diff --git a/Makefile.in b/Makefile.in
index 5216fb2f7..ac14877b8 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -65,6 +65,8 @@ CHECK_SYMLINKS = testsuite/chown-fake_test.py testsuite/devices-fake_test.py \
 
 # Objects for CHECK_PROGS to clean
 CHECK_OBJS=tls.o testrun.o getgroups.o getfsdev.o t_stub.o t_unsafe.o t_chmod_secure.o t_secure_relpath.o trimslash.o wildtest.o
+# Compile-only feature-shape checks.
+CHECK_COMPILE_OBJS=syscall-no-at-fdcwd.o
 
 # note that the -I. is needed to handle config.h when using VPATH
 .c.o:
@@ -77,6 +79,10 @@ CHECK_OBJS=tls.o testrun.o getgroups.o getfsdev.o t_stub.o t_unsafe.o t_chmod_se
 all: Makefile rsync$(EXEEXT) stunnel-rsyncd.conf @MAKE_RRSYNC@ @MAKE_MAN@
 .PHONY: all
 
+syscall-no-at-fdcwd.o: syscall.c $(HEADERS)
+	$(CC) -I. -I$(srcdir) $(CFLAGS) $(CPPFLAGS) \
+	    -DRSYNC_TEST_NO_AT_FDCWD -c $(srcdir)/syscall.c -o $@
+
 .PHONY: install
 install: all
 	-$(MKDIR_P) $(DESTDIR)$(bindir)
@@ -300,7 +306,7 @@ rrsync.1: support/rrsync.1.md md-convert Makefile
 
 .PHONY: clean
 clean: cleantests
-	rm -f *~ $(OBJS) $(CHECK_PROGS) $(CHECK_OBJS) $(CHECK_SYMLINKS) @MAKE_RRSYNC@ \
+	rm -f *~ $(OBJS) $(CHECK_PROGS) $(CHECK_OBJS) $(CHECK_COMPILE_OBJS) $(CHECK_SYMLINKS) @MAKE_RRSYNC@ \
 		git-version.h rounding rounding.h *.old rsync*.1 rsync*.5 @MAKE_RRSYNC_1@ \
 		*.html daemon-parm.h help-*.h default-*.h proto.h proto.h-tstamp
 	rm -f *.gcno *.gcda lib/*.gcno lib/*.gcda zlib/*.gcno zlib/*.gcda popt/*.gcno popt/*.gcda
@@ -378,18 +384,18 @@ COVERAGE_EXCLUDE = -e '(^|/)zlib/' -e '(^|/)popt/' \
 # WITHOUT running it. Used by CI jobs that invoke runtests.py directly with
 # custom options (e.g. the version-mix workflow's --rsync-bin2/--expect-result).
 .PHONY: check-progs
-check-progs: all $(CHECK_PROGS) $(CHECK_SYMLINKS)
+check-progs: all $(CHECK_PROGS) $(CHECK_COMPILE_OBJS) $(CHECK_SYMLINKS)
 
 .PHONY: check
-check: all $(CHECK_PROGS) $(CHECK_SYMLINKS)
+check: all $(CHECK_PROGS) $(CHECK_COMPILE_OBJS) $(CHECK_SYMLINKS)
 	$(srcdir)/runtests.py --rsync-bin=`pwd`/rsync$(EXEEXT) -j $(CHECK_J)
 
 .PHONY: check29
-check29: all $(CHECK_PROGS) $(CHECK_SYMLINKS)
+check29: all $(CHECK_PROGS) $(CHECK_COMPILE_OBJS) $(CHECK_SYMLINKS)
 	$(srcdir)/runtests.py --rsync-bin=`pwd`/rsync$(EXEEXT) -j $(CHECK_J) --protocol=29
 
