[dso_demo] Add some non-trivial constructions (e.g. using TLS)

The use of TLS from DSOs presents particularly interesting technical
challenges.

The use of TLS in general triggers a certain problem in Bazel on MacOS
at this point, which can be avoided by compiling with
--dynamic_mode=off or by disabling the dynamic mode feature
entirely (see bazelbuild/bazel#4341 (comment)).

sandbox/BUILD

@@ -25,4 +25,8 @@ cc_binary(
     srcs = ["dso_demo.cc"],
     linkshared = 1,
     linkstatic = 1,
+    deps = [
+        "@com_google_absl//absl/container:flat_hash_map",
+        "@com_google_absl//absl/strings",
+    ],
 )

sandbox/dso_demo.cc

@@ -1,5 +1,23 @@
 #include <cstdio>
+#include <cstdlib>
+#include <string>
+#include <string_view>
 
-extern "C" void demo_function() {
-  std::puts("I am a function.");
+#include "absl/strings/string_view.h"
+#include "absl/container/flat_hash_map.h"
+
+extern "C" void demo_function(std::size_t n, const char* args[]) {
+  std::puts("I am a function. Here come my arguments.");
+
+  absl::flat_hash_map<std::string, std::vector<std::string>> m;
+
+  for (std::size_t i = 0; i != n; ++i) {
+    absl::string_view arg = args[i];
+    m[arg].emplace_back().assign(arg);
+  }
+
+  for (const auto& [k, v] : m) {
+    std::printf("Argument '%*s' appears %zu times.\n",
+                static_cast<int>(k.size()), k.data(), v.size());
+  }
 }

sandbox/dso_loader.cc

@@ -1,10 +1,11 @@
 #include <dlfcn.h>
 
 #include <cstdio>
+#include <cstdlib>
 #include <cstring>
 
 extern "C" {
-using FunPtr = void();
+using FunPtr = void(std::size_t, const char*[]);
 }  // extern "C"
 
 int main(int argc, char* argv[]) {
@@ -23,7 +24,7 @@ int main(int argc, char* argv[]) {
     } else {
       FunPtr* f;
       *reinterpret_cast<void**>(&f) = s;
-      f();
+      f(argc, const_cast<const char**>(argv));
     }
 
     if (dlclose(h) != 0) {