程序辅助评分

1. 构造学生的文件路径

读取/etc/passwd文件，对班级内所有学生依次构造出源文件和可执行文件路径，关键代码如下：

with open('/etc/passwd', 'r') as f:
    for line in f.readlines():
        l = line.strip().split(':')

        username = l[0]
        uid = int(l[2])
        gid = int(l[3])
        user_info = l[4]
        home_dir = l[5]

        if gid == 201112:
            class_id, student_id, chinese_name = user_info.split('-')

            ## empty source file
            filename = os.path.join(home_dir, 'os_exp/exp4_sync_pv/process_synchronization_pv.c')
            original_filename = '/home/qiankun/os_exp/exp4_sync_pv/process_synchronization_pv.c'

            filename2 = os.path.join(home_dir, 'os_exp/exp4_sync_pv/semaphore.h')
            original_filename2 = '/home/qiankun/os_exp/exp4_sync_pv/semaphore.h'

            program = os.path.join(home_dir, 'os_exp/exp4_sync_pv/process_synchronization_pv')

2. 辅助评分

对于文件大小没有任何改变，没有可执行文件，直接判定为零分，代码如下：

if os.stat(filename).st_size == os.stat(original_filename).st_size and \
os.stat(filename2).st_size == os.stat(original_filename2).st_size:
    lists.append([class_id, student_id, chinese_name, 0, 'Nothing added'])
    continue

try:
    output = subprocess.check_output([program], stderr=subprocess.STDOUT)
    # If returncode is non-zero, raise a CalledProcessError.
except OSError as e:
    if e.errno == 2:
    lists.append([class_id, student_id, chinese_name, 0, 'no executable file'])
    continue

接着。运行用户的可执行程序，将程序输出结果与正确结果相比较，以进程同步的实验为例，子进程先运行，子进程输出的父进程ID应该与父进程输出的进程ID相等，关键代码如下：

output = subprocess.check_output([program], stderr=subprocess.STDOUT)

# run correct
try:
    l = output.strip().decode().split()
    if l[1] == l[2]:
        lists.append([class_id, student_id, chinese_name, 10, 'Match'])
    else:
        lists.append([class_id, student_id, chinese_name, 0, 'Wrong output: {}'.format(output)])
except Exception as e:
    lists.append([class_id, student_id, chinese_name, 0, str(e)])

完整的代码见get_scores_exp4.py。