diff --git a/c/include/prima/prima_internal.h b/c/include/prima/prima_internal.h
new file mode 100644
index 0000000000..c577ada223
--- /dev/null
+++ b/c/include/prima/prima_internal.h
@@ -0,0 +1,38 @@
+#ifndef PRIMA_INTERNAL_H
+#define PRIMA_INTERNAL_H
+
+#include "prima/prima.h"
+
+unsigned int get_random_seed(void);
+
+// 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[],
+             const int m_eq, const double Aeq[], const double beq[],
+             const double xl[], const double xu[],
+             const double f0, const double nlconstr0[],
+             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int iprint, const double ctol,
+             const prima_callback_t callback, int *const info);
+
+int bobyqa_c(prima_obj_t calfun, const void *data, const int n, double x[], double *const f, const double xl[], const double xu[],
+             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int npt, const int iprint, const prima_callback_t callback, int *const info);
+
+int newuoa_c(prima_obj_t calfun, const void *data, const int n, double x[], double *const f,
+             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int npt, const int iprint, const prima_callback_t callback, int *const info);
+
+int uobyqa_c(prima_obj_t calfun, const void *data, const int n, double x[], double *const f,
+             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int iprint, const prima_callback_t callback, int *const info);
+
+int lincoa_c(prima_obj_t calfun, const void *data, const int n, double x[], double *const f,
+             double *const cstrv, const int m_ineq, const double Aineq[], const double bineq[],
+             const int m_eq, const double Aeq[], const double beq[], const double xl[], const double xu[],
+             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int npt, const int iprint, const double ctol,
+             const prima_callback_t callback, int *const info);
+
+#endif
diff --git a/c/prima.c b/c/prima.c
index bfa45e02d7..188abe91e5 100644
--- a/c/prima.c
+++ b/c/prima.c
@@ -1,7 +1,7 @@
 // Dedicated to the late Professor M. J. D. Powell FRS (1936--2015).
 
 
-#include "prima/prima.h"
+#include "prima/prima_internal.h"
 #include <limits.h>
 #include <math.h>
 #include <stdio.h>
@@ -203,32 +203,6 @@ const char *prima_get_rc_string(const prima_rc_t rc)
     }
 }
 
-
-// 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[],
-             const int m_eq, const double Aeq[], const double beq[],
-             const double xl[], const double xu[],
-             const double f0, const double nlconstr0[],
-             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int iprint, const double ctol,
-             const prima_callback_t callback, int *const info);
-
-int bobyqa_c(prima_obj_t calfun, const void *data, const int n, double x[], double *const f, const double xl[], const double xu[],
-             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int npt, const int iprint, const prima_callback_t callback, int *const info);
-
-int newuoa_c(prima_obj_t calfun, const void *data, const int n, double x[], double *const f,
-             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int npt, const int iprint, const prima_callback_t callback, int *const info);
-
-int uobyqa_c(prima_obj_t calfun, const void *data, const int n, double x[], double *const f,
-             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int iprint, const prima_callback_t callback, int *const info);
-
-int lincoa_c(prima_obj_t calfun, const void *data, const int n, double x[], double *const f,
-             double *const cstrv, const int m_ineq, const double Aineq[], const double bineq[],
-             const int m_eq, const double Aeq[], const double beq[], const double xl[], const double xu[],
-             int *const nf, const double rhobeg, const double rhoend, const double ftarget, const int maxfun, const int npt, const int iprint, const double ctol,
-             const prima_callback_t callback, int *const info);
-
-
 // The function that does the minimization using a PRIMA solver
 prima_rc_t prima_minimize(const prima_algorithm_t algorithm, const prima_problem_t problem, const prima_options_t options, prima_result_t *const result)
 {
diff --git a/c/tests/stress.c b/c/tests/stress.c
index d66d685046..ccd7f44a3a 100644
--- a/c/tests/stress.c
+++ b/c/tests/stress.c
@@ -7,7 +7,14 @@
 #include <time.h>
 
 // Make PRIMA available
-#include "prima/prima.h"
+#include "prima/prima_internal.h"
+
+// Thread-safe version of localtime
+#ifdef _WIN32
+#define localtime_safe(a, b) localtime_s(a, b)
+#else
+#define localtime_safe(a, b) localtime_r(b, a)
+#endif
 
 #define MIN(x, y) (((x) < (y)) ? (x) : (y))
 #define N_MAX 2000
@@ -65,8 +72,9 @@ unsigned int get_random_seed(void) {
     // Set the random seed to year/week
     char buf[10] = {0};
     time_t t = time(NULL);
-    struct tm *tmp = localtime(&t);
-    int rc = strftime(buf, 10, "%y%W", tmp);
+    struct tm timeinfo;
+    localtime_safe(&timeinfo, &t);
+    int rc = strftime(buf, 10, "%y%W", &timeinfo);
     if (!rc)
         return 42;
     else
@@ -87,7 +95,7 @@ int main(int argc, char * argv[])
     printf("Debug = %d\n", debug);
 
     unsigned int seed = get_random_seed();
-    printf("Random seed = %d\n", seed);
+    printf("Random seed = %u\n", seed);
     srand(seed);
 
     // Set up the options