diff --git a/.github/workflows/cross-bootstrap-tools.yml b/.github/workflows/cross-bootstrap-tools.yml
index 8f0dd933ef09fb..929811db2c4e2d 100644
--- a/.github/workflows/cross-bootstrap-tools.yml
+++ b/.github/workflows/cross-bootstrap-tools.yml
@@ -26,13 +26,13 @@ jobs:
             cross-bindir: /usr/lib/llvm-14/bin
             pkgs: bmake libarchive-dev clang-14 lld-14
           - os: ubuntu-24.04
-            compiler: clang-18
-            cross-bindir: /usr/lib/llvm-18/bin
-            pkgs: bmake libarchive-dev clang-18 lld-18
+            compiler: clang-20
+            cross-bindir: /usr/lib/llvm-20/bin
+            pkgs: bmake libarchive-dev clang-20 lld-20
           - os: macos-latest
-            compiler: clang-18
-            cross-toolchain: llvm@18
-            pkgs: bmake libarchive llvm@18
+            compiler: clang-21
+            cross-toolchain: llvm@21
+            pkgs: bmake libarchive llvm@21 lld@21
           - target_arch: amd64
             target: amd64
           - target_arch: aarch64
diff --git a/bin/sh/options.h b/bin/sh/options.h
index a059e19707b197..2ce2e6ebedf277 100644
--- a/bin/sh/options.h
+++ b/bin/sh/options.h
@@ -74,7 +74,7 @@ extern char optval[NOPTS];
 extern const char optletter[NSHORTOPTS];
 #ifdef DEFINE_OPTIONS
 char optval[NOPTS];
-const char optletter[NSHORTOPTS] = "efIimnsxvVECabupTPh";
+const char optletter[NSHORTOPTS] __nonstring = "efIimnsxvVECabupTPh";
 static const unsigned char optname[] =
 	"\007errexit"
 	"\006noglob"
diff --git a/bin/test/test.c b/bin/test/test.c
index a35c5b424d3f98..5ba5df5d7e443d 100644
--- a/bin/test/test.c
+++ b/bin/test/test.c
@@ -116,7 +116,7 @@ enum token {
 #define TOKEN_TYPE(token) ((token) & 0xff00)
 
 static const struct t_op {
-	char op_text[2];
+	char op_text[2] __nonstring;
 	short op_num;
 } ops1[] = {
 	{"=",	STREQ},
diff --git a/contrib/bmake/var.c b/contrib/bmake/var.c
index 66b18f7693710d..6ef525801ada7c 100644
--- a/contrib/bmake/var.c
+++ b/contrib/bmake/var.c
@@ -1866,7 +1866,13 @@ QuoteShell(const char *str, bool quoteDollar, LazyBuf *buf)
 static char *
 Hash(const char *str)
 {
-	static const char hexdigits[16] = "0123456789abcdef";
+	/*
+	 * Temporary FreeBSD diff: Disable -Wunterminated-string-initialization
+	 * warning by adding 1 to size. We can't use __nonstring unconditionally
+	 * in bmake since the bmake bootstrap headers don't define it.
+	 * TODO: add the __nonstring attribute and change size back to 16.
+	 */
+	static const char hexdigits[16 + 1] = "0123456789abcdef";
 	const unsigned char *ustr = (const unsigned char *)str;
 
 	uint32_t h = 0x971e137bU;
diff --git a/contrib/ncurses/progs/infocmp.c b/contrib/ncurses/progs/infocmp.c
index 7932203ac2b1aa..f61633cfad8047 100644
--- a/contrib/ncurses/progs/infocmp.c
+++ b/contrib/ncurses/progs/infocmp.c
@@ -687,7 +687,7 @@ compare_predicate(PredType type, PredIdx idx, const char *name)
 #define DATAX()        DATA("", "")
 
 typedef struct {
-    const char from[4];
+    const char from[8];
     const char to[12];
 } assoc;
 
diff --git a/krb5/include/autoconf.h b/krb5/include/autoconf.h
index ed0bf8cacc1438..76a0d548244c69 100644
--- a/krb5/include/autoconf.h
+++ b/krb5/include/autoconf.h
@@ -691,7 +691,18 @@
 #define STDC_HEADERS 1
 
 /* Define to 1 if strerror_r returns char *. */
+/*
+ * glibc defines two versions of strerror_r, the char* returning one is used
+ * when _GNU_SOURCE is defined.
+ */
+#if defined(__linux__) && __has_include(<features.h>)
+#include <features.h>  /* get __GLIBC__ */
+#endif
+#if defined(__GLIBC__) && defined(__USE_GNU)
+#define STRERROR_R_CHAR_P 1
+#else
 /* #undef STRERROR_R_CHAR_P */
+#endif
 
 /* Define if sys_errlist is defined in errno.h */
 #define SYS_ERRLIST_DECLARED 1
diff --git a/lib/libcuse/cuse_lib.c b/lib/libcuse/cuse_lib.c
index 3f040d0eeeda75..ba66c0a28f8929 100644
--- a/lib/libcuse/cuse_lib.c
+++ b/lib/libcuse/cuse_lib.c
@@ -92,7 +92,7 @@ static struct cuse_vm_allocation a_cuse[CUSE_ALLOC_UNIT_MAX]
 #define	CUSE_UNLOCK() \
 	pthread_mutex_unlock(&m_cuse)
 
