diff --git a/c/include/prima/prima.h b/c/include/prima/prima.h
index 9893108747..172e15e545 100644
--- a/c/include/prima/prima.h
+++ b/c/include/prima/prima.h
@@ -73,11 +73,6 @@ typedef enum {
 } prima_rc_t;
 
 
-// Function to get the message string corresponding to a return code
-PRIMAC_API
-const char *prima_get_rc_string(const prima_rc_t rc);
-
-
 /*
  * Objective function required by UOBYQA, NEWUOA, BOBYQA, and LINCOA
  * x     : on input, the vector of variables (should not be modified)
diff --git a/c/include/prima/prima_internal.h b/c/include/prima/prima_internal.h
index 707658c105..f4b8f26e78 100644
--- a/c/include/prima/prima_internal.h
+++ b/c/include/prima/prima_internal.h
@@ -3,12 +3,6 @@
 
 #include "prima/prima.h"
 
-// Function to check whether the problem matches the algorithm
-prima_rc_t prima_check_problem(const prima_problem_t problem, const prima_algorithm_t algorithm);
-
-// Function to initialize the result
-prima_rc_t prima_init_result(prima_result_t *const result, const prima_problem_t problem);
-
 // Functions implemented in Fortran (*_c.f90)
 int cobyla_c(const int m_nlcon, const prima_objcon_t calcfc, const void *data, const int n, double x[], double *const f, double *const cstrv, double nlconstr[],
              const int m_ineq, const double Aineq[], const double bineq[],
diff --git a/c/prima.c b/c/prima.c
index 3fd09acfe1..e6fce82711 100644
--- a/c/prima.c
+++ b/c/prima.c
@@ -63,7 +63,7 @@ prima_rc_t prima_init_options(prima_options_t *const options)
 
 
 // Function to check whether the problem matches the algorithm
-prima_rc_t prima_check_problem(const prima_problem_t problem, const prima_algorithm_t algorithm)
+static prima_rc_t prima_check_problem(const prima_problem_t problem, const prima_algorithm_t algorithm)
 {
     if (!problem.x0)
         return PRIMA_NULL_X0;
@@ -76,7 +76,7 @@ prima_rc_t prima_check_problem(const prima_problem_t problem, const prima_algori
 
 
 // Function to get the string corresponding to the return code
-const char *prima_get_rc_string(const prima_rc_t rc)
+static const char *prima_get_rc_string(const prima_rc_t rc)
 {
     switch (rc) {
         case PRIMA_SMALL_TR_RADIUS:
@@ -130,7 +130,7 @@ const char *prima_get_rc_string(const prima_rc_t rc)
 
 
 // Function to initialize the result
-prima_rc_t prima_init_result(prima_result_t *const result, const prima_problem_t problem)
+static prima_rc_t prima_init_result(prima_result_t *const result, const prima_problem_t problem)
 {
     if (!result)
         return PRIMA_NULL_RESULT;