diff --git a/psutil/_psutil_sunos.c b/psutil/_psutil_sunos.c
index 11335993f..54f353c10 100644
--- a/psutil/_psutil_sunos.c
+++ b/psutil/_psutil_sunos.c
@@ -51,6 +51,7 @@
 #include <arpa/inet.h>
 #include <net/if.h>
 #include <math.h> // fabs()
+#include <unistd.h>
 
 #include "_psutil_common.h"
 #include "_psutil_posix.h"
diff --git a/psutil/arch/linux/net.c b/psutil/arch/linux/net.c
index 522a55dfc..d193e9408 100644
--- a/psutil/arch/linux/net.c
+++ b/psutil/arch/linux/net.c
@@ -10,6 +10,7 @@
 #include <sys/ioctl.h>
 #include <linux/if.h>
 #include <linux/version.h>
+#include <unistd.h>
 
 // see: https://github.com/giampaolo/psutil/issues/659
 #ifdef PSUTIL_ETHTOOL_MISSING_TYPES
diff --git a/psutil/arch/linux/proc.c b/psutil/arch/linux/proc.c
index ac87af917..b58a3ce2a 100644
--- a/psutil/arch/linux/proc.c
+++ b/psutil/arch/linux/proc.c
@@ -7,6 +7,7 @@
 #include <Python.h>
 #include <sys/syscall.h>
 #include <sched.h>
+#include <unistd.h>
 
 #include "proc.h"
 #include "../../_psutil_common.h"
diff --git a/psutil/arch/solaris/environ.c b/psutil/arch/solaris/environ.c
index dd627eb00..4b4e041a4 100644
--- a/psutil/arch/solaris/environ.c
+++ b/psutil/arch/solaris/environ.c
@@ -19,6 +19,7 @@
 #include <sys/procfs.h>
 #include <sys/stat.h>
 #include <fcntl.h>
+#include <unistd.h>
 
 #include "environ.h"