diff --git a/caca/figfont.c b/caca/figfont.c
index caa44d5..815c5c4 100644
--- a/caca/figfont.c
+++ b/caca/figfont.c
@@ -30,8 +30,12 @@
 #include "caca_internals.h"
 
 #if defined _WIN32 && defined __GNUC__ && __GNUC__ >= 3
+#   if !HAVE_SPRINTF_S
 int sprintf_s(char *s, size_t n, const char *fmt, ...) CACA_WEAK;
+#   endif
+#   if !HAVE_VSNPRINTF
 int vsnprintf(char *s, size_t n, const char *fmt, va_list ap) CACA_WEAK;
+#   endif
 #endif
 
 struct caca_charfont
@@ -637,6 +641,7 @@ static uint32_t hsmush(uint32_t ch1, uint32_t ch2, int rule)
  */
 
 #if defined _WIN32 && defined __GNUC__ && __GNUC__ >= 3
+#   if !HAVE_SPRINTF_S
 int sprintf_s(char *s, size_t n, const char *fmt, ...)
 {
     va_list args;
@@ -646,11 +651,14 @@ int sprintf_s(char *s, size_t n, const char *fmt, ...)
     va_end(args);
     return ret;
 }
+#   endif
 
+#   if !HAVE_VSNPRINTF
 int vsnprintf(char *s, size_t n, const char *fmt, va_list ap)
 {
     return 0;
 }
+#   endif
 #endif
 
 /*
diff --git a/caca/string.c b/caca/string.c
index 22e0af2..963e349 100644
--- a/caca/string.c
+++ b/caca/string.c
@@ -37,9 +37,13 @@
 #include "caca_internals.h"
 
 #if defined _WIN32 && defined __GNUC__ && __GNUC__ >= 3
+#   if !HAVE_VSNPRINTF_S
 int vsnprintf_s(char *s, size_t n, size_t c,
                 const char *fmt, va_list ap) CACA_WEAK;
+#   endif
+#   if !HAVE_VSNPRINTF
 int vsnprintf(char *s, size_t n, const char *fmt, va_list ap) CACA_WEAK;
+#   endif
 #endif
 
 /** \brief Set cursor position.
@@ -611,15 +615,19 @@ int caca_set_canvas_boundaries(caca_canvas_t *cv, int x, int y, int w, int h)
  */
 
 #if defined _WIN32 && defined __GNUC__ && __GNUC__ >= 3
+#   if !HAVE_VSNPRINTF_S
 int vsnprintf_s(char *s, size_t n, size_t c, const char *fmt, va_list ap)
 {
     return vsnprintf(s, n, fmt, ap);
 }
+#   endif
 
+#   if !HAVE_VSNPRINTF
 int vsnprintf(char *s, size_t n, const char *fmt, va_list ap)
 {
     return 0;
 }
+#   endif
 #endif
 
 /*