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    use crate::fs::{open_file, OpenFlags};
    use crate::mm::{translated_ref, translated_refmut, translated_str};
    use crate::task::{
        add_task, current_task, current_user_token, exit_current_and_run_next, pid2task,
        suspend_current_and_run_next, SignalFlags, SignalAction, MAX_SIG,
    };
    use crate::timer::get_time_ms;
    use alloc::string::String;
    use alloc::sync::Arc;
    use alloc::vec::Vec;

    pub fn sys_exit(exit_code: i32) -> ! {
        exit_current_and_run_next(exit_code);
        panic!("Unreachable in sys_exit!");
    }

    pub fn sys_yield() -> isize {
        suspend_current_and_run_next();
        0
    }

    pub fn sys_get_time() -> isize {
        get_time_ms() as isize
    }

    pub fn sys_getpid() -> isize {
        current_task().unwrap().pid.0 as isize
    }

    pub fn sys_fork() -> isize {
        let current_task = current_task().unwrap();
        let new_task = current_task.fork();
        let new_pid = new_task.pid.0;
        // modify trap context of new_task, because it returns immediately after switching
        let trap_cx = new_task.inner_exclusive_access().get_trap_cx();
        // we do not have to move to next instruction since we have done it before
        // for child process, fork returns 0
        trap_cx.x[10] = 0;
        // add new task to scheduler
        add_task(new_task);
        new_pid as isize
    }

    pub fn sys_exec(path: *const u8, mut args: *const usize) -> isize {
        let token = current_user_token();
        let path = translated_str(token, path);
        let mut args_vec: Vec<String> = Vec::new();
        loop {
            let arg_str_ptr = *translated_ref(token, args);
            if arg_str_ptr == 0 {
                break;
            }
            args_vec.push(translated_str(token, arg_str_ptr as *const u8));
            unsafe {
                args = args.add(1);
            }
        }
        if let Some(app_inode) = open_file(path.as_str(), OpenFlags::RDONLY) {
            let all_data = app_inode.read_all();
            let task = current_task().unwrap();
            let argc = args_vec.len();
            task.exec(all_data.as_slice(), args_vec);
            // return argc because cx.x[10] will be covered with it later
            argc as isize
        } else {
            -1
        }
    }

    /// If there is not a child process whose pid is same as given, return -1.
    /// Else if there is a child process but it is still running, return -2.
    pub fn sys_waitpid(pid: isize, exit_code_ptr: *mut i32) -> isize {
        let task = current_task().unwrap();
        // find a child process

        // ---- access current PCB exclusively
        let mut inner = task.inner_exclusive_access();
        if !inner
            .children
            .iter()
            .any(|p| pid == -1 || pid as usize == p.getpid())
        {
            return -1;
            // ---- release current PCB
        }
        let pair = inner.children.iter().enumerate().find(|(_, p)| {
            // ++++ temporarily access child PCB exclusively
            p.inner_exclusive_access().is_zombie() && (pid == -1 || pid as usize == p.getpid())
            // ++++ release child PCB
        });
        if let Some((idx, _)) = pair {
            let child = inner.children.remove(idx);
            // confirm that child will be deallocated after being removed from children list
            assert_eq!(Arc::strong_count(&child), 1);
            let found_pid = child.getpid();
            // ++++ temporarily access child PCB exclusively
            let exit_code = child.inner_exclusive_access().exit_code;
            // ++++ release child PCB
            *translated_refmut(inner.memory_set.token(), exit_code_ptr) = exit_code;
            found_pid as isize
        } else {
            -2
        }
        // ---- release current PCB automatically
    }

    pub fn sys_kill(pid: usize, signum: i32) -> isize {
        if let Some(task) = pid2task(pid) {
            if let Some(flag) = SignalFlags::from_bits(1 << signum) {
                // insert the signal if legal
                let mut task_ref = task.inner_exclusive_access();
                if task_ref.signals.contains(flag) {
                    return -1;
                }
                task_ref.signals.insert(flag);
                0
            } else {
                -1
            }
        } else {
            -1
        }
    }

    pub fn sys_sigprocmask(mask: u32) -> isize {
        if let Some(task) = current_task() {
            let mut inner = task.inner_exclusive_access();
            let old_mask = inner.signal_mask;
            if let Some(flag) = SignalFlags::from_bits(mask) {
                inner.signal_mask = flag;
                old_mask.bits() as isize
            } else {
                -1
            }
        } else {
            -1
        }
    }

    pub fn sys_sigretrun() -> isize {
        if let Some(task) = current_task() {
            let mut inner = task.inner_exclusive_access();
            inner.handling_sig = -1;
            // restore the trap context
            let trap_ctx = inner.get_trap_cx();
            *trap_ctx = inner.trap_ctx_backup.unwrap();
            0
        } else {
            -1
        }
    }

    fn check_sigaction_error(signal: SignalFlags, action: usize, old_action: usize) -> bool {
        if action == 0 || old_action == 0 || signal == SignalFlags::SIGKILL ||
            signal == SignalFlags::SIGSTOP {
            true
        } else {
            false
        }
    }

    pub fn sys_sigaction(signum: i32, action: *const SignalAction, old_action: *mut SignalAction) -> isize {
        let token = current_user_token();
        if let Some(task) = current_task() {
            let mut inner = task.inner_exclusive_access();
            if signum as usize > MAX_SIG {
                return -1;
            }
            if let Some(flag) = SignalFlags::from_bits(1 << signum) {
                if check_sigaction_error(flag, action as usize, old_action as usize) {
                    return -1;
                }
                let old_kernel_action = inner.signal_actions.table[signum as usize];
                if old_kernel_action.mask != SignalFlags::from_bits(40).unwrap() {
                    *translated_refmut(token, old_action) = old_kernel_action;
                } else {
                    let mut ref_old_action = *translated_refmut(token, old_action);
                    ref_old_action.handler = old_kernel_action.handler;
                }
                let ref_action = translated_ref(token, action);
                inner.signal_actions.table[signum as usize] = *ref_action;
                return 0;
            }
        }
        -1
    }