diff --git a/src/internal/slibtool_libmeta_impl.c b/src/internal/slibtool_libmeta_impl.c
index 18984bd..a379352 100644
--- a/src/internal/slibtool_libmeta_impl.c
+++ b/src/internal/slibtool_libmeta_impl.c
@@ -4,6 +4,10 @@
 /*  Released under the Standard MIT License; see COPYING.SLIBTOOL. */
 /*******************************************************************/
 
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+
 #include <fcntl.h>
 #include <stdio.h>
 #include <string.h>
diff --git a/src/internal/slibtool_mapfile_impl.c b/src/internal/slibtool_mapfile_impl.c
index a9c1d61..3a086de 100644
--- a/src/internal/slibtool_mapfile_impl.c
+++ b/src/internal/slibtool_mapfile_impl.c
@@ -1,3 +1,7 @@
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+
 #include <fcntl.h>
 #include <errno.h>
 #include <unistd.h>
diff --git a/src/internal/slibtool_objmeta_impl.c b/src/internal/slibtool_objmeta_impl.c
index b38cf75..e723a35 100644
--- a/src/internal/slibtool_objmeta_impl.c
+++ b/src/internal/slibtool_objmeta_impl.c
@@ -4,6 +4,10 @@
 /*  Released under the Standard MIT License; see COPYING.SLIBTOOL. */
 /*******************************************************************/
 
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+
 #include <fcntl.h>
 #include <stdio.h>
 #include <slibtool/slibtool.h>
diff --git a/src/logic/slbt_exec_install.c b/src/logic/slbt_exec_install.c
index 32c1ac6..5f82f7e 100644
--- a/src/logic/slbt_exec_install.c
+++ b/src/logic/slbt_exec_install.c
@@ -4,6 +4,10 @@
 /*  Released under the Standard MIT License; see COPYING.SLIBTOOL. */
 /*******************************************************************/
 
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+
 #include <fcntl.h>
 #include <stdio.h>
 #include <string.h>
diff --git a/src/logic/slbt_exec_link.c b/src/logic/slbt_exec_link.c
index 0a1b5bd..267833e 100644
--- a/src/logic/slbt_exec_link.c
+++ b/src/logic/slbt_exec_link.c
@@ -4,6 +4,10 @@
 /*  Released under the Standard MIT License; see COPYING.SLIBTOOL. */
 /*******************************************************************/
 
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>