diff --git a/src/frontend/numparam/general.h b/src/frontend/numparam/general.h
index a3309eb34..7f4fb3667 100644
--- a/src/frontend/numparam/general.h
+++ b/src/frontend/numparam/general.h
@@ -13,25 +13,25 @@ typedef char string[258];
 
 
 void sfix(SPICE_DSTRINGPTR dstr_p, int len);
-char *pscopy(SPICE_DSTRINGPTR s, char *a, int i, int j);
-char *pscopy_up(SPICE_DSTRINGPTR s, char *a, int i, int j);
+char *pscopy(SPICE_DSTRINGPTR s, const char *a, int i, int j);
+char *pscopy_up(SPICE_DSTRINGPTR s, const char *a, int i, int j);
 bool scopyd(SPICE_DSTRINGPTR a, SPICE_DSTRINGPTR b);
-bool scopys(SPICE_DSTRINGPTR a, char *b);
-bool scopy_up(SPICE_DSTRINGPTR a, char *str);
-bool scopy_lower(SPICE_DSTRINGPTR a, char *str);
+bool scopys(SPICE_DSTRINGPTR a, const char *b);
+bool scopy_up(SPICE_DSTRINGPTR a, const char *str);
+bool scopy_lower(SPICE_DSTRINGPTR a, const char *str);
 bool ccopy(SPICE_DSTRINGPTR a, char c);
-bool sadd(SPICE_DSTRINGPTR s, char *t);
+bool sadd(SPICE_DSTRINGPTR s, const char *t);
 bool nadd(SPICE_DSTRINGPTR s, long n);
 bool cadd(SPICE_DSTRINGPTR s, char c);
 bool naddll(SPICE_DSTRINGPTR s, long long n);
 bool cins(SPICE_DSTRINGPTR s, char c);
-bool sins(SPICE_DSTRINGPTR s, char *t);
+bool sins(SPICE_DSTRINGPTR s, const char *t);
 int cpos(char c, char *s);
-int spos_(char *sub, char *s);
-bool ci_prefix(register char *p, register char *s);
-int length(char *s);
-bool steq(char *s, char *t);
-bool stne(char *s, char *t);
+int spos_(char *sub, const char *s);
+bool ci_prefix(const char *p, const char *s);
+int length(const char *s);
+bool steq(const char *s, const char *t);
+bool stne(const char *s, const char *t);
 void stri(long n, SPICE_DSTRINGPTR s);
 
 char upcase(char c);
diff --git a/src/frontend/numparam/mystring.c b/src/frontend/numparam/mystring.c
index daa716929..c7d4fc891 100644
--- a/src/frontend/numparam/mystring.c
+++ b/src/frontend/numparam/mystring.c
@@ -25,7 +25,7 @@
 /***** primitive input-output ***/
 
 bool
-ci_prefix(register char *p, register char *s)
+ci_prefix(const char *p, const char *s)
 {
     while (*p) {
         if ((isupper(*p) ? tolower(*p) : *p) !=
@@ -93,7 +93,7 @@ sfix(SPICE_DSTRINGPTR dstr_p, int len)
 
 
 int
-length(char *s)
+length(const char *s)
 {
     return (int) strlen(s);
 }
@@ -103,7 +103,7 @@ length(char *s)
  * Function: add string t to dynamic string dstr_p.
  * ----------------------------------------------------------------- */
 bool
-sadd(SPICE_DSTRINGPTR dstr_p, char *t)
+sadd(SPICE_DSTRINGPTR dstr_p, const char *t)
 {
     spice_dstring_append(dstr_p, t, -1);
     return 1;
@@ -148,7 +148,7 @@ cins(SPICE_DSTRINGPTR dstr_p, char c)
  * Function: insert string t at front of dynamic string dstr_p
  * ----------------------------------------------------------------- */
 bool
-sins(SPICE_DSTRINGPTR dstr_p, char *t)
+sins(SPICE_DSTRINGPTR dstr_p, const char *t)
 {
     int i;
     int ls;
@@ -214,7 +214,7 @@ scopyd(SPICE_DSTRINGPTR s, SPICE_DSTRINGPTR t)  /* returns success flag */
  * are always NULLterminated.
  * ----------------------------------------------------------------- */
 bool
-scopys(SPICE_DSTRINGPTR s, char *t)     /* returns success flag */
+scopys(SPICE_DSTRINGPTR s, const char *t)     /* returns success flag */
 {
     spice_dstring_reinit(s);
     spice_dstring_append(s, t, -1);
@@ -227,10 +227,10 @@ scopys(SPICE_DSTRINGPTR s, char *t)     /* returns success flag */
  * Dynamic strings are always NULL * terminated.
  * ----------------------------------------------------------------- */
 bool
-scopy_up(SPICE_DSTRINGPTR dstr_p, char *str)    /* returns success flag */
+scopy_up(SPICE_DSTRINGPTR dstr_p, const char *str)    /* returns success flag */
 {
     char up[2];                 /* short string */
-    char *ptr;                  /* position in string */
+    const char *ptr;            /* position in string */
 
