreeBSD__
+#if defined(__linux__) || defined(__FreeBSD__)
 static int unix_close_range(unsigned int first, unsigned int last, int flags)
 {
 #if !HAVE_CLOSE_RANGE
@@ -179,7 +179,7 @@ void unix::closeExtraFDs()
     constexpr int MAX_KEPT_FD = 2;
     static_assert(std::max({STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO}) == MAX_KEPT_FD);
 
-#if __linux__ || __FreeBSD__
+#if defined(__linux__) || defined(__FreeBSD__)
     // first try to close_range everything we don't care about. if this
     // returns an error with these parameters we're running on a kernel
     // that does not implement close_range (i.e. pre 5.9) and fall back
@@ -189,7 +189,7 @@ void unix::closeExtraFDs()
     }
 #endif
 
-#if __linux__
+#ifdef __linux__
     try {
         for (auto & s : std::filesystem::directory_iterator{"/proc/self/fd"}) {
             checkInterrupt();
diff --git a/src/libutil/unix/processes.cc b/src/libutil/unix/processes.cc
index 4df0a7777..198243c20 100644
--- a/src/libutil/unix/processes.cc
+++ b/src/libutil/unix/processes.cc
@@ -190,7 +190,7 @@ static pid_t doFork(bool allowVfork, ChildWrapperFunction & fun)
 }
 
 
-#if __linux__
+#ifdef __linux__
 static int childEntry(void * arg)
 {
     auto & fun = *reinterpret_cast<ChildWrapperFunction*>(arg);
@@ -213,7 +213,7 @@ pid_t startProcess(std::function<void()> fun, const ProcessOptions & options)
             logger = makeSimpleLogger();
         }
         try {
-#if __linux__
+#ifdef __linux__
             if (options.dieWithParent && prctl(PR_SET_PDEATHSIG, SIGKILL) == -1)
                 throw SysError("setting death signal");
 #endif
diff --git a/src/libutil/unix/signals.cc b/src/libutil/unix/signals.cc
index f1cb28527..665b9b096 100644
--- a/src/libutil/unix/signals.cc
+++ b/src/libutil/unix/signals.cc
@@ -105,7 +105,7 @@ void unix::setChildSignalMask(sigset_t * sigs)
 {
     assert(sigs); // C style function, but think of sigs as a reference
 
-#if _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
+#if (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 1) || (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE) || (defined(_POSIX_SOURCE) && _POSIX_SOURCE)
     sigemptyset(&savedSignalMask);
     // There's no "assign" or "copy" function, so we rely on (math) idempotence
     // of the or operator: a or a = a.
diff --git a/src/nix/main.cc b/src/nix/main.cc
index 6470213a2..a2c9dcf68 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -36,7 +36,7 @@
 # include <netinet/in.h>
 #endif
 
-#if __linux__
+#ifdef __linux__
 # include "nix/util/namespaces.hh"
 #endif
 
@@ -384,7 +384,7 @@ void mainWrapped(int argc, char * * argv)
         "__build-remote",
     });
 
-    #if __linux__
+    #ifdef __linux__
     if (isRootUser()) {
         try {
             saveMountNamespace();
diff --git a/src/nix/run.cc b/src/nix/run.cc
index 64eab3ff3..146ae9ec9 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -12,7 +12,7 @@
 #include "nix/expr/eval.hh"
 #include <filesystem>
 
-#if __linux__
+#ifdef __linux__
 # include <sys/mount.h>
 # include "nix/store/personality.hh"
 #endif
@@ -59,7 +59,7 @@ void execProgramInStore(ref<Store> store,
         throw SysError("could not execute chroot helper");
     }
 
-#if __linux__
+#ifdef __linux__
     if (system)
         linux::setPersonality(*system);
 #endif
@@ -153,7 +153,7 @@ void chrootHelper(int argc, char * * argv)
     while (p < argc)
         args.push_back(argv[p++]);
 
-#if __linux__
+#ifdef __linux__
     uid_t uid = getuid();
     uid_t gid = getgid();
 
@@ -212,7 +212,7 @@ void chrootHelper(int argc, char * * argv)
     writeFile(fs::path{"/proc/self/uid_map"}, fmt("%d %d %d", uid, uid, 1));
     writeFile(fs::path{"/proc/self/gid_map"}, fmt("%d %d %d", gid, gid, 1));
 
-#if __linux__
+#ifdef __linux__
     if (system != "")
         linux::setPersonality(system);
 #endif
diff --git a/src/nix/unix/daemon.cc b/src/nix/unix/daemon.cc
index 1acf2bd5b..607a7bb01 100644
--- a/src/nix/unix/daemon.cc
+++ b/src/nix/unix/daemon.cc
@@ -34,7 +34,7 @@
 #include <grp.h>
 #include <fcntl.h>
 
-#if __linux__
+#ifdef __linux__
 #include "nix/util/cgroup.hh"
 #endif
 
@@ -317,7 +317,7 @@ static void daemonLoop(std::optional<TrustedFlag> forceTrustClientOpt)
     //  Get rid of children automatically; don't let them become zombies.
     setSigChldAction(true);
 
-    #if __linux__
+    #ifdef __linux__
     if (settings.useCgroups) {
         experimentalFeatureSettings.require(Xp::Cgroups);