fix sensor example (doc)

plus add build instructions (in doc/counter_ex.c)

doc/counter_ex.c

+/*
+ * compilation:
+ *
+
+# normally, this part is done with `configure.sh`, but here we need to
+# do this manually
+cat <<EOF > src/sensors.h
 #include "counter_ex.h"
 
-static int acc;
+#define NB_SENSOR 1
+#define NB_SENSOR_OPT 1
+
+void init_sensors(Optparse *opts, Sensor *sensors, size_t len, size_t offset, int *nb_defined)
+{
+    int opt_idx = offset;
+    for (int i = 0; i < counter_ex.nb_opt; i++) {
+        opts[opt_idx++] = counter_ex_opt[i];
+    }
+    sensors[(*nb_defined)++] = counter_ex;
+    assert((offset + *nb_defined) <= len);
+}
+EOF
+
+# idem
+echo 'CAPTOR_OBJ = doc/counters_ex.o' > sensors.mk
+
+# actual compilation here
+gcc -std=gnu99 -Wall -Wpedantic -I./lib -I./doc -I./src -g -Og -c doc/counter_ex.c -o obj/counter_ex.o
+gcc -std=gnu99 -Wall -Wpedantic -I./lib -I./doc -I./src -g -Og -c doc/util.c -o obj/util.o
+gcc -std=gnu99 -Wall -Wpedantic -I./lib -I./doc -I./src -g -Og -c doc/mojitos.c -o obj/mojitos.o
+gcc -std=gnu99 -Wall -Wpedantic -I./lib -I./doc -I./src -g -Og obj/util.o obj/mojitos.o obj/counter_ex.o -o bin/mojitos
 
-unsigned int init_acc(char *a, void **b)
+ *
+**/
+
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "util.h"
+
+#define NB_SENSOR 3
+
+struct accumulator_t {
+    int v[NB_SENSOR];
+};
+
+void _get_acc(int v[NB_SENSOR])
 {
-    acc = 0;
+    for (int i = 0; i < NB_SENSOR; i++) {
+        v[i]++;
+    }
 }
 
-unsigned int get_acc(uint64_t *results, void *none)
+unsigned int init_acc(char *none, void **ptr)
 {
+    /* "none" refers to an optionnal command-line argument */
+    /* there is none in this case, so this parameter is not used */
     UNUSED(none);
-    return a++;
+
+    struct accumulator_t *state = malloc(sizeof(struct accumulator_t));
+
+    for (int i = 0; i < NB_SENSOR; i++) {
+        state->v[i] = -1;
+    }
+
+    *ptr = (void *)state;
+    _get_acc(state->v);
+
+    return NB_SENSOR;
+}
+
+unsigned int get_acc(uint64_t *results, void *ptr)
+{
+    struct accumulator_t *state = (struct accumulator_t *)ptr;
+
+    _get_acc(state->v);
+
+    for (int i = 0; i < NB_SENSOR; i++) {
+        results[i] = state->v[i];
+    }
+
+    return NB_SENSOR;
 }
 
+char *_labels_accumulator[NB_SENSOR] = {"acc1", "acc2", "acc3"};
 void label_acc(char **labels, void *none)
 {
     UNUSED(none);
-    labels[0] = "acc";
+    for (int i = 0; i < NB_SENSOR; i++) {
+        labels[i] = _labels_accumulator[i];
+    }
 }
 
-void clean_acc(void *none)
+void clean_acc(void *ptr)
 {
-    /* That's a no-op for this example */
-    UNUSED(none);
+    struct accumulator_t *state = (struct accumulator_t *)ptr;
+
+    if (state == NULL) {
+        return;
+    }
+
+    free(state);
 }

doc/counter_ex.h

@@ -7,7 +7,7 @@ unsigned int get_acc(uint64_t *results, void *);
 void clean_acc(void *);
 void label_acc(char **labels, void *);
 
-Sensor rapl = {
+Sensor counter_ex = {
     .init = init_acc,
     .get = get_acc,
     .clean = clean_acc,
@@ -15,7 +15,7 @@ Sensor rapl = {
     .nb_opt = 1,
 };
 
-Optparse rapl_opt[1] = {
+Optparse counter_ex_opt[1] = {
     {
         .longname = "accumulator",
         .shortname = 'a',