     spice_dstring_reinit(dstr_p);
     up[1] = '\0';
@@ -247,10 +247,10 @@ scopy_up(SPICE_DSTRINGPTR dstr_p, char *str)    /* returns success flag */
  * Dynamic strings are always NULL * terminated.
  * ----------------------------------------------------------------- */
 bool
-scopy_lower(SPICE_DSTRINGPTR dstr_p, char *str) /* returns success flag */
+scopy_lower(SPICE_DSTRINGPTR dstr_p, const char *str) /* returns success flag */
 {
     char low[2];                /* short string */
-    char *ptr;                  /* position in string */
+    const char *ptr;            /* position in string */
 
     spice_dstring_reinit(dstr_p);
     low[1] = '\0';
@@ -275,7 +275,7 @@ ccopy(SPICE_DSTRINGPTR dstr_p, char c)  /* returns success flag */
 
 
 char *
-pscopy(SPICE_DSTRINGPTR dstr_p, char *t, int start, int leng)
+pscopy(SPICE_DSTRINGPTR dstr_p, const char *t, int start, int leng)
 /* partial string copy, with C-based start - Because we now have a 0 based
  * start and string may copy outselves, we may need to restore the first
  * character of the original dstring because resetting string will wipe
@@ -313,7 +313,7 @@ pscopy(SPICE_DSTRINGPTR dstr_p, char *t, int start, int leng)
 
 
 char *
-pscopy_up(SPICE_DSTRINGPTR dstr_p, char *t, int start, int leng)
+pscopy_up(SPICE_DSTRINGPTR dstr_p, const char *t, int start, int leng)
 /* partial string copy to upper case, with C convention for start. */
 {
     int i;                      /* counter */
@@ -439,14 +439,14 @@ stri(long n, SPICE_DSTRINGPTR dstr_p)
 
 
 bool
-steq(char *a, char *b)          /* string a==b test */
+steq(const char *a, const char *b)  /* string a==b test */
 {
     return strcmp(a, b) == 0;
 }
 
 
 bool
-stne(char *s, char *t)
+stne(const char *s, const char *t)
 {
     return strcmp(s, t) != 0;
 }
@@ -576,7 +576,7 @@ absi(long i)
 
 
 int
-spos_(char *sub, char *s)
+spos_(char *sub, const char *s)
 /* equivalent to Turbo Pascal pos().
    BUG: counts 1 ... length(s), not from 0 like C
 */
diff --git a/src/frontend/numparam/xpressn.c b/src/frontend/numparam/xpressn.c
index 2bf417b6e..4d37e2c28 100644
--- a/src/frontend/numparam/xpressn.c
+++ b/src/frontend/numparam/xpressn.c
@@ -736,7 +736,7 @@ keyword(SPICE_DSTRINGPTR keys_p, SPICE_DSTRINGPTR tstr_p)
 
 
 static double
-parseunit(char *s)
+parseunit(const char *s)
 /* the Spice suffixes */
 {
     switch (toupper(s[0]))
@@ -754,7 +754,7 @@ parseunit(char *s)
 
 
 static int
-fetchid(char *s, SPICE_DSTRINGPTR t, int ls, int i)
+fetchid(const char *s, SPICE_DSTRINGPTR t, int ls, int i)
 /* copy next identifier from s into t, advance and return scan index i */
 {
     char c;
@@ -791,7 +791,7 @@ fetchid(char *s, SPICE_DSTRINGPTR t, int ls, int i)
 
 
 static double
-exists(tdico *d, char *s, int *pi, bool *perror)
+exists(tdico *d, const char *s, int *pi, bool *perror)
 /* check if s in simboltable 'defined': expect (ident) and return 0 or 1 */
 {
     bool error = *perror;
@@ -853,7 +853,7 @@ exists(tdico *d, char *s, int *pi, bool *perror)
 
 
 static double
-fetchnumber(tdico *dico, char *s, int *pi, bool *perror)
+fetchnumber(tdico *dico, const char *s, int *pi, bool *perror)
 /* parse a Spice number in string s */
 {
     double u;
@@ -887,7 +887,7 @@ fetchnumber(tdico *dico, char *s, int *pi, bool *perror)
 
