-extern void header_add(int, char *, ...);
-extern void header_add_at_position(BOOL, uschar *, BOOL, int, char *, ...);
-extern void header_remove(int, uschar *);
-extern BOOL header_testname(header_line *, uschar *, int, BOOL);
-extern BOOL header_testname_incomplete(header_line *, uschar *, int, BOOL);
-extern void log_write(unsigned int, int, char *format, ...);
+extern void header_add(int, const char *, ...);
+extern void header_add_at_position(BOOL, uschar *, BOOL, int, const char *, ...);
+extern void header_remove(int, const uschar *);
+extern BOOL header_testname(header_line *, const uschar *, int, BOOL);
+extern BOOL header_testname_incomplete(header_line *, const uschar *, int, BOOL);
+extern void log_write(unsigned int, int, const char *format, ...) PRINTF_FUNCTION(3,4);