-int
+int __no_lock_analysis /* No need for locking during init */
 cuse_init(void)
 {
 	pthread_mutexattr_t attr;
diff --git a/lib/libfetch/common.c b/lib/libfetch/common.c
index 786d5647d99350..2e1364edff04de 100644
--- a/lib/libfetch/common.c
+++ b/lib/libfetch/common.c
@@ -135,7 +135,7 @@ static struct fetcherr socks5_errlist[] = {
 };
 
 /* End-of-Line */
-static const char ENDL[2] = "\r\n";
+static const char ENDL[2] = { '\r', '\n' };
 
 
 /*** Error-reporting functions ***********************************************/
diff --git a/lib/libsysdecode/utrace.c b/lib/libsysdecode/utrace.c
index 67ba86bfbf7b92..8932f4b7e58fd0 100644
--- a/lib/libsysdecode/utrace.c
+++ b/lib/libsysdecode/utrace.c
@@ -169,7 +169,8 @@ sysdecode_utrace(FILE *fp, void *p, size_t len)
 	struct utrace_malloc um;
 	struct utrace_malloc32 *pm;
 #endif
-	static const char rtld_utrace_sig[RTLD_UTRACE_SIG_SZ] = RTLD_UTRACE_SIG;
+	static const char rtld_utrace_sig[RTLD_UTRACE_SIG_SZ] __nonstring =
+	    RTLD_UTRACE_SIG;
 
 	if (len == sizeof(struct utrace_rtld) && bcmp(p, rtld_utrace_sig,
 	    sizeof(rtld_utrace_sig)) == 0)
diff --git a/lib/libthr/thread/thr_printf.c b/lib/libthr/thread/thr_printf.c
index e1edffe5acb2d4..b7cb6ce81bda13 100644
--- a/lib/libthr/thread/thr_printf.c
+++ b/lib/libthr/thread/thr_printf.c
@@ -61,7 +61,7 @@ _thread_printf(int fd, const char *fmt, ...)
 void
 _thread_vprintf(int fd, const char *fmt, va_list ap)
 {
-	static const char digits[16] = "0123456789abcdef";
+	static const char digits[16] __nonstring = "0123456789abcdef";
 	char buf[20];
 	char *s;
 	unsigned long r, u;
@@ -147,4 +147,3 @@ pstr(int fd, const char *s)
 
 	__sys_write(fd, s, strlen(s));
 }
-
diff --git a/libexec/rtld-elf/rtld.c b/libexec/rtld-elf/rtld.c
index d27af520c21d1d..226835d4faa685 100644
--- a/libexec/rtld-elf/rtld.c
+++ b/libexec/rtld-elf/rtld.c
@@ -348,7 +348,8 @@ ld_utrace_log(int event, void *handle, void *mapbase, size_t mapsize,
     int refcnt, const char *name)
 {
 	struct utrace_rtld ut;
-	static const char rtld_utrace_sig[RTLD_UTRACE_SIG_SZ] = RTLD_UTRACE_SIG;
+	static const char rtld_utrace_sig[RTLD_UTRACE_SIG_SZ] __nonstring =
+	    RTLD_UTRACE_SIG;
 
 	memset(&ut, 0, sizeof(ut));	/* clear holes */
 	memcpy(ut.sig, rtld_utrace_sig, sizeof(ut.sig));
diff --git a/libexec/rtld-elf/rtld_utrace.h b/libexec/rtld-elf/rtld_utrace.h
index c14a62ddcb0ba6..dee807cc00980c 100644
--- a/libexec/rtld-elf/rtld_utrace.h
+++ b/libexec/rtld-elf/rtld_utrace.h
@@ -49,7 +49,7 @@
 #define	RTLD_UTRACE_SIG			"RTLD"
 
 struct utrace_rtld {
-	char sig[RTLD_UTRACE_SIG_SZ];
+	char sig[RTLD_UTRACE_SIG_SZ] __nonstring;
 	int event;
 	void *handle;
 	void *mapbase;			/* Used for 'parent' and 'init/fini' */
diff --git a/sbin/fsck_msdosfs/dir.c b/sbin/fsck_msdosfs/dir.c
index 19516d882e2a13..2d7e4bcdc38ffa 100644
--- a/sbin/fsck_msdosfs/dir.c
+++ b/sbin/fsck_msdosfs/dir.c
@@ -463,8 +463,8 @@ checksize(struct fat_descriptor *fat, u_char *p, struct dosDirEntry *dir)
 	return FSOK;
 }
 
-static const u_char dot_name[11]    = ".          ";
-static const u_char dotdot_name[11] = "..         ";
+static const u_char dot_name[11] __nonstring    = ".          ";
+static const u_char dotdot_name[11] __nonstring = "..         ";
 
