Question

以下是我得到的序列

socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 7

    setsockopt(7, SOL_TCP, TCP_NODELAY, [1], 4) = 0
    setsockopt(7, SOL_SOCKET, SO_SNDBUF, [32120], 4) = 0
    getsockopt(7, SOL_SOCKET, SO_SNDBUF, [30064835312], [4]) = 0
    setsockopt(7, SOL_SOCKET, SO_SNDBUF, [64240], 4) = 0
    getsockopt(7, SOL_SOCKET, SO_SNDBUF, [30064899552], [4]) = 0
    stat("/etc/localtime", {st_dev=makedev(8, 1), st_ino=229001, st_mode=S_IFREG|0644, st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=8, st_size=265, st_atime=2013/07/15-06:30:03, st_mtime=2012/06/25-23:46:43, st_ctime=2012/06/25-23:46:43}) = 0
    write(1, "[info 2013/07/16 05:53:24.622210"..., 114) = 114
    setsockopt(7, SOL_SOCKET, SO_RCVBUF, [32120], 4) = 0
    getsockopt(7, SOL_SOCKET, SO_RCVBUF, [30064835312], [4]) = 0
    setsockopt(7, SOL_SOCKET, SO_RCVBUF, [64240], 4) = 0
    getsockopt(7, SOL_SOCKET, SO_RCVBUF, [30064899552], [4]) = 0
    fcntl(7, F_GETFL)                       = 0x2 (flags O_RDWR)
    fcntl(7, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
    rt_sigaction(SIGPIPE, {SIG_IGN, [PIPE], SA_RESTORER|SA_RESTART, 0x33b3632920}, {SIG_DFL, [], 0}, 8) = 0
    fcntl(7, F_GETFL)                       = 0x802 (flags O_RDWR|O_NONBLOCK)
    fcntl(7, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
    connect(7, {sa_family=AF_INET, sin_port=htons(50505), sin_addr=inet_addr("1.2.3.4")}, 16) = -1 EINPROGRESS (Operation now in progress)
    poll([{fd=7, events=POLLIN|POLLOUT}], 1, 59000) = 1 ([{fd=7, revents=POLLOUT}])
    fcntl(7, F_GETFL)                       = 0x802 (flags O_RDWR|O_NONBLOCK)
    fcntl(7, F_SETFL, O_RDWR)               = 0
    getsockname(7, {sa_family=AF_INET, sin_port=htons(33220), sin_addr=inet_addr("10.112.204.215")}, [16]) = 0
    fcntl(7, F_GETFL)                       = 0x2 (flags O_RDWR)
    fcntl(7, F_GETFL)                       = 0x2 (flags O_RDWR)
    fcntl(7, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
    write(7, "d\23;\177\377\330\357\1&W\1\\\4\np\314\327\0\0\0\2W\0\rpnq-gst-"..., 103) = 103
    fcntl(7, F_GETFL)                       = 0x802 (flags O_RDWR|O_NONBLOCK)
    fcntl(7, F_SETFL, O_RDWR)               = 0
    fcntl(7, F_GETFL)                       = 0x2 (flags O_RDWR)
    fcntl(7, F_GETFL)                       = 0x2 (flags O_RDWR)
    fcntl(7, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
    read(7, 0x9d9f90, 1)                    = -1 EAGAIN (Resource temporarily unavailable)

为什么要调用此读取，我假设poll只有在有数据需要读取时才会被唤醒

Answer 1

poll用revents = POLLOUT醒来，这意味着套接字已准备就绪，无法读取。代码显然没有检查这个标志，并试图读取。

这可能是故意的。尽管poll没有说套接字已准备好读取，但它在编写时可能已经准备就绪。因此，只要有东西出现，它就会调用。如果没有，它将返回poll再次等待。这使它能够更快地处理传入的数据，因为它可以在一次调用而不是两次调用中获取它。

为什么STRACE显示EAGAIN（资源暂时不可用）

1 个答案: