为什么STRACE显示EAGAIN(资源暂时不可用) [英] Why STRACE shows EAGAIN (Resource temporarily unavailable)

问题描述

以下是我得到的顺序

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)

为什么调用此读取，我认为poll仅在有要读取的数据时才唤醒

Why this read is getting called, I assume that poll should wake up only when there is data to read

推荐答案

poll被revents = POLLOUT唤醒，这意味着套接字已准备好进行写操作，而不准备进行读取.该代码显然没有检查此标志，而是试图进行读取.

poll woke up with revents = POLLOUT, which means that the socket is ready to write, not ready to read. The code is apparently not checking this flag, and trying to read anyway.

这可能是故意的.即使poll并未说套接字已准备好读取，但它在写入时可能已经准备就绪.因此，它会在出现某些情况时发出"ready"通知.如果不是，它将返回poll以再次等待.这样一来，它就可以更快地处理传入数据，因为它可以一次调用而不是两次调用.

This might be intentional. Even though poll didn't say the socket is ready to read, it might have become ready while it was writing. So it calls ready just in case something has shown up. If not, it will go back into poll to wait again. This allows it to process incoming data more quickly, since it can get it in one call rather than two.

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