 /*
  * Basic sanity check if the subdirectory have good '.' and '..' entries,
diff --git a/share/mk/bsd.sys.mk b/share/mk/bsd.sys.mk
index 2134886abcf501..09dbbe645489fe 100644
--- a/share/mk/bsd.sys.mk
+++ b/share/mk/bsd.sys.mk
@@ -93,6 +93,10 @@ CWARNFLAGS.clang+=	-Wno-error=unused-but-set-parameter
 # Similar to gcc >= 8.1 -Wno-error=cast-function-type below
 CWARNFLAGS.clang+=	-Wno-error=cast-function-type-mismatch
 .endif
+.if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 210000
+# Until gtest is updated: https://github.com/google/googletest/issues/4762
+CXXWARNFLAGS.clang+=	-Wno-error=character-conversion
+.endif
 .endif # WARNS <= 6
 .if ${WARNS} <= 3
 CWARNFLAGS.clang+=	-Wno-tautological-compare -Wno-unused-value\
diff --git a/sys/compat/linux/linux_timer.c b/sys/compat/linux/linux_timer.c
index ed9133359302ce..e9eb9cc99f9ab5 100644
--- a/sys/compat/linux/linux_timer.c
+++ b/sys/compat/linux/linux_timer.c
@@ -131,7 +131,7 @@ linux_timer_settime(struct thread *td, struct linux_timer_settime_args *uap)
 		return (error);
 	error = kern_ktimer_settime(td, uap->timerid, flags, &val, ovalp);
 	if (error == 0 && uap->old != NULL) {
-		error = native_to_linux_itimerspec(&l_val, &val);
+		error = native_to_linux_itimerspec(&l_oval, &val);
 		if (error == 0)
 			error = copyout(&l_oval, uap->old, sizeof(l_oval));
 	}
@@ -158,7 +158,7 @@ linux_timer_settime64(struct thread *td, struct linux_timer_settime64_args *uap)
 		return (error);
 	error = kern_ktimer_settime(td, uap->timerid, flags, &val, ovalp);
 	if (error == 0 && uap->old != NULL) {
-		error = native_to_linux_itimerspec64(&l_val, &val);
+		error = native_to_linux_itimerspec64(&l_oval, &val);
 		if (error == 0)
 			error = copyout(&l_oval, uap->old, sizeof(l_oval));
 	}
diff --git a/sys/contrib/openzfs/include/os/freebsd/spl/sys/cmn_err.h b/sys/contrib/openzfs/include/os/freebsd/spl/sys/cmn_err.h
index 93dd1196d977ca..fa55b391766915 100644
--- a/sys/contrib/openzfs/include/os/freebsd/spl/sys/cmn_err.h
+++ b/sys/contrib/openzfs/include/os/freebsd/spl/sys/cmn_err.h
@@ -52,28 +52,28 @@ extern "C" {
 #ifndef _ASM
 
 extern void cmn_err(int, const char *, ...)
-    __attribute__((format(printf, 2, 3)));
+    __attribute__((format(__printf__, 2, 3)));
 
 extern void vzcmn_err(zoneid_t, int, const char *, __va_list)
-    __attribute__((format(printf, 3, 0)));
+    __attribute__((format(__printf__, 3, 0)));
 
 extern void vcmn_err(int, const char *, __va_list)
-    __attribute__((format(printf, 2, 0)));
+    __attribute__((format(__printf__, 2, 0)));
 
 extern void zcmn_err(zoneid_t, int, const char *, ...)
-    __attribute__((format(printf, 3, 4)));
+    __attribute__((format(__printf__, 3, 4)));
 
 extern void vzprintf(zoneid_t, const char *, __va_list)
-    __attribute__((format(printf, 2, 0)));
+    __attribute__((format(__printf__, 2, 0)));
 
 extern void zprintf(zoneid_t, const char *, ...)
-    __attribute__((format(printf, 2, 3)));
+    __attribute__((format(__printf__, 2, 3)));
 
 extern void vuprintf(const char *, __va_list)
-    __attribute__((format(printf, 1, 0)));
+    __attribute__((format(__printf__, 1, 0)));
 
 extern void panic(const char *, ...)
-    __attribute__((format(printf, 1, 2), __noreturn__));
+    __attribute__((format(__printf__, 1, 2), __noreturn__));
 
 #define	cmn_err_once(ce, ...)				\
 do {							\
diff --git a/sys/contrib/openzfs/include/os/freebsd/spl/sys/kmem.h b/sys/contrib/openzfs/include/os/freebsd/spl/sys/kmem.h
index a9619d7d021900..4ae96c91a5f3e6 100644
--- a/sys/contrib/openzfs/include/os/freebsd/spl/sys/kmem.h
+++ b/sys/contrib/openzfs/include/os/freebsd/spl/sys/kmem.h
@@ -55,9 +55,9 @@ MALLOC_DECLARE(M_SOLARIS);
 typedef struct vmem vmem_t;
 
 extern char	*kmem_asprintf(const char *, ...)
-    __attribute__((format(printf, 1, 2)));
+    __attribute__((format(__printf__, 1, 2)));
 extern char *kmem_vasprintf(const char *fmt, va_list ap)
-    __attribute__((format(printf, 1, 0)));
+    __attribute__((format(__printf__, 1, 0)));
 
 extern int kmem_scnprintf(char *restrict str, size_t size,
     const char *restrict fmt, ...);
diff --git a/sys/contrib/openzfs/include/sys/spa.h b/sys/contrib/openzfs/include/sys/spa.h
index 66db16b33c51de..22f6ddc1a9d53b 100644
--- a/sys/contrib/openzfs/include/sys/spa.h
+++ b/sys/contrib/openzfs/include/sys/spa.h
@@ -1122,9 +1122,9 @@ extern void spa_set_allocator(spa_t *spa, const char *allocator);
 
