diff --git a/configure b/configure
index b56575d7b7..78347b1dad 100755
--- a/configure
+++ b/configure
@@ -3564,6 +3564,7 @@ echores "$_vsscanf"
 
 echocheck "swab()"
 cat > $TMPC << EOF
+#define _XOPEN_SOURCE
 #include <unistd.h>
 int main(void) { swab(0, 0, 0); return 0; }
 EOF
diff --git a/libmpcodecs/ad_hwac3.c b/libmpcodecs/ad_hwac3.c
index 1757018796..d58de77610 100644
--- a/libmpcodecs/ad_hwac3.c
+++ b/libmpcodecs/ad_hwac3.c
@@ -5,6 +5,7 @@
    (see http://www.dtek.chalmers.se/~dvd/)
 */
 
+#define _XOPEN_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
diff --git a/libmpcodecs/ad_liba52.c b/libmpcodecs/ad_liba52.c
index d886ad6a6a..73b43dcaab 100644
--- a/libmpcodecs/ad_liba52.c
+++ b/libmpcodecs/ad_liba52.c
@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>