 .PHONY: check30
-check30: all $(CHECK_PROGS) $(CHECK_SYMLINKS)
+check30: all $(CHECK_PROGS) $(CHECK_COMPILE_OBJS) $(CHECK_SYMLINKS)
 	$(srcdir)/runtests.py --rsync-bin=`pwd`/rsync$(EXEEXT) -j $(CHECK_J) --protocol=30
 
 # Whole-suite gcov coverage report (HTML, with branch + decision coverage).
diff --git a/syscall.c b/syscall.c
index 243f7f243..c7f12d2c2 100644
--- a/syscall.c
+++ b/syscall.c
@@ -22,6 +22,16 @@
 
 #include "rsync.h"
 
+/* Exercise the pre-*at() portability tier on modern build hosts. */
+#ifdef RSYNC_TEST_NO_AT_FDCWD
+#undef AT_FDCWD
+#undef AT_SYMLINK_NOFOLLOW
+#undef HAVE_LINKAT
+#undef HAVE_OPENAT2
+#undef HAVE_UTIMENSAT
+#undef O_RESOLVE_BENEATH
+#endif
+
 #if !defined MKNOD_CREATES_SOCKETS && defined HAVE_SYS_UN_H
 #include <sys/un.h>
 #endif
@@ -426,7 +436,7 @@ int do_lchown(const char *path, uid_t owner, gid_t group)
 */
 int do_lchown_at(const char *fname, uid_t owner, gid_t group)
 {
-#ifdef AT_FDCWD
+#if defined AT_FDCWD && defined AT_SYMLINK_NOFOLLOW
 	extern int am_daemon, am_chrooted;
 	char dirpath[MAXPATHLEN];
 	const char *bname;
@@ -1181,6 +1191,7 @@ static int do_xstat_at(const char *path, STRUCT_STAT *st, int at_flags, int (*fa
 	errno = e;
 	return ret;
 #else
+	(void)at_flags;
 	return fallback(path, st);
 #endif
 }
@@ -1192,8 +1203,10 @@ int do_stat_at(const char *path, STRUCT_STAT *st)
 
 int do_lstat_at(const char *path, STRUCT_STAT *st)
 {
-#ifdef SUPPORT_LINKS
+#if defined SUPPORT_LINKS && defined AT_FDCWD && defined AT_SYMLINK_NOFOLLOW
 	return do_xstat_at(path, st, AT_SYMLINK_NOFOLLOW, do_lstat);
+#elif defined SUPPORT_LINKS
+	return do_lstat(path, st);
 #else
 	return do_xstat_at(path, st, 0, do_stat);
 #endif
@@ -2007,6 +2020,7 @@ int secure_relative_open(const char *basedir, const char *relpath, int flags, mo
 #endif // O_NOFOLLOW, O_DIRECTORY
 }
 
+#if defined O_NOFOLLOW && defined O_DIRECTORY && defined AT_FDCWD
 /* Fill buf with len random bytes.  Prefers /dev/urandom for cryptographic
  * quality; falls back to rand() if /dev/urandom cannot be opened or read
  * (e.g. inside a chroot or container without /dev populated). */
@@ -2027,6 +2041,7 @@ static void rand_bytes(unsigned char *buf, size_t len)
 		buf[i] = (unsigned char)rand();
 	}
 }
+#endif
 
 /*
   Secure version of mkstemp that prevents symlink attacks on parent directories.
diff --git a/wildtest.c b/wildtest.c
index df87f9240..5867bb1cc 100644
--- a/wildtest.c
+++ b/wildtest.c
@@ -23,6 +23,7 @@
 #include "lib/wildmatch.c"
 
 #include <popt.h>
+#include <stdbool.h>
 
 #ifdef COMPARE_WITH_FNMATCH
 #include <fnmatch.h>
@@ -32,10 +33,6 @@ int fnmatch_errors = 0;
 
 int wildmatch_errors = 0;
 
-#if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 202311L
-typedef char bool;
-#endif
-
 int output_iterations = 0;
 int explode_mod = 0;
 int empties_mod = 0;