 static char
 fetchoperator(tdico *dico,
-              char *s, int ls,
+              const char *s, int ls,
               int *pi,
               unsigned char *pstate, unsigned char *plevel,
               bool *perror)
@@ -1122,7 +1122,7 @@ operate(char op, double x, double y)
 
 
 static double
-formula(tdico *dico, char *s, bool *perror)
+formula(tdico *dico, const char *s, bool *perror)
 {
     /* Expression parser.
        s is a formula with parentheses and math ops +-* / ...
diff --git a/src/include/ngspice/dstring.h b/src/include/ngspice/dstring.h
index 443970971..6ac03e39e 100644
--- a/src/include/ngspice/dstring.h
+++ b/src/include/ngspice/dstring.h
@@ -25,10 +25,10 @@ typedef struct spice_dstring {
  * spice_dstring_xxxx routines.  Used to manipulate dynamic strings.
 ----------------------------------------------------------------- */
 extern void spice_dstring_init(SPICE_DSTRINGPTR dsPtr) ;
-extern char *spice_dstring_append(SPICE_DSTRINGPTR dsPtr,char *string,int length) ;
-extern char *spice_dstring_append_lower(SPICE_DSTRINGPTR dsPtr,char *string,int length) ;
+extern char *spice_dstring_append(SPICE_DSTRINGPTR dsPtr,const char *string,int length) ;
+extern char *spice_dstring_append_lower(SPICE_DSTRINGPTR dsPtr,const char *string,int length) ;
 extern char *spice_dstring_append_char(SPICE_DSTRINGPTR dsPtr,char c) ;
-extern char *spice_dstring_print(SPICE_DSTRINGPTR dsPtr,char *format, ... ) ;
+extern char *spice_dstring_print(SPICE_DSTRINGPTR dsPtr,const char *format, ... ) ;
 extern char *spice_dstring_setlength(SPICE_DSTRINGPTR dsPtr,int length) ;
 extern char *_spice_dstring_setlength(SPICE_DSTRINGPTR dsPtr,int length) ;
 extern void spice_dstring_free(SPICE_DSTRINGPTR dsPtr) ;
diff --git a/src/misc/dstring.c b/src/misc/dstring.c
index 6b79ce723..3aca84a55 100644
--- a/src/misc/dstring.c
+++ b/src/misc/dstring.c
@@ -59,12 +59,12 @@ void spice_dstring_init(SPICE_DSTRINGPTR dsPtr)
  *
  *----------------------------------------------------------------------
  */
-char *spice_dstring_append(SPICE_DSTRINGPTR dsPtr,char *string,int length)
+char *spice_dstring_append(SPICE_DSTRINGPTR dsPtr, const char *string, int length)
 {
     int newSize ;			/* needed size */
     char *newString ;			/* newly allocated string buffer */
     char *dst ;				/* destination */
-    char *end ;				/* end of string */
+    const char *end ;				/* end of string */
 
     if( length < 0){
 	length = (int) strlen(string) ;
@@ -124,12 +124,12 @@ char *spice_dstring_append(SPICE_DSTRINGPTR dsPtr,char *string,int length)
  *
  *----------------------------------------------------------------------
  */
-char *spice_dstring_append_lower(SPICE_DSTRINGPTR dsPtr,char *string,int length)
+char *spice_dstring_append_lower(SPICE_DSTRINGPTR dsPtr, const char *string, int length)
 {
     int newSize ;                       /* needed size */
     char *newString ;                   /* newly allocated string buffer */
     char *dst ;                         /* destination */
-    char *end ;                         /* end of string */
+    const char *end ;                   /* end of string */
 
     if( length < 0){
         length = (int) strlen(string) ;
@@ -178,7 +178,7 @@ char *spice_dstring_append_char( SPICE_DSTRINGPTR dstr_p, char c)
   return spice_dstring_append( dstr_p, &c, 1 ) ;
 } /* end spice_dstring_append_char() */
 
-static int spice_format_length( va_list args, char *fmt )
+static int spice_format_length( va_list args, const char *fmt )
 {
     int i ;					/* integer */
     int len ;					/* length of format */
@@ -263,7 +263,7 @@ static int spice_format_length( va_list args, char *fmt )
 } /* end Ymessage_format_length() */
 
 
-char *spice_dstring_print( SPICE_DSTRINGPTR dsPtr,char *format, ... )
+char *spice_dstring_print( SPICE_DSTRINGPTR dsPtr, const char *format, ... )
 {
     va_list args ;
     int format_len ;				/* length of format */