diff --git a/ChangeLog.d/posix-define.txt b/ChangeLog.d/posix-define.txt
new file mode 100644
index 0000000000..98cf2d0122
--- /dev/null
+++ b/ChangeLog.d/posix-define.txt
@@ -0,0 +1,6 @@
+Bugfix
+   * In library/net_sockets.c, _POSIX_C_SOURCE and _XOPEN_SOURCE are
+     defined to specific values.  If the code is used in a context
+     where these are already defined, this can result in a compilation
+     error.  Instead, assume that if they are defined, the values will
+     be adequate to build Mbed TLS.
diff --git a/library/net_sockets.c b/library/net_sockets.c
index 671115f15f..3874783750 100644
--- a/library/net_sockets.c
+++ b/library/net_sockets.c
@@ -47,11 +47,15 @@
 /* Enable definition of getaddrinfo() even when compiling with -std=c99. Must
  * be set before config.h, which pulls in glibc's features.h indirectly.
  * Harmless on other platforms. */
+#ifndef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200112L
+#endif
 
 #if defined(__NetBSD__)
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 600 /* sockaddr_storage */
 #endif
+#endif
 
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"