diff --git a/usr.bin/m4/extern.h b/usr.bin/m4/extern.h
index 165f57387bb..7e994cc301a 100644
--- a/usr.bin/m4/extern.h
+++ b/usr.bin/m4/extern.h
@@ -105,16 +105,16 @@ extern void	pbnumbase(int, int, int);
 extern void	pbunsigned(unsigned long);
 extern void	pbstr(const char *);
 extern void	pushback(int);
-extern void	*xalloc(size_t, const char *, ...);
-extern void	*xcalloc(size_t, size_t, const char *, ...);
-extern void	*xrealloc(void *, size_t, const char *, ...);
-extern void	*xreallocarray(void *, size_t, size_t, const char *, ...);
+extern void	*xalloc(size_t, const char *, ...) __printf0like(2, 3);
+extern void	*xcalloc(size_t, size_t, const char *, ...) __printf0like(3, 4);
+extern void	*xrealloc(void *, size_t, const char *, ...) __printf0like(3, 4);
+extern void	*xreallocarray(void *, size_t, size_t, const char *, ...) __printf0like(4, 5);
 extern char	*xstrdup(const char *);
 extern void	usage(void);
 extern void	resizedivs(int);
 extern size_t	buffer_mark(void);
 extern void	dump_buffer(FILE *, size_t);
-extern void	m4errx(int, const char *, ...);
+extern void	m4errx(int, const char *, ...) __dead2 __printf0like(2, 3);
 
 extern int	obtain_char(struct input_file *);
 extern void	set_input(struct input_file *, FILE *, const char *);