diff --git a/ChangeLog b/ChangeLog
index 517381bc5..4d5f5829f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,7 @@ Changes
    * Allow overriding the time on Windows via the platform-time abstraction.
      Fixed by Nick Wilson.
    * Use gmtime_r/gmtime_s for thread-safety. Fixed by Nick Wilson.
+   * Fix build using -std=c99. Fixed by Nick Wilson.
 
 = mbed TLS 2.11.0 branch released 2018-06-18
 
diff --git a/library/entropy_poll.c b/library/entropy_poll.c
index fd96258ce..31f608b83 100644
--- a/library/entropy_poll.c
+++ b/library/entropy_poll.c
@@ -19,6 +19,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+#if defined(__linux__)
+/* Ensure that syscall() is available even when compiling with -std=c99 */
+#define _GNU_SOURCE
+#endif
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/library/net_sockets.c b/library/net_sockets.c
index 202da0171..4b267cf35 100644
--- a/library/net_sockets.c
+++ b/library/net_sockets.c
@@ -19,6 +19,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/* 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. */
+#define _POSIX_C_SOURCE 200112L
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/library/x509.c b/library/x509.c
index 906d1714b..b47599b0d 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -29,6 +29,10 @@
  *  http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
  */
 
+/* Ensure gmtime_r is available even with -std=c99; must be included before
+ * config.h, which pulls in glibc's features.h. Harmless on other platforms. */
+#define _XOPEN_SOURCE 500
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/programs/aes/aescrypt2.c b/programs/aes/aescrypt2.c
index 4acf38dd7..36dabe940 100644
--- a/programs/aes/aescrypt2.c
+++ b/programs/aes/aescrypt2.c
@@ -19,6 +19,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/* Enable definition of fileno() even when compiling with -std=c99. Must be
+ * set before config.h, which pulls in glibc's features.h indirectly.
+ * Harmless on other platforms. */
+#define _POSIX_C_SOURCE 1
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/programs/aes/crypt_and_hash.c b/programs/aes/crypt_and_hash.c
index 0e272ebe4..49c43b321 100644
--- a/programs/aes/crypt_and_hash.c
+++ b/programs/aes/crypt_and_hash.c
@@ -20,6 +20,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/* Enable definition of fileno() even when compiling with -std=c99. Must be
+ * set before config.h, which pulls in glibc's features.h indirectly.
+ * Harmless on other platforms. */
+#define _POSIX_C_SOURCE 1
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/programs/ssl/ssl_mail_client.c b/programs/ssl/ssl_mail_client.c
index 04b847a69..74d5d7270 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -19,6 +19,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/* Enable definition of hostname() even when compiling with -std=c99. Must be
+ * set before config.h, which pulls in glibc's features.h indirectly.
+ * Harmless on other platforms. */
+#define _POSIX_C_SOURCE 200112L
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index f630edb83..084da59f4 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -15,6 +15,11 @@ if(NOT PERL_FOUND)
     message(FATAL_ERROR "Cannot build test suites without Perl")
 endif()
 
+# Enable definition of various functions used throughout the testsuite
+# (hostname, strdup, fileno...) even when compiling with -std=c99. Harmless
+# on non-POSIX platforms.
+add_definitions("-D_POSIX_C_SOURCE=200809L")
+
 function(add_test_suite suite_name)
     if(ARGV1)
         set(data_name ${ARGV1})
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index f82694ada..8f04885a5 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -36,6 +36,7 @@ typedef UINT32 uint32_t;
 
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 #include <unistd.h>
+#include <strings.h>
 #endif
 
 /*----------------------------------------------------------------------------*/