diff --git a/lib/src/util/bsd/scoped_ifaddrs.cc b/lib/src/util/bsd/scoped_ifaddrs.cc
index d61d8a4..fbbea6d 100644
--- a/lib/src/util/bsd/scoped_ifaddrs.cc
+++ b/lib/src/util/bsd/scoped_ifaddrs.cc
@@ -1,3 +1,4 @@
+#include <cstddef>
 #include <internal/util/bsd/scoped_ifaddrs.hpp>
 
 using namespace std;
diff --git a/lib/src/util/posix/scoped_addrinfo.cc b/lib/src/util/posix/scoped_addrinfo.cc
index 1dc60b4..bf5bf00 100644
--- a/lib/src/util/posix/scoped_addrinfo.cc
+++ b/lib/src/util/posix/scoped_addrinfo.cc
@@ -1,3 +1,5 @@
+#include <cstddef>
+
 #include <internal/util/posix/scoped_addrinfo.hpp>
 
 using namespace std;
diff --git a/lib/src/util/scoped_bio.cc b/lib/src/util/scoped_bio.cc
index cdf25d6..dfb7f09 100644
--- a/lib/src/util/scoped_bio.cc
+++ b/lib/src/util/scoped_bio.cc
@@ -1,4 +1,5 @@
 #pragma clang diagnostic ignored "-Wdeprecated-declarations"
+#include <cstddef>
 #include <internal/util/scoped_bio.hpp>
 
 using namespace std;
diff --git a/lib/src/util/posix/scoped_descriptor.cc b/lib/src/util/posix/scoped_descriptor.cc
index 3ba2d4e..7a131d3 100644
--- a/lib/src/util/posix/scoped_descriptor.cc
+++ b/lib/src/util/posix/scoped_descriptor.cc
@@ -1,3 +1,4 @@
+#include <cstddef>
 #include <internal/util/posix/scoped_descriptor.hpp>
 
 using namespace std;
diff --git a/lib/src/util/scoped_file.cc b/lib/src/util/scoped_file.cc
index 440d8e3..2369987 100644
--- a/lib/src/util/scoped_file.cc
+++ b/lib/src/util/scoped_file.cc
@@ -1,3 +1,4 @@
+#include <cstddef>
 #include <internal/util/scoped_file.hpp>
 #include <boost/nowide/cstdio.hpp>