Added Process exercise's chapter

ex12.c

@@ -0,0 +1,39 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/wait.h>
+
+void add_children(pid_t children[], pid_t pid);
+
+int main(void) 
+{
+  pid_t parent_pid = getpid();
+  pid_t children[1024] = {-1};
+  for (int i = 0; i < 4; i++) {
+    pid_t pid = fork();
+    add_children(children, pid);
+  }
+
+  if (parent_pid = getpid()) {
+    wait(NULL);
+    printf("pids: ");
+    for (int i = 0; i < 1024; i++) {
+      if (children[i] == -1) {
+        printf("\n");
+        break;
+      }
+      printf("%d ", children[i]);
+    }
+  }
+
+  return 0;
+}
+
+void add_children(pid_t children[], pid_t pid)
+{
+  for (int i = 0; i < 1024; i++) {
+    if (children[i] < 0) {
+      children[i] = pid;
+      break;
+    }
+  }
+}