extern enum iostat_mode_t iostat_mode;
 
-typedef void (*iostat_print_counter_t)(struct perf_stat_config *, struct evsel *, char *);
+typedef void (*iostat_print_counter_t)(struct perf_stat_config *, struct evsel *, const char *);
 
 int iostat_prepare(struct evlist *evlist, struct perf_stat_config *config);
 int iostat_parse(const struct option *opt, const char *str,
 
 
 static void printout(struct perf_stat_config *config, struct aggr_cpu_id id, int nr,
                     struct evsel *counter, double uval,
-                    char *prefix, u64 run, u64 ena, double noise,
+                    const char *prefix, u64 run, u64 ena, double noise,
                     struct runtime_stat *st, int map_idx)
 {
        struct perf_stat_output_ctx out;
 
 static void print_counter_aggrdata(struct perf_stat_config *config,
                                   struct evsel *counter, int s,
-                                  char *prefix)
+                                  const char *prefix)
 {
        FILE *output = config->output;
        u64 ena, run, val;
 
 static void print_metric_begin(struct perf_stat_config *config,
                               struct evlist *evlist,
-                              char *prefix, int aggr_idx,
+                              const char *prefix, int aggr_idx,
                               struct cgroup *cgrp)
 {
        struct perf_stat_aggr *aggr;
 
 static void print_aggr(struct perf_stat_config *config,
                       struct evlist *evlist,
-                      char *prefix)
+                      const char *prefix)
 {
        struct evsel *counter;
        int s;
 
 static void print_aggr_cgroup(struct perf_stat_config *config,
                              struct evlist *evlist,
-                             char *prefix)
+                             const char *prefix)
 {
        struct evsel *counter, *evsel;
        struct cgroup *cgrp = NULL;
 }
 
 static void print_counter(struct perf_stat_config *config,
-                         struct evsel *counter, char *prefix)
+                         struct evsel *counter, const char *prefix)
 {
        int s;
 
 
 static void print_no_aggr_metric(struct perf_stat_config *config,
                                 struct evlist *evlist,
-                                char *prefix)
+                                const char *prefix)
 {
        int all_idx;
        struct perf_cpu cpu;
 }
 
 static void print_percore(struct perf_stat_config *config,
-                         struct evsel *counter, char *prefix)
+                         struct evsel *counter, const char *prefix)
 {
        bool metric_only = config->metric_only;
        FILE *output = config->output;
 }
 
 static void print_cgroup_counter(struct perf_stat_config *config, struct evlist *evlist,
-                                char *prefix)
+                                const char *prefix)
 {
        struct cgroup *cgrp = NULL;
        struct evsel *counter;