static int
sys_ipc_try_send(envid_t envid, uint32_t value, void *srcva, unsigned perm)
{
    // LAB 4: Your code here.
    struct Env *env = NULL;
    struct PageInfo *page = NULL;
    int r;

    if ((r = envid2env(envid, &env, 0)) < 0) {
        return r;
    }
    /* make sure that dst is waiting for us */
    if (env->env_ipc_recving != 1) {
        return -E_IPC_NOT_RECV;
    }

    if (((uintptr_t)srcva > UTOP) || ((uintptr_t)srcva % PGSIZE) != 0)
        return -E_INVAL;

    if ((uintptr_t)srcva < UTOP &&  (perm & PTE_U) != PTE_U) {
        cprintf("perm not valid\n");
        return -E_INVAL;
    }
    if (env->env_ipc_dstva != (void *) UTOP && srcva != (void *)UTOP) {
        if ((perm & PTE_W) == PTE_W) {
            pte_t *pte = NULL;
            if ((page = page_lookup(curenv->env_pgdir, srcva, &pte)) == NULL)
                return -E_INVAL;
            if ((*pte & PTE_W) != PTE_W)
                return -E_INVAL;
        }
        /* insert page into dst env's pgdir */ 
        if (page_insert(env->env_pgdir, page, env->env_ipc_dstva, perm) < 0) {
            return -E_NO_MEM;
        }
        env->env_ipc_perm = perm;
    }
    else {
        env->env_ipc_perm = 0;
    }

    /* now set dst env safely... */
    env->env_ipc_recving = 0;
    env->env_ipc_from = curenv->env_id;
    env->env_ipc_value = value;
    if (page != NULL)
        env->env_ipc_perm = perm;
    else
        env->env_ipc_perm = 0;

    env->env_tf.tf_regs.reg_eax = 0;
    /* set des env not block, RUNNABLE now ... */
    //cprintf("sys_ipc_try_send: %x send to %x, set recver runnalbe\n", curenv->env_id, env->env_id);
    env->env_status = ENV_RUNNABLE;
    return 0;
}

int32_t
ipc_recv(envid_t *from_env_store, void *pg, int *perm_store)
{
    // LAB 4: Your code here.
    int r;

    if (!pg) pg = (void *)UTOP;
    if ((r = sys_ipc_recv(pg)) < 0) {
        if (from_env_store) 
            *from_env_store = 0;
        if (perm_store) 
            *perm_store = 0;
        return r;
    }
    //cprintf("ipc_recv: env=%x receive finish\n", thisenv->env_id);
    if (from_env_store) 
        *from_env_store = thisenv->env_ipc_from;
    if (perm_store) 
        *perm_store = thisenv->env_ipc_perm;
    return thisenv->env_ipc_value;
}

void
ipc_send(envid_t to_env, uint32_t val, void *pg, int perm)
{
    // LAB 4: Your code here.
    int r;

    if (!pg) pg = (void *)UTOP;

    while ((r = sys_ipc_try_send(to_env, val, pg, perm)) < 0) {
        if (r != -E_IPC_NOT_RECV)
            panic("ipc_send panic\n");
        sys_yield();
    }
}