如何非阻塞地读取命名FIFO？

Question

我创建了一个FIFO,并定期从a.py以只读和非阻塞模式打开它:

os.mkfifo(cs_cmd_fifo_file, 0777)
io = os.open(fifo, os.O_RDONLY | os.O_NONBLOCK)
buffer = os.read(io, BUFFER_SIZE)

从b.py,打开fifo进行写作:

out = open(fifo, 'w')
out.write('sth')

然后a.py将引发错误:

buffer = os.read(io, BUFFER_SIZE)

OSError: [Errno 11] Resource temporarily unavailable

谁知道什么是错的？

Answer 1

根据以下手册read(2):

   EAGAIN or EWOULDBLOCK
          The  file  descriptor  fd refers to a socket and has been marked
          nonblocking   (O_NONBLOCK),   and   the   read   would    block.
          POSIX.1-2001  allows  either error to be returned for this case,
          and does not require these constants to have the same value,  so
          a portable application should check for both possibilities.

所以你得到的是没有可供阅读的数据.处理这样的错误是安全的:

try:
    buffer = os.read(io, BUFFER_SIZE)
except OSError as err:
    if err.errno == errno.EAGAIN or err.errno == errno.EWOULDBLOCK:
        buffer = None
    else:
        raise  # something else has happened -- better reraise

if buffer is None: 
    # nothing was received -- do something else
else:
    # buffer contains some received data -- do something with it

确保已导入errno模块:import errno.