From b96ecd621a269a100805474709ea0c359f3a4e58 Mon Sep 17 00:00:00 2001
From: Chun-Chieh Li <ccli8@nuvoton.com>
Date: Thu, 24 Oct 2024 10:57:47 +0800
Subject: [PATCH] Support WiFi as default network interface

This requires support for weak symbol override. Object files arcived in
static library don't always participate in linking, dependent on linker
and link order. The steps below pull in the override object file anyway
even though it comes from static library:

1. Add e.g. GCC linker option "--undefined=<LINK_FOO>"
2. Add <LINK_FOO> function with 'extern "C"' in override source file

See: https://stackoverflow.com/questions/42588983/what-does-the-gnu-ld-undefined-option-do
---
 .../COMPONENT_ESPRESSIF_ESP8266/CMakeLists.txt     | 10 ++++++++++
 .../ESP8266Interface.cpp                           | 14 ++++++++++++++
 .../TARGET_STM/COMPONENT_EMW3080B/CMakeLists.txt   | 10 ++++++++++
 .../COMPONENT_EMW3080B/EMW3080BInterface.cpp       | 14 ++++++++++++++
 4 files changed, 48 insertions(+)

diff --git a/connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/CMakeLists.txt b/connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/CMakeLists.txt
index 682139f0e84..b0d5e9fbd52 100644
--- a/connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/CMakeLists.txt
+++ b/connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/CMakeLists.txt
@@ -12,3 +12,13 @@ target_include_directories(mbed-wifi
         .
         ./ESP8266
 )
+
+# Link override object file from static library
+if("MBED_CONF_ESP8266_PROVIDE_DEFAULT=1" IN_LIST MBED_CONFIG_DEFINITIONS)
+    if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
+        target_link_options(mbed-wifi
+            INTERFACE
+                LINKER:--undefined=LINK_ESP8266INTERFACE_CPP
+        )
+    endif()
+endif()
diff --git a/connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/ESP8266Interface.cpp b/connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/ESP8266Interface.cpp
index ed040325cea..f8eb93f9b5e 100644
--- a/connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/ESP8266Interface.cpp
+++ b/connectivity/drivers/wifi/COMPONENT_ESPRESSIF_ESP8266/ESP8266Interface.cpp
@@ -1149,6 +1149,20 @@ WiFiInterface *WiFiInterface::get_default_instance()
     return &esp;
 }
 
+/*
+ * With e.g. GCC linker option "--undefined=<LINK_FOO>", pull in this
+ * object file anyway for being able to override weak symbol successfully
+ * even though from static library. See:
+ * https://stackoverflow.com/questions/42588983/what-does-the-gnu-ld-undefined-option-do
+ *
+ * NOTE: For C++ name mangling, 'extern "C"' is necessary to match the
+ *       <LINK_FOO> symbol correctly.
+ */
+extern "C"
+void LINK_ESP8266INTERFACE_CPP(void)
+{
+}
+
 #endif
 
 void ESP8266Interface::refresh_conn_state_cb()
diff --git a/connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/CMakeLists.txt b/connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/CMakeLists.txt
index a91dc52b119..8dda87839c6 100644
--- a/connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/CMakeLists.txt
+++ b/connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/CMakeLists.txt
@@ -21,3 +21,13 @@ target_sources(mbed-wifi
         mx_wifi/core/mx_wifi_ipc.c
         mx_wifi/core/mx_wifi_slip.c
 )
+
+# Link override object file from static library
+if("MBED_CONF_EMW3080B_PROVIDE_DEFAULT=1" IN_LIST MBED_CONFIG_DEFINITIONS)
+    if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
+        target_link_options(mbed-wifi
+            INTERFACE
+                LINKER:--undefined=LINK_EMW3080BINTERFACE_CPP
+        )
+    endif()
+endif()
diff --git a/connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/EMW3080BInterface.cpp b/connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/EMW3080BInterface.cpp
index c59407d7ffc..1773b9282e5 100644
--- a/connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/EMW3080BInterface.cpp
+++ b/connectivity/drivers/wifi/TARGET_STM/COMPONENT_EMW3080B/EMW3080BInterface.cpp
@@ -352,6 +352,20 @@ WiFiInterface *WiFiInterface::get_default_instance()
     static EMW3080BInterface emw;
     return &emw;
 }
+
+/*
+ * With e.g. GCC linker option "--undefined=<LINK_FOO>", pull in this
+ * object file anyway for being able to override weak symbol successfully
+ * even though from static library. See:
+ * https://stackoverflow.com/questions/42588983/what-does-the-gnu-ld-undefined-option-do
+ *
+ * NOTE: For C++ name mangling, 'extern "C"' is necessary to match the
+ *       <LINK_FOO> symbol correctly.
+ */
+extern "C"
+void LINK_EMW3080BINTERFACE_CPP(void)
+{
+}
 #endif /* MBED_CONF_EMW3080B_PROVIDE_DEFAULT */