diff --git a/libsrc/common/_printf.c b/libsrc/common/_printf.c
index 5bb72e3d9..b76f39a3b 100644
--- a/libsrc/common/_printf.c
+++ b/libsrc/common/_printf.c
@@ -18,7 +18,7 @@
 
 
 
-int _printf (struct outdesc* d, char* f, va_list ap)
+int _printf (struct outdesc* d, const char* f, va_list ap)
 {
     outfunc fout;    		/* Output function */
     unsigned char type;	      	/* variable argument type */
@@ -39,7 +39,7 @@ int _printf (struct outdesc* d, char* f, va_list ap)
     register char* s;          	/* work pointer to argument string */
 
     /* Remember the format string in a register variable for shorter code */
-    register char* format = f;
+    register const char* format = f;
 
     /* Remember the output function in a local variable for speed and size */
     fout = d->fout;
diff --git a/libsrc/common/_printf.h b/libsrc/common/_printf.h
index d4d5ed607..6ce43439a 100644
--- a/libsrc/common/_printf.h
+++ b/libsrc/common/_printf.h
@@ -30,7 +30,7 @@ struct outdesc {
 
 
 /* Internal formatting routine */
-int _printf (struct outdesc* d, char* format, va_list ap);
+int _printf (struct outdesc* d, const char* format, va_list ap);