diff --git a/psutil/_psutil_linux.c b/psutil/_psutil_linux.c
index a6ee602..56fd4f6 100644
--- a/psutil/_psutil_linux.c
+++ b/psutil/_psutil_linux.c
@@ -24,6 +24,7 @@
 #include <linux/sockios.h>
 #include <linux/if.h>
 #include <sys/resource.h>
+#include <unistd.h>
 
 // see: https://github.com/giampaolo/psutil/issues/659
 #ifdef PSUTIL_ETHTOOL_MISSING_TYPES