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--- os_cp:semaphores:solutions [2023/05/10 07:38] – utworzono jkonczak
+++ os_cp:semaphores:solutions [2023/09/15 13:18] (current) – jkonczak
@@ Line 1: / Line 1: @@
 <html><div style="line-height:1em"></html>
+~~Exercise.#~~
+<code c>
+#include <fcntl.h>
+#include <semaphore.h>
+#include <stdio.h>
+int main() {
+  sem_t *mySemaphore = sem_open("/mySemaphore", O_CREAT | O_EXCL, 0666, 0);
+  if (mySemaphore == SEM_FAILED) {
+    perror("Creating the semaphore failed");
+    return 1;
+  }
+  sem_wait(mySemaphore);
+  return 0;
+}
+</code>
+<code c>
+#include <fcntl.h>
+#include <semaphore.h>
+#include <stdio.h>
+int main() {
+  sem_t *mySemaphore = sem_open("/mySemaphore", 0, 0666, 0);
+  if (mySemaphore == SEM_FAILED) {
+    perror("Opening the semaphore failed");
+    return 1;
+  }
+  sem_post(mySemaphore);
+  return 0;
+}
+</code>
+~~Exercise.#~~
 <code c>
 #include <fcntl.h>
@@ Line 79: / Line 115: @@
 }
 </code>
+~~Exercise.#~~
+Line:
+<code c> if (createSucceeded) sleep(10);</code>
+added anywhere after line 13 and before line 22 delays the first process from
+initializing the semaphore for 10 seconds. If another process attempts to wait
+on uninitialized semaphore, it is likely to get stuck forever.
+~~Exercise.#~~
+Lines between ''sem_wait'' and ''sem_post'' are the critical region.
+~~Exercise.#~~
+When the program is run concurrently with arguments ''0 1'' and ''1 0'', it will
+deadlock if both processes manage to acquire the first semaphore (respectively
+''0'' for the first process, and ''1'' for the second) and then try to acquire
+the second one (respectively ''1'' and ''0'').
+~~Exercise.#~~
+One of the simpler solutions is to wait on semaphores in a predefined order,
+that is instead of:
+<code c>
+    sem_wait(&data->item[argOne].sem);
+    sem_wait(&data->item[argTwo].sem);
+</code>
+one may just:
+<code c>
+    sem_wait(&data->item[argOne<argTwo?argOne:argTwo].sem);
+    sem_wait(&data->item[argOne<argTwo?argTwo:argOne].sem);
+</code>
 <html></div></html>
+~~META:
+language = en
+~~