diff --git a/tests/ctest_lib.c b/tests/ctest_lib.c
index ae9c9542..176a7df2 100644
--- a/tests/ctest_lib.c
+++ b/tests/ctest_lib.c
@@ -11,6 +11,7 @@
 #endif
 
 #include <stdio.h>
+#include <stdarg.h>
 
 #include "ctest_lib.h"
 
@@ -55,6 +56,18 @@ void ctest_diagnostic(const char *line)
     printf("# %s\n", line);
 }
 
+void ctest_diagnostic_printf(const char *format, ...)
+{
+    va_list ap;
+    va_start(ap, format);
+
+    printf("# ");
+    vprintf(format, ap);
+    printf("\n");
+
+    va_end(ap);
+}
+
 void ctest_bail_out(const char *reason)
 {
     ctest_g_bailed_out = 1;
diff --git a/tests/ctest_lib.h b/tests/ctest_lib.h
index 3a7072bd..69eb22ab 100644
--- a/tests/ctest_lib.h
+++ b/tests/ctest_lib.h
@@ -14,6 +14,7 @@ void ctest_fin(void);
 
 void ctest_test(const char *desc, int res);
 void ctest_diagnostic(const char *line);
+void ctest_diagnostic_printf(const char *format, ...);
 void ctest_bail_out(const char *reason);
 int  ctest_bailed_out(void);