diff --git a/src/stdio/asprintf.c b/src/stdio/asprintf.c
index 79e59c2d..4ec83534 100644
--- a/src/stdio/asprintf.c
+++ b/src/stdio/asprintf.c
@@ -1,8 +1,7 @@
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <stdarg.h>
 
-int vasprintf(char **, const char *, va_list);
-
 int asprintf(char **s, const char *fmt, ...)
 {
 	int ret;
diff --git a/src/stdio/vasprintf.c b/src/stdio/vasprintf.c
index f2bbc7aa..68b7246b 100644
--- a/src/stdio/vasprintf.c
+++ b/src/stdio/vasprintf.c
@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <stdarg.h>
 #include <stdlib.h>