diff --git a/include/villas/hist.h b/include/villas/hist.h
index 477470956..b3043deb8 100644
--- a/include/villas/hist.h
+++ b/include/villas/hist.h
@@ -79,34 +79,34 @@ void hist_reset(struct hist *h);
 void hist_put(struct hist *h, double value);
 
 /** Calcluate the variance of all counted values. */
-double hist_var(struct hist *h);
+double hist_var(const struct hist *h);
 
 /** Calculate the mean average of all counted values. */
-double hist_mean(struct hist *h);
+double hist_mean(const struct hist *h);
 
 /** Calculate the standard derivation of all counted values. */
-double hist_stddev(struct hist *h);
+double hist_stddev(const struct hist *h);
 
 /** Print all statistical properties of distribution including a graphilcal plot of the histogram. */
-void hist_print(struct hist *h, int details);
+void hist_print(const struct hist *h, int details);
 
 /** Print ASCII style plot of histogram */
-void hist_plot(struct hist *h);
+void hist_plot(const struct hist *h);
 
 /** Dump histogram data in Matlab format.
  *
  * @return The string containing the dump. The caller is responsible to free() the buffer.
  */
-char * hist_dump(struct hist *h);
+char * hist_dump(const struct hist *h);
 
 /** Prints Matlab struct containing all infos to file. */
-int hist_dump_matlab(struct hist *h, FILE *f);
+int hist_dump_matlab(const struct hist *h, FILE *f);
 
 /** Write the histogram in JSON format to fiel \p f. */
-int hist_dump_json(struct hist *h, FILE *f);
+int hist_dump_json(const struct hist *h, FILE *f);
 
 /** Build a libjansson / JSON object of the histogram. */
-json_t * hist_json(struct hist *h);
+json_t * hist_json(const struct hist *h);
 
 #ifdef __cplusplus
 }
diff --git a/include/villas/mapping.h b/include/villas/mapping.h
index 0ab06db7a..89ec23817 100644
--- a/include/villas/mapping.h
+++ b/include/villas/mapping.h
@@ -99,9 +99,9 @@ struct mapping_entry {
 	};
 };
 
-int mapping_remap(struct list *m, struct sample *remapped, struct sample *original, struct stats *s);
+int mapping_remap(const struct list *m, struct sample *remapped, const struct sample *original, const struct stats *s);
 
-int mapping_update(struct mapping_entry *e, struct sample *remapped, struct sample *original, struct stats *s);
+int mapping_update(const struct mapping_entry *e, struct sample *remapped, const struct sample *original, const struct stats *s);
 
 int mapping_parse(struct mapping_entry *e, json_t *cfg, struct list *nodes);
 
@@ -109,7 +109,7 @@ int mapping_parse_str(struct mapping_entry *e, const char *str, struct list *nod
 
 int mapping_parse_list(struct list *l, json_t *cfg, struct list *nodes);
 
-int mapping_to_str(struct mapping_entry *me, unsigned index, char **str);
+int mapping_to_str(const struct mapping_entry *me, unsigned index, char **str);
 
 #ifdef __cplusplus
 }
diff --git a/lib/hist.c b/lib/hist.c
index 0d969ef00..ac1b4def7 100644
--- a/lib/hist.c
+++ b/lib/hist.c
@@ -119,22 +119,22 @@ void hist_reset(struct hist *h)
 		memset(h->data, 0, h->length * sizeof(unsigned));
 }
 
-double hist_mean(struct hist *h)
+double hist_mean(const struct hist *h)
 {
 	return (h->total > 0) ? h->_m[0] : NAN;
 }
 
-double hist_var(struct hist *h)
+double hist_var(const struct hist *h)
 {
 	return (h->total > 1) ? h->_s[0] / (h->total - 1) : NAN;
 }
 
-double hist_stddev(struct hist *h)
+double hist_stddev(const struct hist *h)
 {
 	return sqrt(hist_var(h));
 }
 
-void hist_print(struct hist *h, int details)
+void hist_print(const struct hist *h, int details)
 {
 	if (h->total > 0) {
 		hist_cnt_t missed = h->total - h->higher - h->lower;
@@ -159,7 +159,7 @@ void hist_print(struct hist *h, int details)
 		stats("Counted values: %ju", h->total);
 }
 
-void hist_plot(struct hist *h)
+void hist_plot(const struct hist *h)
 {
 	hist_cnt_t max = 1;
 
@@ -200,7 +200,7 @@ void hist_plot(struct hist *h)
 	table_footer(&table);
 }
 
-char * hist_dump(struct hist *h)
+char * hist_dump(const struct hist *h)
 {
 	char *buf = alloc(128);
 
@@ -214,7 +214,7 @@ char * hist_dump(struct hist *h)
 	return buf;
 }
 
-json_t * hist_json(struct hist *h)
+json_t * hist_json(const struct hist *h)
 {
 	json_t *json_buckets, *json_hist;
 
@@ -248,7 +248,7 @@ json_t * hist_json(struct hist *h)
 	return json_hist;
 }
 
-int hist_dump_json(struct hist *h, FILE *f)
+int hist_dump_json(const struct hist *h, FILE *f)
 {
 	json_t *j = hist_json(h);
 
@@ -259,7 +259,7 @@ int hist_dump_json(struct hist *h, FILE *f)
 	return ret;
 }
 
-int hist_dump_matlab(struct hist *h, FILE *f)
+int hist_dump_matlab(const struct hist *h, FILE *f)
 {
 	fprintf(f, "struct(");
 	fprintf(f, "'low', %f, ", h->low);
diff --git a/lib/mapping.c b/lib/mapping.c
index 9db88f2b5..4b7f16269 100644
--- a/lib/mapping.c
+++ b/lib/mapping.c
@@ -240,7 +240,7 @@ out:	json_decref(json_mapping);
 	return ret;
 }
 
-int mapping_update(struct mapping_entry *me, struct sample *remapped, struct sample *original, struct stats *s)
+int mapping_update(const struct mapping_entry *me, struct sample *remapped, const struct sample *original, const struct stats *s)
 {
 	int len = me->length;
 	int off = me->offset;
@@ -257,7 +257,7 @@ int mapping_update(struct mapping_entry *me, struct sample *remapped, struct sam
 
 	switch (me->type) {
 		case MAPPING_TYPE_STATS: {
-			struct hist *h = &s->histograms[me->stats.id];
+			const struct hist *h = &s->histograms[me->stats.id];
 
 			switch (me->stats.type) {
 				case MAPPING_STATS_TYPE_TOTAL:
@@ -288,7 +288,7 @@ int mapping_update(struct mapping_entry *me, struct sample *remapped, struct sam
 		}
 
 		case MAPPING_TYPE_TIMESTAMP: {
-			struct timespec *ts;
+			const struct timespec *ts;
 
 			switch (me->timestamp.type) {
 				case MAPPING_TIMESTAMP_TYPE_RECEIVED:
@@ -347,7 +347,7 @@ int mapping_update(struct mapping_entry *me, struct sample *remapped, struct sam
 	return 0;
 }
 
-int mapping_remap(struct list *m, struct sample *remapped, struct sample *original, struct stats *s)
+int mapping_remap(const struct list *m, struct sample *remapped, const struct sample *original, const struct stats *s)
 {
 	int ret;
 
@@ -366,7 +366,7 @@ int mapping_remap(struct list *m, struct sample *remapped, struct sample *origin
 	return 0;
 }
 
-int mapping_to_str(struct mapping_entry *me, unsigned index, char **str)
+int mapping_to_str(const struct mapping_entry *me, unsigned index, char **str)
 {
 	const char *type;