diff --git a/crypto/bio/bio_test.c b/crypto/bio/bio_test.c
index fd88b55f..86363f9c 100644
--- a/crypto/bio/bio_test.c
+++ b/crypto/bio/bio_test.c
@@ -12,7 +12,9 @@
  * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
 
+#if !defined(_POSIX_C_SOURCE)
 #define _POSIX_C_SOURCE 201410L
+#endif
 
 #include <openssl/base.h>
 
diff --git a/crypto/lhash/lhash_test.c b/crypto/lhash/lhash_test.c
index e47e96fa..cf5e99b6 100644
--- a/crypto/lhash/lhash_test.c
+++ b/crypto/lhash/lhash_test.c
@@ -12,7 +12,9 @@
  * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
 
+#if !defined(_POSIX_C_SOURCE)
 #define _POSIX_C_SOURCE 201410L
+#endif
 
 #include <openssl/crypto.h>
 #include <openssl/lhash.h>
diff --git a/crypto/mem.c b/crypto/mem.c
index b16b3104..11ef3352 100644
--- a/crypto/mem.c
+++ b/crypto/mem.c
@@ -53,7 +53,9 @@
  * copied and put under another distribution licence
  * [including the GNU Public Licence.] */
 
+#if !defined(_POSIX_C_SOURCE)
 #define _POSIX_C_SOURCE 201410L  /* needed for strdup, snprintf, vprintf etc */
+#endif
 
 #include <openssl/mem.h>
 
diff --git a/crypto/time_support.c b/crypto/time_support.c
index 4537e82b..bbfe3036 100644
--- a/crypto/time_support.c
+++ b/crypto/time_support.c
@@ -55,7 +55,9 @@
  * (eay@cryptsoft.com).  This product includes software written by Tim
  * Hudson (tjh@cryptsoft.com). */
 
+#if !defined(_POSIX_C_SOURCE)
 #define _POSIX_C_SOURCE 201410L  /* for gmtime_r */
+#endif
 
 #include <openssl/time_support.h>