diff --git a/src/eccodes.c b/src/eccodes.c
index fb56decc3..a7d351f0b 100644
--- a/src/eccodes.c
+++ b/src/eccodes.c
@@ -96,7 +96,7 @@ int codes_fieldset_count(grib_fieldset* set)
 
 /* Indexing */
 /******************************************************************************/
-grib_index* codes_index_new_from_file(grib_context* c, char* filename, const char* keys, int* err)
+grib_index* codes_index_new_from_file(grib_context* c, const char* filename, const char* keys, int* err)
 {
     return grib_index_new_from_file(c, filename, keys, err);
 }
@@ -140,7 +140,7 @@ int codes_index_select_double(grib_index* index, const char* key, double value)
 {
     return grib_index_select_double(index, key, value);
 }
-int codes_index_select_string(grib_index* index, const char* key, char* value)
+int codes_index_select_string(grib_index* index, const char* key, const char* value)
 {
     return grib_index_select_string(index, key, value);
 }
diff --git a/src/eccodes.h b/src/eccodes.h
index a310dbbfb..1c0958490 100644
--- a/src/eccodes.h
+++ b/src/eccodes.h
@@ -214,7 +214,7 @@ typedef struct grib_index codes_index;
  * @param err         :  0 if OK, integer value on error
  * @return            the newly created index
  */
-codes_index* codes_index_new_from_file(codes_context* c, char* filename, const char* keys, int* err);
+codes_index* codes_index_new_from_file(codes_context* c, const char* filename, const char* keys, int* err);
 
 /**
  *  Create a new index based on a set of keys.
@@ -320,7 +320,7 @@ int codes_index_select_double(codes_index* index, const char* key, double value)
  * @param value       : value of the key to select
  * @return            0 if OK, integer value on error
  */
-int codes_index_select_string(codes_index* index, const char* key, char* value);
+int codes_index_select_string(codes_index* index, const char* key, const char* value);
 
 /**
  *  Create a new handle from an index after having selected the key values.
diff --git a/src/grib_api.h b/src/grib_api.h
index 44b0e7ebd..34e3a49fb 100644
--- a/src/grib_api.h
+++ b/src/grib_api.h
@@ -266,7 +266,7 @@ typedef struct grib_index grib_index;
  * @return            the newly created index
  */
 grib_index* grib_index_new_from_file(grib_context* c,
-                                     char* filename, const char* keys, int* err);
+                                     const char* filename, const char* keys, int* err);
 /**
  *  Create a new index based on a set of keys.
  *
@@ -374,7 +374,7 @@ int grib_index_select_double(grib_index* index, const char* key, double value);
  * @param value       : value of the key to select
  * @return            0 if OK, integer value on error
  */
-int grib_index_select_string(grib_index* index, const char* key, char* value);
+int grib_index_select_string(grib_index* index, const char* key, const char* value);
 
 /**
  *  Create a new handle from an index after having selected the key values.
diff --git a/src/grib_api_prototypes.h b/src/grib_api_prototypes.h
index 12ce195ee..871e6201e 100644
--- a/src/grib_api_prototypes.h
+++ b/src/grib_api_prototypes.h
@@ -731,14 +731,14 @@ int grib_index_add_file(grib_index* index, const char* filename);
 grib_handle* new_message_from_file(int message_type, grib_context* c, FILE* f, int* error);
 int _codes_index_add_file(grib_index* index, const char* filename, int message_type);
 int grib_index_add_file(grib_index* index, const char* filename);
-grib_index* grib_index_new_from_file(grib_context* c, char* filename, const char* keys, int* err);
+grib_index* grib_index_new_from_file(grib_context* c, const char* filename, const char* keys, int* err);
 int grib_index_get_size(const grib_index* index, const char* key, size_t* size);
 int grib_index_get_string(const grib_index* index, const char* key, char** values, size_t* size);
 int grib_index_get_long(const grib_index* index, const char* key, long* values, size_t* size);
 int grib_index_get_double(const grib_index* index, const char* key, double* values, size_t* size);
 int grib_index_select_long(grib_index* index, const char* skey, long value);
 int grib_index_select_double(grib_index* index, const char* skey, double value);
-int grib_index_select_string(grib_index* index, const char* skey, char* value);
+int grib_index_select_string(grib_index* index, const char* skey, const char* value);
 grib_handle* grib_index_get_handle(grib_field* field, int* err);
 grib_handle* codes_index_get_handle(grib_field* field, int message_type, int* err);
 int grib_index_dump_file(FILE* fout, const char* filename);
diff --git a/src/grib_index.c b/src/grib_index.c
index 4fd0cb2cc..09e138666 100644
--- a/src/grib_index.c
+++ b/src/grib_index.c
@@ -1400,7 +1400,7 @@ int grib_index_add_file(grib_index* index, const char* filename)
 }
 #endif
 
-grib_index* grib_index_new_from_file(grib_context* c, char* filename, const char* keys, int* err)
+grib_index* grib_index_new_from_file(grib_context* c, const char* filename, const char* keys, int* err)
 {
     grib_index* index = NULL;
 
@@ -1576,7 +1576,7 @@ int grib_index_select_double(grib_index* index, const char* skey, double value)
     return 0;
 }
 
-int grib_index_select_string(grib_index* index, const char* skey, char* value)
+int grib_index_select_string(grib_index* index, const char* skey, const char* value)
 {
     grib_index_key* key = NULL;
     int err             = GRIB_NOT_FOUND;