diff --git a/src/args.c b/src/args.c
index 6e873be..0cdd152 100644
--- a/src/args.c
+++ b/src/args.c
@@ -71,17 +71,17 @@ static void print_help(int status_code) {
   exit(status_code);
 }
 
-static void invalid_arg(char *arg) {
+static void invalid_arg(const char *arg) {
   log_error("invalid argument: '%s'", arg);
   print_help(EXIT_FAILURE);
 }
 
-static void invalid_value(char *arg, char *value) {
+static void invalid_value(const char *arg, const char *value) {
   log_error("invalid value for argument '%s': '%s'", arg, value);
   print_help(EXIT_FAILURE);
 }
 
-static bool is_arg(char *arg, const char *ref) {
+static bool is_arg(const char *arg, const char *ref) {
   return strcoll(arg, ref) == 0;
 }
 
@@ -93,7 +93,7 @@ static char *split_arg_value(char *arg) {
   return rest;
 }
 
-static unsigned int parse_uint(char *arg, char *value) {
+static unsigned int parse_uint(const char *arg, const char *value) {
   unsigned long long tmp_value;
 
   if (!string_is_number(value)) {
diff --git a/src/string.c b/src/string.c
index 7239526..4f52680 100644
--- a/src/string.c
+++ b/src/string.c
@@ -39,7 +39,7 @@ unsigned int string_trim(char *str) {
 
 static bool is_digit(char c) { return c >= '0' && c <= '9'; }
 
-bool string_is_number(char *value) {
+bool string_is_number(const char *value) {
   unsigned long value_len;
 
   if (value == NULL) {
@@ -57,8 +57,8 @@ bool string_is_number(char *value) {
   return true;
 }
 
-char *string_replace_at(char *src, unsigned int from, unsigned int to,
-                        char *rpl) {
+char *string_replace_at(const char *src, unsigned int from, unsigned int to,
+                        const char *rpl) {
   unsigned long src_len, rpl_len;
   char *dst;
 
diff --git a/src/string.h b/src/string.h
index 8734b1d..df35d19 100644
--- a/src/string.h
+++ b/src/string.h
@@ -5,9 +5,9 @@
 
 unsigned int string_trim(char *str);
 
-bool string_is_number(char *value);
+bool string_is_number(const char *value);
 
-char *string_replace_at(char *src, unsigned int from, unsigned int to,
-                        char *rpl);
+char *string_replace_at(const char *src, unsigned int from, unsigned int to,
+                        const char *rpl);
 
 #endif /* STRINGS_H */
\ No newline at end of file