userland: move more multithreading from cpp-cheat!

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

README.adoc

@@ -12845,7 +12845,7 @@ Programs under link:userland/c/[] are examples of https://en.wikipedia.org/wiki/
 *** File IO
 **** link:userland/c/file_write_read.c[]
 * Fun
-** link:userland/c/infinite_loop.c[]
+** link:userland/c/loop.c[]
 
 ==== malloc
 
@@ -12949,6 +12949,16 @@ It then walks over every page and writes a value in it to ensure that it is used
 
 Algorithm used by the OOM: https://unix.stackexchange.com/questions/153585/how-does-the-oom-killer-decide-which-process-to-kill-first
 
+==== C multithreading
+
+Added in C11!
+
+* link:userland/c/atomic.c[]: `atomic_int` and `thrd_create`
+
+Bibliography:
+
+* https://stackoverflow.com/questions/3908031/how-to-multithread-c-code/52453354#52453354
+
 ==== GCC C extensions
 
 ===== C empty struct
@@ -12986,8 +12996,6 @@ Programs under link:userland/cpp/[] are examples of https://en.wikipedia.org/wik
 ** link:userland/cpp/template.cpp[]: basic example
 ** link:userland/cpp/template_class_with_static_member.cpp[]: https://stackoverflow.com/questions/3229883/static-member-initialization-in-a-class-template
 ** link:userland/cpp/if_constexpr.cpp[]: C++17 `if constexpr`
-*** https://stackoverflow.com/questions/12160765/if-else-at-compile-time-in-c/54647315#54647315
-*** https://stackoverflow.com/questions/37617677/implementing-a-compile-time-static-if-logic-for-different-string-types-in-a-co
 
 [[cpp-multithreading]]
 ==== C++ multithreading
@@ -12995,6 +13003,8 @@ Programs under link:userland/cpp/[] are examples of https://en.wikipedia.org/wik
 * https://en.cppreference.com/w/cpp/header/thread[`<thread>`]
 ** link:userland/cpp/count.cpp[] Exemplifies: `std::this_thread::sleep_for`
 ** link:userland/cpp/thread_hardware_concurrency.cpp[] `std::thread::hardware_concurrency`
+** link:userland/cpp/thread_get_id.cpp[] `std::thread::get_id`
+** link:userland/cpp/thread_return_value.cpp[]: how to return a value from a thread
 * https://en.cppreference.com/w/cpp/header/atomic[`<atomic>`]: <<cpp17>> 32 "Atomic operations library"
 ** link:userland/cpp/atomic.cpp[]
 
@@ -13093,6 +13103,7 @@ Bibliography: https://stackoverflow.com/questions/6988487/what-does-the-brk-syst
 The following sections are related to multithreading in userland:
 
 * language topics:
+** <<c-multithreading>>
 ** <<cpp-multithreading>>
 ** <<pthreads>>
 * ISA topics:

path_properties.py

@@ -475,6 +475,7 @@ path_properties_tuples = (
                     },
                     {
                         'abort.c': {'signal_received': signal.Signals.SIGABRT},
+                        'atomic.c': {'baremetal': False},
                         'assert_fail.c': {'signal_received': signal.Signals.SIGABRT},
                         # This has complex failure modes, too hard to assert.
                         'smash_stack.c': {'skip_run_unclassified': True},
@@ -483,7 +484,6 @@ path_properties_tuples = (
                         'false.c': {'exit_status': 1},
                         'file_write_read.c': {'baremetal': False},
                         'getchar.c': {'interactive': True},
-                        'infinite_loop.c': {'test_run_args': {'userland_args': '1 10'}},
                         'malloc_max.c': {'disrupts_system': True},
                         'return1.c': {'exit_status': 1},
                         'return2.c': {'exit_status': 2},

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);
+    }
+}