 /* Miscellaneous support routines */
 extern void spa_load_failed(spa_t *spa, const char *fmt, ...)
-    __attribute__((format(printf, 2, 3)));
+    __attribute__((format(__printf__, 2, 3)));
 extern void spa_load_note(spa_t *spa, const char *fmt, ...)
-    __attribute__((format(printf, 2, 3)));
+    __attribute__((format(__printf__, 2, 3)));
 extern void spa_activate_mos_feature(spa_t *spa, const char *feature,
     dmu_tx_t *tx);
 extern void spa_deactivate_mos_feature(spa_t *spa, const char *feature);
diff --git a/sys/contrib/openzfs/include/sys/vdev.h b/sys/contrib/openzfs/include/sys/vdev.h
index 510474d6c085e0..86f2235f03aca8 100644
--- a/sys/contrib/openzfs/include/sys/vdev.h
+++ b/sys/contrib/openzfs/include/sys/vdev.h
@@ -54,7 +54,7 @@ extern int zfs_nocacheflush;
 typedef boolean_t vdev_open_children_func_t(vdev_t *vd);
 
 extern void vdev_dbgmsg(vdev_t *vd, const char *fmt, ...)
-    __attribute__((format(printf, 2, 3)));
+    __attribute__((format(__printf__, 2, 3)));
 extern void vdev_dbgmsg_print_tree(vdev_t *, int);
 extern int vdev_open(vdev_t *);
 extern void vdev_open_children(vdev_t *);
diff --git a/sys/contrib/openzfs/include/sys/zfs_debug.h b/sys/contrib/openzfs/include/sys/zfs_debug.h
index 4d4cd4c39e9776..87fafec0c9e71f 100644
--- a/sys/contrib/openzfs/include/sys/zfs_debug.h
+++ b/sys/contrib/openzfs/include/sys/zfs_debug.h
@@ -67,7 +67,7 @@ extern int zfs_dbgmsg_enable;
 extern void __set_error(const char *file, const char *func, int line, int err);
 extern void __zfs_dbgmsg(char *buf);
 extern void __dprintf(boolean_t dprint, const char *file, const char *func,
-    int line, const char *fmt, ...)  __attribute__((format(printf, 5, 6)));
+    int line, const char *fmt, ...)  __attribute__((format(__printf__, 5, 6)));
 
 /*
  * Some general principles for using zfs_dbgmsg():
diff --git a/sys/crypto/chacha20/chacha.c b/sys/crypto/chacha20/chacha.c
index 52f7e18c651cee..b5f51bd10cc98e 100644
--- a/sys/crypto/chacha20/chacha.c
+++ b/sys/crypto/chacha20/chacha.c
@@ -50,8 +50,8 @@ typedef struct chacha_ctx chacha_ctx;
   a = PLUS(a,b); d = ROTATE(XOR(d,a), 8); \
   c = PLUS(c,d); b = ROTATE(XOR(b,c), 7);
 
-static const char sigma[16] = "expand 32-byte k";
-static const char tau[16] = "expand 16-byte k";
+static const char sigma[16] __nonstring = "expand 32-byte k";
+static const char tau[16] __nonstring = "expand 16-byte k";
 
 LOCAL void
 chacha_keysetup(chacha_ctx *x,const u8 *k,u32 kbits)
diff --git a/sys/dev/netmap/netmap_kloop.c b/sys/dev/netmap/netmap_kloop.c
index ba9e67076e72d1..ac3ca2d8d21f50 100644
--- a/sys/dev/netmap/netmap_kloop.c
+++ b/sys/dev/netmap/netmap_kloop.c
@@ -161,7 +161,8 @@ netmap_sync_kloop_tx_ring(const struct sync_kloop_ring_args *a)
 	struct netmap_kring *kring = a->kring;
 	struct nm_csb_atok *csb_atok = a->csb_atok;
 	struct nm_csb_ktoa *csb_ktoa = a->csb_ktoa;
-	struct netmap_ring shadow_ring; /* shadow copy of the netmap_ring */
+	/* shadow copy of the netmap_ring */
+	struct netmap_ring shadow_ring = {0};
 #ifdef SYNC_KLOOP_POLL
 	bool more_txspace = false;
 #endif /* SYNC_KLOOP_POLL */
@@ -317,7 +318,8 @@ netmap_sync_kloop_rx_ring(const struct sync_kloop_ring_args *a)
 	struct netmap_kring *kring = a->kring;
 	struct nm_csb_atok *csb_atok = a->csb_atok;
 	struct nm_csb_ktoa *csb_ktoa = a->csb_ktoa;
-	struct netmap_ring shadow_ring; /* shadow copy of the netmap_ring */
+	/* shadow copy of the netmap_ring */
+	struct netmap_ring shadow_ring = {0};
 	int dry_cycles = 0;
 #ifdef SYNC_KLOOP_POLL
 	bool some_recvd = false;
diff --git a/sys/dev/ocs_fc/ocs_ddump.h b/sys/dev/ocs_fc/ocs_ddump.h
index b7d7eefad8136c..3cffff6d9670c9 100644
--- a/sys/dev/ocs_fc/ocs_ddump.h
+++ b/sys/dev/ocs_fc/ocs_ddump.h
@@ -46,8 +46,7 @@ extern void ocs_ddump_startfile(ocs_textbuf_t *textbuf);
 extern void ocs_ddump_endfile(ocs_textbuf_t *textbuf);
 extern void ocs_ddump_section(ocs_textbuf_t *textbuf, const char *name, uint32_t instance);
 extern void ocs_ddump_endsection(ocs_textbuf_t *textbuf, const char *name, uint32_t instance);
-__attribute__((format(printf,3,4)))
-extern void ocs_ddump_value(ocs_textbuf_t *textbuf, const char *name, const char *fmt, ...);
+extern void ocs_ddump_value(ocs_textbuf_t *textbuf, const char *name, const char *fmt, ...) __printflike(3, 4);
 extern void ocs_ddump_buffer(ocs_textbuf_t *textbuf, const char *name, uint32_t instance, void *buffer, uint32_t size);
 extern int32_t ocs_save_ddump(ocs_t *ocs, uint32_t flags, uint32_t qentries);
 extern int32_t ocs_get_saved_ddump(ocs_t *ocs, ocs_textbuf_t *textbuf);
diff --git a/sys/dev/ocs_fc/ocs_mgmt.h b/sys/dev/ocs_fc/ocs_mgmt.h
index 7b59a3916ca85e..9781aec18462e7 100644
--- a/sys/dev/ocs_fc/ocs_mgmt.h
+++ b/sys/dev/ocs_fc/ocs_mgmt.h
@@ -95,8 +95,7 @@ extern void ocs_mgmt_end_section(ocs_textbuf_t *textbuf, const char *name, int i
 extern void ocs_mgmt_end_unnumbered_section(ocs_textbuf_t *textbuf, const char *name);
 extern void ocs_mgmt_emit_property_name(ocs_textbuf_t *textbuf, int access, const char *name);
 extern void ocs_mgmt_emit_string(ocs_textbuf_t *textbuf, int access, const char *name, const char *value);
-__attribute__((format(printf,4,5)))
-extern void ocs_mgmt_emit_int(ocs_textbuf_t *textbuf, int access, const char *name, const char *fmt, ...);
+extern void ocs_mgmt_emit_int(ocs_textbuf_t *textbuf, int access, const char *name, const char *fmt, ...) __printflike(4, 5);
 extern void ocs_mgmt_emit_boolean(ocs_textbuf_t *textbuf, int access, const char *name, const int value);
 extern int parse_wwn(char *wwn_in, uint64_t *wwn_out);
 
