diff --git a/src/include/ngspice/ngspice.h b/src/include/ngspice/ngspice.h
index 3c3070a9a..c9f0ba291 100644
--- a/src/include/ngspice/ngspice.h
+++ b/src/include/ngspice/ngspice.h
@@ -241,7 +241,7 @@ extern double x_atanh(double);
 extern char *gettok_noparens(char **s);
 extern char *gettok_node(char **s);
 extern char *gettok_iv(char **s);
-extern char *nexttok(char *s);
+extern char *nexttok(const char *s);
 extern int get_l_paren(char **s);
 extern int get_r_paren(char **s);
 
diff --git a/src/include/ngspice/stringskip.h b/src/include/ngspice/stringskip.h
index ee553d043..7e3de7b4f 100644
--- a/src/include/ngspice/stringskip.h
+++ b/src/include/ngspice/stringskip.h
@@ -1,13 +1,13 @@
 #ifndef ngspice_STRINGSKIP_H
 #define ngspice_STRINGSKIP_H
 
-static inline char *skip_non_ws(char *s)  { while (*s && !isspace_c(*s)) s++; return s; }
-static inline char *skip_ws(char *s)      { while (       isspace_c(*s)) s++; return s; }
+static inline char *skip_non_ws(const char *s)  { while (*s && !isspace_c(*s)) s++; return (char *) s; }
+static inline char *skip_ws(const char *s)      { while (       isspace_c(*s)) s++; return (char *) s; }
 
 static inline char *depreciated_skip_back_non_ws(char *s) { while (s[-1] && !isspace_c(s[-1])) s--; return s; }
 static inline char *depreciated_skip_back_ws(char *s)     { while (isspace_c(s[-1]))           s--; return s; }
 
-static inline char *skip_back_non_ws(char *s, char *start) { while (s > start && !isspace_c(s[-1])) s--; return s; }
-static inline char *skip_back_ws(char *s, char *start)     { while (s > start && isspace_c(s[-1])) s--; return s; }
+static inline char *skip_back_non_ws(const char *s, const char *start) { while (s > start && !isspace_c(s[-1])) s--; return (char *) s; }
+static inline char *skip_back_ws(const char *s, const char *start)     { while (s > start && isspace_c(s[-1])) s--; return (char *) s; }
 
 #endif
diff --git a/src/misc/string.c b/src/misc/string.c
index 4148a7d47..95dae1a8e 100644
--- a/src/misc/string.c
+++ b/src/misc/string.c
@@ -316,7 +316,7 @@ gettok(char **s)
 * you have parens or commas anywhere in the nodelist.
 *-------------------------------------------------------------------------*/
 char *
-nexttok(char *s)
+nexttok(const char *s)
 {
     int paren = 0;
 
@@ -335,7 +335,7 @@ nexttok(char *s)
     while (isspace_c(*s) || *s == ',')
         s++;
 
-    return s;
+    return (char *) s;
 }