userland: move more multithreading from cpp-cheat!

Convert infinite_loop.c into loop.c. Keep all examples fast by default!

userland/c/atomic.c

@@ -0,0 +1,50 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#c-multithreading */
+
+#if __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
+#include <stdatomic.h>
+#include <stdio.h>
+#include <threads.h>
+#include <string.h>
+#include <stdlib.h>
+
+atomic_int acnt;
+int cnt;
+size_t niters;
+
+int f(void *thr_data) {
+    (void)thr_data;
+    for (size_t i = 0; i < niters; ++i) {
+        ++cnt;
+        ++acnt;
+    }
+    return 0;
+}
+#endif
+
+int main(int argc, char **argv) {
+#if __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
+    size_t nthreads;
+    thrd_t *threads;
+    if (argc > 1) {
+        nthreads = strtoull(argv[1], NULL, 0);
+    } else {
+        nthreads = 2;
+    }
+    if (argc > 2) {
+        niters = strtoull(argv[2], NULL, 0);
+    } else {
+        niters = 10;
+    }
+    threads = malloc(sizeof(thrd_t) * nthreads);
+    for(size_t i = 0; i < nthreads; ++i)
+        thrd_create(threads + i, f, NULL);
+    for(size_t i = 0; i < nthreads; ++i)
+        thrd_join(threads[i], NULL);
+    free(threads);
+    printf("atomic %u\n", acnt);
+    printf("non-atomic %u\n", cnt);
+#else
+    (void)argc;
+    (void)argv;
+#endif
+}

userland/c/loop.c

@@ -3,7 +3,7 @@
  * Loop and print an integer whenever a period is reached:
  *
  * ....
- * ./infinite_loop [period=100000000 [max=0]]
+ * ./loop.out [max=10 [period=1]]
  * ....
  *
  * * period: period for printing integers to stdout
@@ -19,27 +19,25 @@
 
 int main(int argc, char **argv) {
     uintmax_t i, j, period, max;
-    int max_given;
     if (argc > 1) {
-        period = strtoumax(argv[1], NULL, 10);
+        max = strtoumax(argv[1], NULL, 10);
     } else {
-        period = 100000000;
+        max = 10;
     }
     if (argc > 2) {
-        max = strtoumax(argv[2], NULL, 10);
-        max_given = 1;
+        period = strtoumax(argv[2], NULL, 10);
     } else {
-        max_given = 0;
+        period = 1;
     }
     i = 0;
     j = 0;
     while (1) {
-        i++;
         if (period != 0 && i % period == 0) {
             printf("%ju\n", j);
             j++;
         }
-        if (max_given && i == max)
+        i++;
+        if (i == max)
             break;
     }
 }

userland/cpp/atomic.cpp

@@ -79,7 +79,7 @@ int main(int argc, char **argv) {
     if (argc > 2) {
         niters = std::stoull(argv[2], NULL, 0);
     } else {
-        niters = 10000;
+        niters = 10;
     }
     std::vector<std::thread> threads(nthreads);
     for (size_t i = 0; i < nthreads; ++i)

userland/cpp/thread_get_id.cpp

@@ -0,0 +1,29 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-multithreading
+//
+// Spawn some threads and print their ID.
+//
+// On Ubuntu 19.04, they ar large possibly non-consecutive numbers.
+
+#include <iostream>
+#include <mutex>
+#include <thread>
+#include <vector>
+
+std::mutex mutex;
+
+void myfunc(int i) {
+    mutex.lock();
+    std::cout << i << " " << std::this_thread::get_id() << std::endl;
+    mutex.unlock();
+}
+
+int main() {
+    std::cout << "main " << std::this_thread::get_id() << std::endl;
+    std::vector<std::thread> threads;
+    for (unsigned int i = 0; i < 4; ++i) {
+        threads.push_back(std::thread(myfunc, i));
+    }
+    for (auto& thread : threads) {
+        thread.join();
+    }
+}

userland/cpp/thread_hardware_concurrency.cpp

@@ -1,3 +1,5 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-multithreading
+//
 // http://stackoverflow.com/questions/150355/programmatically-find-the-number-of-cores-on-a-machine
 //
 // Not affected by taskset: https://stackoverflow.com/questions/1006289/how-to-find-out-the-number-of-cpus-using-python/55423170#55423170

userland/cpp/thread_return_value.cpp

@@ -0,0 +1,61 @@
+// http://stackoverflow.com/questions/7686939/c-simple-return-value-from-stdthread
+// http://stackoverflow.com/questions/28950835/c-error-no-type-named-type-in-class-stdresult-ofvoid-stdunordered
+// http://stackoverflow.com/questions/21048906/stdthread-pass-by-reference-calls-copy-constructor
+// http://stackoverflow.com/questions/8299545/passing-arguments-to-thread-function
+// http://stackoverflow.com/questions/5116756/difference-between-pointer-and-reference-as-thread-parameter
+
+#include <cassert>
+#include <future>
+#include <iostream>
+#include <thread>
+#include <vector>
+
+int myfunc(int i) {
+    return i + 1;
+}
+
+void myfunc_reference(int& i) {
+    i = myfunc(i);
+}
+
+int main() {
+    unsigned int nthreads = 4;
+    std::vector<int> inputs{1, 2, 3, 4};
+    std::vector<int> outputs_expect{2, 3, 4, 5};
+
+    // future and sync. Nirvana. When you are not fighting to death with types:
+    // https://stackoverflow.com/questions/10620300/can-stdasync-be-use-with-template-functions
+    {
+        std::vector<std::future<int>> futures(nthreads);
+        std::vector<int> outputs(nthreads);
+        for (decltype(futures)::size_type i = 0; i < nthreads; ++i) {
+            futures[i] = std::async(
+                myfunc,
+                inputs[i]
+            );
+        }
+        for (decltype(futures)::size_type i = 0; i < nthreads; ++i) {
+            outputs[i] = futures[i].get();
+        }
+        assert(outputs_expect == outputs);
+    }
+
+    // Reference arguments.
+    //
+    // Annoying because requires:
+    //
+    // - wrapping the return function to accept references
+    // - keeping an array of outputs
+    // - std::ref
+    {
+        std::vector<std::thread> threads(nthreads);
+        std::vector<int> inouts(inputs);
+        for (decltype(threads)::size_type i = 0; i < nthreads; ++i) {
+            threads[i] = std::thread(myfunc_reference, std::ref(inouts[i]));
+        }
+        for (auto& thread : threads) {
+            thread.join();
+        }
+        assert(outputs_expect == inouts);
+    }
+}