diff --git a/sys/dev/ocs_fc/ocs_os.h b/sys/dev/ocs_fc/ocs_os.h
index 331d5b07aecde5..55ee98b3cc58ee 100644
--- a/sys/dev/ocs_fc/ocs_os.h
+++ b/sys/dev/ocs_fc/ocs_os.h
@@ -711,7 +711,7 @@ typedef struct {
  * @return returns 0 for success, a negative error code value for failure.
  */
 
-extern int ocs_sem_init(ocs_sem_t *sem, int val, const char *name, ...) __attribute__((format(printf, 3, 4)));
+extern int ocs_sem_init(ocs_sem_t *sem, int val, const char *name, ...) __printflike(3, 4);
 
 /**
  * @brief execute a P (decrement) operation
diff --git a/sys/dev/ocs_fc/ocs_utils.h b/sys/dev/ocs_fc/ocs_utils.h
index e3a952e0cf265d..7c9c9ae85b742e 100644
--- a/sys/dev/ocs_fc/ocs_utils.h
+++ b/sys/dev/ocs_fc/ocs_utils.h
@@ -103,10 +103,8 @@ extern int32_t ocs_textbuf_init(ocs_t *ocs, ocs_textbuf_t *textbuf, void *buffer
 extern void ocs_textbuf_free(ocs_t *ocs, ocs_textbuf_t *textbuf);
 extern void ocs_textbuf_putc(ocs_textbuf_t *textbuf, uint8_t c);
 extern void ocs_textbuf_puts(ocs_textbuf_t *textbuf, char *s);
-__attribute__((format(printf,2,3)))
-extern void ocs_textbuf_printf(ocs_textbuf_t *textbuf, const char *fmt, ...);
-__attribute__((format(printf,2,0)))
-extern void ocs_textbuf_vprintf(ocs_textbuf_t *textbuf, const char *fmt, va_list ap);
+extern void ocs_textbuf_printf(ocs_textbuf_t *textbuf, const char *fmt, ...) __printflike(2, 3);
+extern void ocs_textbuf_vprintf(ocs_textbuf_t *textbuf, const char *fmt, va_list ap) __printflike(2, 0);
 extern void ocs_textbuf_buffer(ocs_textbuf_t *textbuf, uint8_t *buffer, uint32_t buffer_length);
 extern void ocs_textbuf_copy(ocs_textbuf_t *textbuf, uint8_t *buffer, uint32_t buffer_length);
 extern int32_t ocs_textbuf_remaining(ocs_textbuf_t *textbuf);
@@ -325,10 +323,8 @@ typedef struct ocs_ramlog_s ocs_ramlog_t;
 extern ocs_ramlog_t *ocs_ramlog_init(ocs_t *ocs, uint32_t buffer_len, uint32_t buffer_count);
 extern void ocs_ramlog_free(ocs_t *ocs, ocs_ramlog_t *ramlog);
 extern void ocs_ramlog_clear(ocs_t *ocs, ocs_ramlog_t *ramlog, int clear_start_of_day, int clear_recent);
-__attribute__((format(printf,2,3)))
-extern int32_t ocs_ramlog_printf(void *os, const char *fmt, ...);
-__attribute__((format(printf,2,0)))
-extern int32_t ocs_ramlog_vprintf(ocs_ramlog_t *ramlog, const char *fmt, va_list ap);
+extern int32_t ocs_ramlog_printf(void *os, const char *fmt, ...) __printflike(2, 3);
+extern int32_t ocs_ramlog_vprintf(ocs_ramlog_t *ramlog, const char *fmt, va_list ap) __printflike(2, 0);
 extern int32_t ocs_ddump_ramlog(ocs_textbuf_t *textbuf, ocs_ramlog_t *ramlog);
 
