From a3befc6dcd67dbfb087d20a164e1b153025c4e32 Mon Sep 17 00:00:00 2001
From: Allen Webster <yoyo4thdimention@gmail.com>
Date: Tue, 23 Apr 2024 12:50:20 -0700
Subject: [PATCH] linux_load_time example

---
 linux_load_time/build.sh       | 26 +++++++++++++++++++++++
 linux_load_time/linux_base.c   | 11 ++++++++++
 linux_load_time/linux_main.c   | 38 ++++++++++++++++++++++++++++++++++
 linux_load_time/linux_plugin.c | 17 +++++++++++++++
 win32_load_time/build.bat      |  6 ++++--
 5 files changed, 96 insertions(+), 2 deletions(-)
 create mode 100644 linux_load_time/build.sh
 create mode 100644 linux_load_time/linux_base.c
 create mode 100644 linux_load_time/linux_main.c
 create mode 100644 linux_load_time/linux_plugin.c

diff --git a/linux_load_time/build.sh b/linux_load_time/build.sh
new file mode 100644
index 0000000..3e659a8
--- /dev/null
+++ b/linux_load_time/build.sh
@@ -0,0 +1,26 @@
+#!/bin/bash
+
+# Path setup
+src=$PWD
+cd ..
+mkdir -p build
+cd build
+
+
+#### THE ARRANGEMENT OF THIS BUILD ####
+# The "base layer" must be built and shared by all dynamic components.
+# In this example the the base layer is linked via load-time linking with the other modules.
+# The "plugin" is loaded at run-time.
+
+
+# Build the base layer as a .so
+gcc -fvisibility=hidden -fPIC -shared $src/linux_base.c -o linux_base.so
+
+
+# Build the executable; provide the base layer's load-time linking data via linux_base.so
+gcc -fvisibility=hidden $src/linux_main.c linux_base.so -o linux_main -Wl,-rpath,\$ORIGIN/
+
+
+# Build the plugin as a .so; provide the base layer's load-time linking data via linux_base.so
+gcc -fvisibility=hidden -fPIC -shared $src/linux_plugin.c linux_base.so -o linux_plugin.so -Wl,-rpath,\$ORIGIN/
+
diff --git a/linux_load_time/linux_base.c b/linux_load_time/linux_base.c
new file mode 100644
index 0000000..45059fb
--- /dev/null
+++ b/linux_load_time/linux_base.c
@@ -0,0 +1,11 @@
+// declare base_func; mark it as an exported symbol
+__attribute__ ((visibility ("default"))) void base_func(void);
+
+// define base_func
+#include <stdio.h>
+int x = 0;
+__attribute__ ((visibility ("default"))) void
+base_func(void){
+  printf("x = %d\n", x);
+  x += 1;
+}
diff --git a/linux_load_time/linux_main.c b/linux_load_time/linux_main.c
new file mode 100644
index 0000000..e2095fc
--- /dev/null
+++ b/linux_load_time/linux_main.c
@@ -0,0 +1,38 @@
+// declare base_func; no mark -> load-time imported symbol
+void base_func(void);
+
+
+// setup plugin_func as a function pointer so that it can be linked at run time
+typedef void PLUGIN_Func(void);
+PLUGIN_Func *plugin_func = 0;
+
+
+// need to include for dlopen and dlsym
+#include <dlfcn.h>
+
+// helper macro for dlsym that handles the casting of function pointers
+#define GET_PROC(var,mod,name) *((void**)(&(var))) = dlsym((mod),(name))
+
+
+int main(){
+  
+  // functions linked by load-time linking can be called directly without further setup
+  base_func();
+  
+  
+  // to call a function with run-time linking, we must manually load and link it
+  void *module = dlopen("./linux_plugin.so", RTLD_NOW);
+  if (module != 0){
+    GET_PROC(plugin_func, module, "plugin_func");
+  }
+  
+  
+  // calls to plugin_func only work after the "plugin" loaded successfully
+  if (plugin_func != 0){
+    plugin_func();
+  }
+  
+  
+  // demonstration: the data structures contained in the 'base' are not duplicated in each binary
+  base_func();
+}
diff --git a/linux_load_time/linux_plugin.c b/linux_load_time/linux_plugin.c
new file mode 100644
index 0000000..2a34763
--- /dev/null
+++ b/linux_load_time/linux_plugin.c
@@ -0,0 +1,17 @@
+// declare base_func; no mark -> load-time imported symbol
+void base_func(void);
+
+// declare base_func; mark it as an exported symbol
+__attribute__ ((visibility ("default"))) void plugin_func(void);
+
+// define plugin_func
+#include <stdio.h>
+__attribute__ ((visibility ("default"))) void
+plugin_func(void){
+  printf("provided by plugin: {\n");
+  printf(" ");
+  base_func();
+  printf(" ");
+  base_func();
+  printf("}\n");
+}
diff --git a/win32_load_time/build.bat b/win32_load_time/build.bat
index d7e5898..2bfbb0a 100644
--- a/win32_load_time/build.bat
+++ b/win32_load_time/build.bat
@@ -1,12 +1,14 @@
 @echo off
 
-SET src=%cd%
+REM: Path setup
 
+SET src=%cd%
 cd ..
 if not exist "build\" mkdir build
 cd build
 
-REM: ## The arrangement of this build ##
+
+REM: ### THE ARRANGEMENT OF THIS BUILD ###
 REM:  The "base layer" must be built and shared by all dynamic components.
 REM:  In this example the the base layer is linked via load-time linking with the other modules.
 REM:  The "plugin" is loaded at run-time.