diff --git a/src/core/iomgr/resolve_address.c b/src/core/iomgr/resolve_address.c
index 01681168ce4..575f884d91a 100644
--- a/src/core/iomgr/resolve_address.c
+++ b/src/core/iomgr/resolve_address.c
@@ -31,7 +31,9 @@
  *
  */
 
+#ifndef _POSIX_SOURCE
 #define _POSIX_SOURCE
+#endif
 
 #include "src/core/iomgr/sockaddr.h"
 #include "src/core/iomgr/resolve_address.h"
diff --git a/src/core/iomgr/socket_utils_linux.c b/src/core/iomgr/socket_utils_linux.c
index f971cb33bcc..7ef58940c24 100644
--- a/src/core/iomgr/socket_utils_linux.c
+++ b/src/core/iomgr/socket_utils_linux.c
@@ -31,7 +31,9 @@
  *
  */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 #include <grpc/support/port_platform.h>
 
 #ifdef GPR_LINUX
diff --git a/src/core/iomgr/socket_utils_posix.c b/src/core/iomgr/socket_utils_posix.c
index 06c5033d457..9184b2a47cf 100644
--- a/src/core/iomgr/socket_utils_posix.c
+++ b/src/core/iomgr/socket_utils_posix.c
@@ -35,7 +35,6 @@
 
 #ifdef GPR_POSIX_SOCKETUTILS
 
-#define _BSD_SOURCE
 #include "src/core/iomgr/socket_utils_posix.h"
 
 #include <fcntl.h>
diff --git a/src/core/iomgr/tcp_server_posix.c b/src/core/iomgr/tcp_server_posix.c
index d169d232718..091f0aab1a6 100644
--- a/src/core/iomgr/tcp_server_posix.c
+++ b/src/core/iomgr/tcp_server_posix.c
@@ -31,11 +31,15 @@
  *
  */
 
+/* FIXME: "posix" files shouldn't be depending on _GNU_SOURCE */
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <grpc/support/port_platform.h>
 
 #ifdef GPR_POSIX_SOCKET
 
-#define _GNU_SOURCE
 #include "src/core/iomgr/tcp_server.h"
 
 #include <limits.h>
diff --git a/src/core/support/log_linux.c b/src/core/support/log_linux.c
index a0307e1a9a4..a64faa98bd9 100644
--- a/src/core/support/log_linux.c
+++ b/src/core/support/log_linux.c
@@ -31,8 +31,14 @@
  *
  */
 
+#ifndef _POSIX_SOURCE
 #define _POSIX_SOURCE
+#endif
+
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
+
 #include <grpc/support/port_platform.h>
 
 #ifdef GPR_LINUX
diff --git a/src/core/support/log_posix.c b/src/core/support/log_posix.c
index ab2d2e5a740..05f45de1308 100644
--- a/src/core/support/log_posix.c
+++ b/src/core/support/log_posix.c
@@ -31,11 +31,16 @@
  *
  */
 
-#ifndef _POSIX_C_SOURCE
+#if !defined _POSIX_C_SOURCE || _POSIX_C_SOURCE < 200112L
+#undef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200112L
 #endif
 
+/* FIXME: "posix" files probably shouldn't depend on _GNU_SOURCE */
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
+
 #include <grpc/support/port_platform.h>
 
 #if defined(GPR_POSIX_LOG)
diff --git a/src/core/support/string_posix.c b/src/core/support/string_posix.c
index 57832810ad3..a6bb8058e6c 100644
--- a/src/core/support/string_posix.c
+++ b/src/core/support/string_posix.c
@@ -33,7 +33,8 @@
 
 /* Posix code for gpr snprintf support. */
 
-#ifndef _POSIX_C_SOURCE
+#if !defined _POSIX_C_SOURCE || _POSIX_C_SOURCE < 200112L
+#undef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200112L
 #endif
 
diff --git a/test/core/echo/echo_test.c b/test/core/echo/echo_test.c
index 83b83ab7ff7..5450dfbef56 100644
--- a/test/core/echo/echo_test.c
+++ b/test/core/echo/echo_test.c
@@ -31,7 +31,10 @@
  *
  */
 
+#ifndef _POSIX_SOURCE
 #define _POSIX_SOURCE
+#endif
+
 #include <unistd.h>
 #include <assert.h>
 #include <stdio.h>
diff --git a/test/core/fling/fling_stream_test.c b/test/core/fling/fling_stream_test.c
index 7f52fb1bad1..1db2f1a7916 100644
--- a/test/core/fling/fling_stream_test.c
+++ b/test/core/fling/fling_stream_test.c
@@ -31,7 +31,10 @@
  *
  */
 
+#ifndef _POSIX_SOURCE
 #define _POSIX_SOURCE
+#endif
+
 #include <unistd.h>
 #include <assert.h>
 #include <stdio.h>
diff --git a/test/core/fling/fling_test.c b/test/core/fling/fling_test.c
index b2272f20c8e..4f41a21aaaf 100644
--- a/test/core/fling/fling_test.c
+++ b/test/core/fling/fling_test.c
@@ -31,7 +31,10 @@
  *
  */
 
+#ifndef _POSIX_SOURCE
 #define _POSIX_SOURCE
+#endif
+
 #include <unistd.h>
 #include <assert.h>
 #include <stdio.h>