 #endif 
diff --git a/sys/dev/xen/blkback/blkback.c b/sys/dev/xen/blkback/blkback.c
index c6cba729b991f9..dcfdcafa9351bd 100644
--- a/sys/dev/xen/blkback/blkback.c
+++ b/sys/dev/xen/blkback/blkback.c
@@ -152,7 +152,7 @@ struct xbb_softc;
 struct xbb_xen_req;
 
 static void xbb_attach_failed(struct xbb_softc *xbb, int err, const char *fmt,
-			      ...) __attribute__((format(printf, 3, 4)));
+			      ...) __printflike(3, 4);
 static int  xbb_shutdown(struct xbb_softc *xbb);
 
 /*------------------------------ Data Structures -----------------------------*/
diff --git a/sys/kern/imgact_elf.c b/sys/kern/imgact_elf.c
index 1bc2491a1a120b..de4599fb07acb8 100644
--- a/sys/kern/imgact_elf.c
+++ b/sys/kern/imgact_elf.c
@@ -2712,12 +2712,15 @@ __elfN(note_procstat_auxv)(void *arg, struct sbuf *sb, size_t *sizep)
 	struct proc *p;
 	size_t size;
 	int structsize;
-
+#if defined(COMPAT_FREEBSD32) && __ELF_WORD_SIZE == 32
+	structsize = sizeof(Elf32_Auxinfo);
+#else
+	structsize = sizeof(Elf_Auxinfo);
+#endif
 	p = arg;
 	if (sb == NULL) {
 		size = 0;
-		sb = sbuf_new(NULL, NULL, AT_COUNT * sizeof(Elf_Auxinfo),
-		    SBUF_FIXEDLEN);
+		sb = sbuf_new(NULL, NULL, AT_COUNT * structsize, SBUF_FIXEDLEN);
 		sbuf_set_drain(sb, sbuf_count_drain, &size);
 		sbuf_bcat(sb, &structsize, sizeof(structsize));
 		PHOLD(p);
@@ -2727,7 +2730,6 @@ __elfN(note_procstat_auxv)(void *arg, struct sbuf *sb, size_t *sizep)
 		sbuf_delete(sb);
 		*sizep = size;
 	} else {
-		structsize = sizeof(Elf_Auxinfo);
 		sbuf_bcat(sb, &structsize, sizeof(structsize));
 		PHOLD(p);
 		proc_getauxv(curthread, p, sb);
diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
index 3bc4389346d295..ea6a92ad005679 100644
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -394,6 +394,13 @@
 #define	__printf0like(fmtarg, firstvararg)
 #endif
 
+/* To silence warnings about null terminator not fitting into an array. */
+#if __has_attribute(__nonstring__)
+#define	__nonstring	__attribute__((__nonstring__))
+#else
+#define	__nonstring
+#endif
+
 #define	__strong_reference(sym,aliassym)	\
 	extern __typeof (sym) aliassym __attribute__ ((__alias__ (#sym)))
 #ifdef __STDC__
diff --git a/sys/sys/font.h b/sys/sys/font.h
index fb5fbbc9e55768..d486dbc3a011fc 100644
--- a/sys/sys/font.h
+++ b/sys/sys/font.h
@@ -108,7 +108,7 @@ typedef STAILQ_HEAD(font_list, fontlist) font_list_t;
 
 #define	FONT_HEADER_MAGIC	"VFNT0002"
 struct font_header {
-	uint8_t		fh_magic[8];
+	uint8_t		fh_magic[8] __nonstring;
 	uint8_t		fh_width;
 	uint8_t		fh_height;
 	uint16_t	fh_pad;
diff --git a/sys/sys/priority.h b/sys/sys/priority.h
index 93dd5aa90d9511..3a3537bee1b25d 100644
--- a/sys/sys/priority.h
+++ b/sys/sys/priority.h
@@ -136,10 +136,10 @@
 #endif
 
 struct priority {
-	u_char	pri_class;	/* Scheduling class. */
-	u_char	pri_level;	/* Normal priority level. */
-	u_char	pri_native;	/* Priority before propagation. */
-	u_char	pri_user;	/* User priority based on p_cpu and p_nice. */
+	unsigned char pri_class;	/* Scheduling class. */
+	unsigned char pri_level;	/* Normal priority level. */
+	unsigned char pri_native;	/* Priority before propagation. */
+	unsigned char pri_user;		/* User priority based on p_cpu and p_nice. */
 };
 
 #endif	/* !_SYS_PRIORITY_H_ */
diff --git a/sys/sys/rtprio.h b/sys/sys/rtprio.h
index 6e0d8c7b0fe742..d6968a9e58a4e0 100644
--- a/sys/sys/rtprio.h
+++ b/sys/sys/rtprio.h
@@ -34,6 +34,7 @@
 #ifndef _SYS_RTPRIO_H_
 #define _SYS_RTPRIO_H_
 
