add lab answers

labcodes_answer/lab6_result/user/forktree.c

@@ -0,0 +1,37 @@
+#include <ulib.h>
+#include <stdio.h>
+#include <string.h>
+
+#define DEPTH 4
+
+void forktree(const char *cur);
+
+void
+forkchild(const char *cur, char branch) {
+    char nxt[DEPTH + 1];
+
+    if (strlen(cur) >= DEPTH)
+        return;
+
+    snprintf(nxt, DEPTH + 1, "%s%c", cur, branch);
+    if (fork() == 0) {
+        forktree(nxt);
+        yield();
+        exit(0);
+    }
+}
+
+void
+forktree(const char *cur) {
+    cprintf("%04x: I am '%s'\n", getpid(), cur);
+
+    forkchild(cur, '0');
+    forkchild(cur, '1');
+}
+
+int
+main(void) {
+    forktree("");
+    return 0;
+}
+