diff --git a/backend/common.c b/backend/common.c
index d9c7606f..0a884120 100644
--- a/backend/common.c
+++ b/backend/common.c
@@ -73,7 +73,7 @@ void to_upper(unsigned char source[]) {
 }
 
 /* Verifies that a string only uses valid characters */
-int is_sane(char test_string[], const unsigned char source[], const int length) {
+int is_sane(const char test_string[], const unsigned char source[], const int length) {
     unsigned int i, j, latch;
     unsigned int lt = strlen(test_string);
 
@@ -94,7 +94,7 @@ int is_sane(char test_string[], const unsigned char source[], const int length)
 }
 
 /* Returns the position of data in set_string */
-int posn(char set_string[], const char data) {
+int posn(const char set_string[], const char data) {
     unsigned int i, n = strlen(set_string);
 
     for (i = 0; i < n; i++) {
@@ -106,7 +106,7 @@ int posn(char set_string[], const char data) {
 }
 
 /* Replaces huge switch statements for looking up in tables */
-void lookup(char set_string[], const char *table[], const char data, char dest[]) {
+void lookup(const char set_string[], const char *table[], const char data, char dest[]) {
     unsigned int i, n = strlen(set_string);
 
     for (i = 0; i < n; i++) {
@@ -132,7 +132,7 @@ void unset_module(struct zint_symbol *symbol, const int y_coord, const int x_coo
 }
 
 /* Expands from a width pattern to a bit pattern */
-void expand(struct zint_symbol *symbol, char data[]) {
+void expand(struct zint_symbol *symbol, const char data[]) {
 
     unsigned int reader, n = strlen(data);
     int writer, i;
diff --git a/backend/common.h b/backend/common.h
index f748a872..a328b42a 100644
--- a/backend/common.h
+++ b/backend/common.h
@@ -56,10 +56,10 @@ extern "C" {
     extern int ctoi(const char source);
     extern char itoc(const int source);
     extern void to_upper(unsigned char source[]);
-    extern int is_sane(char test_string[], const unsigned char source[], const int length);
-    extern void lookup(char set_string[], const char *table[], const char data, char dest[]);
-    extern int posn(char set_string[], const char data);
-    extern void expand(struct zint_symbol *symbol, char data[]);
+    extern int is_sane(const char test_string[], const unsigned char source[], const int length);
+    extern void lookup(const char set_string[], const char *table[], const char data, char dest[]);
+    extern int posn(const char set_string[], const char data);
+    extern void expand(struct zint_symbol *symbol, const char data[]);
     extern int is_stackable(const int symbology);
     extern int is_extendable(const int symbology);
     extern int roundup(const float input);
diff --git a/backend/library.c b/backend/library.c
index 03340f44..269c1589 100644
--- a/backend/library.c
+++ b/backend/library.c
@@ -1099,7 +1099,7 @@ int ZBarcode_Encode_File_and_Buffer(struct zint_symbol *symbol, char *filename,
  * dimensions. The symbol->scale and symbol->height values are totally ignored in this case.
  *
  */
-int ZBarcode_Render(struct zint_symbol *symbol, float width, float height) {
+int ZBarcode_Render(struct zint_symbol *symbol, const float width, const float height) {
     // Send the request to the render_plot method
     return render_plot(symbol, width, height);
 }