+#include <sys/_types.h>
 #include <sys/priority.h>
 
 /*
@@ -71,8 +72,8 @@
  * Scheduling class information.
  */
 struct rtprio {
-	u_short type;			/* scheduling class */
-	u_short prio;
+	unsigned short type;	/* scheduling class */
+	unsigned short prio;
 };
 
 #ifdef _KERNEL
@@ -86,8 +87,8 @@ void	pri_to_rtp(struct thread *, struct rtprio *);
 #include <sys/cdefs.h>
 
 __BEGIN_DECLS
-int	rtprio(int, pid_t, struct rtprio *);
-int	rtprio_thread(int, lwpid_t, struct rtprio *);
+int	rtprio(int, __pid_t, struct rtprio *);
+int	rtprio_thread(int, __lwpid_t, struct rtprio *);
 __END_DECLS
 #endif	/* !_KERNEL */
 #endif	/* !_SYS_RTPRIO_H_ */
diff --git a/sys/xen/xen_intr.h b/sys/xen/xen_intr.h
index 75c3dc7c60600a..4965928cc8b427 100644
--- a/sys/xen/xen_intr.h
+++ b/sys/xen/xen_intr.h
@@ -182,7 +182,7 @@ void xen_intr_unbind(xen_intr_handle_t *handle);
  */
 int
 xen_intr_describe(xen_intr_handle_t port_handle, const char *fmt, ...)
-	__attribute__((format(printf, 2, 3)));
+	__printflike(2, 3);
 
 /**
  * Signal the remote peer of an interrupt source associated with an
diff --git a/sys/xen/xenbus/xenbusvar.h b/sys/xen/xenbus/xenbusvar.h
index ee444c132c0c26..0f4168f17e6a92 100644
--- a/sys/xen/xenbus/xenbusvar.h
+++ b/sys/xen/xenbus/xenbusvar.h
@@ -151,7 +151,7 @@ int xenbus_grant_ring(device_t dev, unsigned long ring_mfn, grant_ref_t *refp);
  *             printf arguments.
  */
 void xenbus_dev_error(device_t dev, int err, const char *fmt, ...)
-	__attribute__((format(printf, 3, 4)));
+	__printflike(3, 4);
 
 /**
  * va_list version of xenbus_dev_error().
@@ -162,7 +162,7 @@ void xenbus_dev_error(device_t dev, int err, const char *fmt, ...)
  * \param ap   Va_list of printf arguments.
  */
 void xenbus_dev_verror(device_t dev, int err, const char *fmt, va_list ap)
-	__attribute__((format(printf, 3, 0)));
+	__printflike(3, 0);
 
 /**
  * Equivalent to xenbus_dev_error(), followed by
@@ -174,7 +174,7 @@ void xenbus_dev_verror(device_t dev, int err, const char *fmt, va_list ap)
  *             printf arguments.
  */
 void xenbus_dev_fatal(device_t dev, int err, const char *fmt, ...)
-	__attribute__((format(printf, 3, 4)));
+	__printflike(3, 4);
 
 /**
  * va_list version of xenbus_dev_fatal().
@@ -185,7 +185,7 @@ void xenbus_dev_fatal(device_t dev, int err, const char *fmt, ...)
  * \param ap   Va_list of printf arguments.
  */
 void xenbus_dev_vfatal(device_t dev, int err, const char *fmt, va_list)
-	__attribute__((format(printf, 3, 0)));
+	__printflike(3, 0);
 
 /**
  * Convert a member of the xenbus_state enum into an ASCII string.
diff --git a/sys/xen/xenstore/xenstorevar.h b/sys/xen/xenstore/xenstorevar.h
index 8b02262396e8ec..f34ad375bc3af4 100644
--- a/sys/xen/xenstore/xenstorevar.h
+++ b/sys/xen/xenstore/xenstorevar.h
@@ -270,7 +270,7 @@ int xs_scanf(struct xs_transaction t,
  */
 int xs_printf(struct xs_transaction t, const char *dir,
     const char *node, const char *fmt, ...)
-    __attribute__((format(printf, 4, 5)));
+    __printflike(4, 5);
 
 /**
  * va_list version of xenbus_printf().
diff --git a/tools/build/cross-build/include/common/sys/cdefs.h b/tools/build/cross-build/include/common/sys/cdefs.h
index 3f9b7866141ffc..8419b53135f6cb 100644
--- a/tools/build/cross-build/include/common/sys/cdefs.h
+++ b/tools/build/cross-build/include/common/sys/cdefs.h
@@ -100,6 +100,14 @@
 	__attribute__((__format__(__printf0__, fmtarg, firstvararg)))
 #endif
 
