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
+}