diff --git a/man2/mprotect.2 b/man2/mprotect.2
index cbbfd5137..866ac7736 100644
--- a/man2/mprotect.2
+++ b/man2/mprotect.2
@@ -38,6 +38,10 @@ mprotect, pkey_mprotect \- set protection on a region of memory
 .B #include <sys/mman.h>
 .PP
 .BI "int mprotect(void *" addr ", size_t " len ", int " prot );
+
+.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
+.B #include <sys/mman.h>
+.PP
 .BI "int pkey_mprotect(void *" addr ", size_t " len ", int " prot ", int " pkey ");
 .fi
 .SH DESCRIPTION
diff --git a/man2/pkey_alloc.2 b/man2/pkey_alloc.2
index ce9bd96cd..d82f316a8 100644
--- a/man2/pkey_alloc.2
+++ b/man2/pkey_alloc.2
@@ -27,6 +27,7 @@
 pkey_alloc, pkey_free \- allocate or free a protection key
 .SH SYNOPSIS
 .nf
+.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
 .B #include <sys/mman.h>
 .PP
 .BI "int pkey_alloc(unsigned int " flags ", unsigned int " access_rights ");"