+#ifndef __nonstring
+#if __has_attribute(__nonstring__)
+#define	__nonstring	__attribute__((__nonstring__))
+#else
+#define	__nonstring
+#endif
+#endif
+
 #ifndef __predict_true
 #define __predict_true(exp) __builtin_expect((exp), 1)
 #endif
diff --git a/tools/build/mk/Makefile.boot b/tools/build/mk/Makefile.boot
index 43246f3ceebf95..6d44bca20c1ec5 100644
--- a/tools/build/mk/Makefile.boot
+++ b/tools/build/mk/Makefile.boot
@@ -38,6 +38,8 @@ CWARNFLAGS.clang+=-Wno-typedef-redefinition
 # bsd.sys.mk explicitly turns on -Wsystem-headers, but that's extremely
 # noisy when building on Linux.
 CWARNFLAGS+=	-Wno-system-headers
+# Catch mismatched int/pointer types (e.g. glibc's char* strerror_r prototype)
+CWARNFLAGS+=	-Werror=int-conversion
 CWARNFLAGS.clang+=-Werror=incompatible-pointer-types-discards-qualifiers
 
 # b64_pton and b64_ntop is in libresolv on MacOS and Linux:
diff --git a/tools/build/test-includes/badfiles.inc b/tools/build/test-includes/badfiles.inc
index 9feb73edc5d03f..39eb7ef882db08 100644
--- a/tools/build/test-includes/badfiles.inc
+++ b/tools/build/test-includes/badfiles.inc
@@ -90,14 +90,12 @@ BADHDRS= \
 	sys/pmckern.h \
 	sys/posix4.h \
 	sys/power.h \
-	sys/priority.h \
 	sys/prng.h \
 	sys/qmath.h \
 	sys/racct.h \
 	sys/resourcevar.h \
 	sys/rman.h \
 	sys/rmlock.h \
-	sys/rtprio.h \
 	sys/runq.h \
 	sys/rwlock.h \
 	sys/sbuf.h \
diff --git a/usr.bin/m4/misc.c b/usr.bin/m4/misc.c
index 1870323ccb301e..3091f2ad1f9e7a 100644
--- a/usr.bin/m4/misc.c
+++ b/usr.bin/m4/misc.c
@@ -120,7 +120,8 @@ pbnum(int n)
 void
 pbnumbase(int n, int base, int d)
 {
-	static char digits[36] = "0123456789abcdefghijklmnopqrstuvwxyz";
+	static char digits[36] __nonstring =
+	    "0123456789abcdefghijklmnopqrstuvwxyz";
 	unsigned int num;
 	int printed = 0;
 
diff --git a/usr.sbin/bsnmpd/tools/libbsnmptools/bsnmptc.c b/usr.sbin/bsnmpd/tools/libbsnmptools/bsnmptc.c
index 354ea2ece83aeb..db836d2e731725 100644
--- a/usr.sbin/bsnmpd/tools/libbsnmptools/bsnmptc.c
+++ b/usr.sbin/bsnmpd/tools/libbsnmptools/bsnmptc.c
@@ -336,7 +336,7 @@ static char *
 snmp_date2asn_oid(char *str, struct asn_oid *oid)
 {
 	char *endptr, *ptr;
-	static const char UTC[3] = "UTC";
+	static const char UTC[] = "UTC";
 	int32_t saved_errno;
 	uint32_t v;
 
@@ -445,8 +445,8 @@ snmp_date2asn_oid(char *str, struct asn_oid *oid)
 
 	/* 'UTC' - optional */
 	ptr = endptr + 1;
-	if (strncmp(ptr, UTC, sizeof(UTC)) == 0)
-		ptr += sizeof(UTC);
+	if (strncmp(ptr, UTC, strlen(UTC)) == 0)
+		ptr += strlen(UTC);
 
 	/* '+/-' */
 	if (*ptr == '-' || *ptr == '+') {
diff --git a/usr.sbin/makefs/msdos/direntry.h b/usr.sbin/makefs/msdos/direntry.h
index 9d6c65dfcc7d00..d7ef79114e80ac 100644
--- a/usr.sbin/makefs/msdos/direntry.h
+++ b/usr.sbin/makefs/msdos/direntry.h
@@ -55,7 +55,7 @@
  * Structure of a dos directory entry.
  */
 struct direntry {
-	uint8_t		deName[11];	/* filename, blank filled */
+	uint8_t 	deName[11] __nonstring;	/* filename, blank filled */
 #define	SLOT_EMPTY	0x00		/* slot has never been used */
 #define	SLOT_E5		0x05		/* the real value is 0xe5 */
 #define	SLOT_DELETED	0xe5		/* file in this slot deleted */
diff --git a/usr.sbin/pmcannotate/pmcannotate.c b/usr.sbin/pmcannotate/pmcannotate.c
index 5958ff615e4ba3..fcfebbe8186d78 100644
--- a/usr.sbin/pmcannotate/pmcannotate.c
+++ b/usr.sbin/pmcannotate/pmcannotate.c
@@ -54,8 +54,10 @@
 	if ((ptr) != NULL)						\
 		perror(ptr);						\
 	fprintf(stderr, ##x);						\
-	remove(tbfl);							\
-	remove(tofl);							\
+	if (tbfl != NULL)						\
+		remove(tbfl);						\
+	if (tofl != NULL)						\
+		remove(tofl);						\
 	exit(EXIT_FAILURE);						\
 } while (0)
 
@@ -692,7 +694,7 @@ int
 main(int argc, char *argv[])
 {
 	char buffer[LNBUFF], fname[FNBUFF];
-	char *tbfl, *tofl, *tmpdir;
+	char *tbfl = NULL, *tofl = NULL, *tmpdir;
 	char tmpf[MAXPATHLEN * 2 + 50];
 	float limit;
 	char *bin, *exec, *kfile, *ofile;