userland: add some random filesystem and random stuff

Some moved from C++, some moved from SO, some I just made up.

userland/cpp/file_write_read.cpp

@@ -1,4 +1,4 @@
-/* https://cirosantilli.com/linux-kernel-module-cheat#cpp */
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp
 
 #include <lkmc.h>
 

userland/cpp/random.cpp

@@ -0,0 +1,22 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+
+#include <iostream>
+#include <random>
+
+int main(int argc, char **argv) {
+    // Potentially expensive /dev/urandom read that waits for entropy.
+    std::random_device dev;
+    std::mt19937 prng(dev());
+    std::uniform_int_distribution<uint64_t> zero_ull_max(0);
+    std::uniform_int_distribution<uint8_t> one_to_six(1, 6);
+
+    unsigned long long loops = 1;
+    if (argc > 1) {
+        loops = std::stoull(argv[1], NULL, 0);
+    }
+
+    for (auto i = 0U; i < loops; ++i) {
+        std::cout << "zero_to_ull_max 0x" << std::hex << zero_ull_max(prng) << std::endl;
+        std::cout << "one_to_six " << std::dec << one_to_six(prng) << std::endl;
+    }
+}

userland/cpp/temporary_directory.cpp

@@ -0,0 +1,61 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+//
+// ./build disabled for now because of GCC std::filesystem mess.
+// Compile manually with:
+// g++-8 -std=c++17 -o temporary_directory.out temporary_directory.cpp -lstdc++fs
+
+#if __cplusplus >= 201703L
+#include <exception>
+#include <fstream>
+#include <iostream>
+#include <random>
+#include <sstream>
+
+#include <filesystem>
+
+std::filesystem::path create_temporary_directory(unsigned long long max_tries = 100) {
+    auto tmp_dir = std::filesystem::temp_directory_path();
+    unsigned long long i = 0;
+    std::random_device dev;
+    std::mt19937 prng(dev());
+    std::uniform_int_distribution<uint64_t> rand(0);
+    std::filesystem::path path;
+    while (true) {
+        std::stringstream ss;
+        ss << std::hex << rand(prng);
+        path = tmp_dir / ss.str();
+        // true if the directory was created.
+        if (std::filesystem::create_directory(path)) {
+            break;
+        }
+        if (i == max_tries) {
+            throw std::runtime_error("could not find non-existing directory");
+        }
+        i++;
+    }
+    return path;
+}
+
+#endif
+
+int main() {
+#if __cplusplus >= 201703L
+    std::cout << "temp_directory_path() = "
+              << std::filesystem::temp_directory_path()
+              << std::endl;
+
+    auto tmpdir = create_temporary_directory();
+    std::cout << "create_temporary_directory() = "
+              << tmpdir
+              << std::endl;
+
+
+    // Use our temporary directory.
+    std::ofstream ofs(tmpdir / "myfile");
+    ofs << "asdf\nqwer\n";
+    ofs.close();
+
+    // Remove the directory and its contents.
+    std::filesystem::remove_all(tmpdir);
+#endif
+}