diff --git a/src/netif/ppp/ccp.c b/src/netif/ppp/ccp.c
index 032ca66d..f8472208 100644
--- a/src/netif/ppp/ccp.c
+++ b/src/netif/ppp/ccp.c
@@ -176,9 +176,7 @@ static void ccp_lowerdown(ppp_pcb *pcb);
 static void ccp_input(ppp_pcb *pcb, u_char *pkt, int len);
 static void ccp_protrej(ppp_pcb *pcb);
 #if PRINTPKT_SUPPORT
-static int  ccp_printpkt (u_char *pkt, int len,
-			      void (*printer) (void *, char *, ...),
-			      void *arg);
+static int ccp_printpkt(u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg);
 #endif /* PRINTPKT_SUPPORT */
 static void ccp_datainput(ppp_pcb *pcb, u_char *pkt, int len);
 
@@ -1449,7 +1447,7 @@ static char *ccp_codenames[] = {
     "ResetReq", "ResetAck",
 };
 
-static int ccp_printpkt(u_char *p, u_char plen, void (*printer) (void *, char *, ...), void *arg) {
+static int ccp_printpkt(u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg) {
     u_char *p0, *optend;
     int code, id, len;
     int optlen;