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@@ Line 1: / Line 1: @@
+<html><div style="line-height:1em"></html>
+~~Exercise.#3~~
+<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>
+#include <semaphore.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#define MAX_ELEMENTS 4
+typedef char item_t[256];
+struct ring_buffer {
+  item_t data[MAX_ELEMENTS];
+  size_t first;
+  size_t last;
+};
+struct ring_buffer *buffer;
+sem_t *storedItemsCount;
+sem_t *emptyItemsCount;
+sem_t *canModify;
+void put(struct ring_buffer *buffer, const item_t *item) {
+  sem_wait(emptyItemsCount);
+  sem_wait(canModify);
+  memcpy(&buffer->data[buffer->last], item, sizeof(*item));
+  buffer->last = (buffer->last + 1) % MAX_ELEMENTS;
+  sem_post(canModify);
+  sem_post(storedItemsCount);
+}
+void get(struct ring_buffer *buffer, item_t *item) {
+  sem_wait(storedItemsCount);
+  sem_wait(canModify);
+  memcpy(item, &buffer->data[buffer->first], sizeof(*item));
+  buffer->first = (buffer->first + 1) % MAX_ELEMENTS;
+  sem_post(canModify);
+  sem_post(emptyItemsCount);
+}
+int main() {
+  int fd = shm_open("/myRingBuffer", O_RDWR | O_CREAT, 0666);
+  if (fd == -1) { perror("shm_open"); exit(1); }
+  ftruncate(fd, sizeof(struct ring_buffer));
+  buffer = mmap(NULL, sizeof(struct ring_buffer), PROT_WRITE | PROT_READ, MAP_SHARED, fd, 0);
+  storedItemsCount = sem_open("myRingBuffer.storedItems", O_RDWR | O_CREAT, 0666, 0);
+  emptyItemsCount  = sem_open("myRingBuffer.emptyItems",  O_RDWR | O_CREAT, 0666, 4);
+  canModify        = sem_open("myRingBuffer.canModify",   O_RDWR | O_CREAT, 0666, 1);
+  printf("g             gets data from buffer\n"
+         "ptext...      puts 'text...' to buffer\n");
+  while (1) {
+    printf("> ");
+    fflush(stdout);
+    char cmd[2], c;
+    item_t item = {0};
+    scanf("%1[^\n]%255[^\n]", cmd, item);
+    do { // this reads all remaining characters in this line including '\n'
+      c = getchar();
+      if (c == -1)
+        return 0;
+    } while (c != '\n');
+    switch (cmd[0]) {
+    case 'p':
+      put(buffer, &item);
+      break;
+    case 'g':
+      get(buffer, &item);
+      printf("%s\n", item);
+      break;
+    }
+  }
+  return 0;
+}
+</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.
+~~META:
+language = en
+~~

Jan Kończak

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