diff --git a/src/frontend/outitf.c b/src/frontend/outitf.c
index a5517a700..697db26d4 100644
--- a/src/frontend/outitf.c
+++ b/src/frontend/outitf.c
@@ -846,7 +846,7 @@ fileInit(runDesc *run)
 
 
 static int
-guess_type(char *name)
+guess_type(const char *name)
 {
     int type;
 
diff --git a/src/include/ngspice/stringutil.h b/src/include/ngspice/stringutil.h
index 8e42dad3b..f715dac5a 100644
--- a/src/include/ngspice/stringutil.h
+++ b/src/include/ngspice/stringutil.h
@@ -12,14 +12,14 @@
 #include <stdarg.h>
 
 
-int prefix(register char *p, register char *s);
+int prefix(const char *p, const char *s);
 char * copy(const char *str);
 char * copy_substring(const char *str, const char *end);
-int substring(register char *sub, register char *str);
+int substring(const char *sub, const char *str);
 void appendc(char *s, char c);
 int scannum(char *str);
-int cieq(register char *p, register char *s);
-int ciprefix(register char *p, register char *s);
+int cieq(const char *p, const char *s);
+int ciprefix(const char *p, const char *s);
 void strtolower(char *str);
 void strtoupper(char *str);
 char * stripWhiteSpacesInsideParens(char *str);
diff --git a/src/misc/string.c b/src/misc/string.c
index 9fa420ae1..323072f29 100644
--- a/src/misc/string.c
+++ b/src/misc/string.c
@@ -14,7 +14,7 @@ Copyright 1990 Regents of the University of California.  All rights reserved.
 
 
 int
-prefix(register char *p, register char *s)
+prefix(const char *p, const char *s)
 {
     while (*p && (*p == *s))
         p++, s++;
@@ -106,9 +106,9 @@ tprintf(const char *fmt, ...)
 /* Like strstr( ) XXX */
 
 int
-substring(register char *sub, register char *str)
+substring(const char *sub, const char *str)
 {
-    char *s, *t;
+    const char *s, *t;
 
     while (*str) {
         if (*str == *sub) {
@@ -157,7 +157,7 @@ scannum(char *str)
 /* Like strcasecmp( ) XXX */
 
 int
-cieq(register char *p, register char *s)
+cieq(const char *p, const char *s)
 {
     while (*p) {
         if ((isupper(*p) ? tolower(*p) : *p) !=
@@ -172,7 +172,7 @@ cieq(register char *p, register char *s)
 /* Case insensitive prefix. */
 
 int
-ciprefix(register char *p, register char *s)
+ciprefix(const char *p, const char *s)
 {
     while (*p) {
         if ((isupper(*p) ? tolower(*p) : *p) !=