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+===== Yet another reason that makes concurrent programming harder. =====
+Below, there is a piece of code that can be compiled by issuing:
+<code bash>gcc -g -O0 sb.c -o sb</code>
+__Analyze the code and tell which results can be output.__
+\\
+Remember that global variables are by default zero-initialized.
+Run the program multiple times with the following command, and observe the results:
+<code bash>( for((i=0;i<10000;++i)); do ./sb; done ) | sort | uniq -c</code>
+Finally, explain the results.
+<html><div style="line-height:1em"></html>
+<code c sb.c>
+#include <pthread.h>
+#include <stdatomic.h>
+#include <stdio.h>
+volatile char data[64 * 3];
+volatile char *const a = data + 64;
+volatile char *const b = data + 64 * 2;
+atomic_char vA, vB, control;
+void *th1(void *arg) {
+  control++;
+  while (control != 2);
+  *b = 1;
+  vA = *a;
+  return NULL;
+}
+void *th2(void *arg) {
+  control++;
+  while (control != 2);
+  volatile int dummy = 44;
+  while (dummy--);
+  *a = 1;
+  vB = *b;
+  return NULL;
+}
+int main() {
+  pthread_t t1, t2;
+  pthread_create(&t1, NULL, th1, NULL);
+  pthread_create(&t2, NULL, th2, NULL);
+  pthread_join(t1, NULL);
+  pthread_join(t2, NULL);
+  printf("%d %d\n", vA, vB);
+  return 0;
+}
+</code>
+<html></div></html>
+~~META:
+language = en
+~~

Jan Kończak