diff --git a/libavutil/ppc/cpu.c b/libavutil/ppc/cpu.c
index 85c1f2ca11c84cead5251aa2e277ff11cb374f40..6b122ed8c4347a4ae52554766b6ed5d40d8d08c3 100644
--- a/libavutil/ppc/cpu.c
+++ b/libavutil/ppc/cpu.c
@@ -20,6 +20,7 @@
 #undef _POSIX_C_SOURCE
 #include <sys/sysctl.h>
 #elif defined(__OpenBSD__)
+#undef _POSIX_C_SOURCE
 #include <sys/param.h>
 #include <sys/sysctl.h>
 #include <machine/cpu.h>