为什么STRACE显示EAGAIN(资源暂时不可用)

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) 

为什么这个读取被调用,我认为poll应该只有当有数据要读取时才会唤醒

poll醒来与revents = POLLOUT ,这意味着套接字准备写入,不准备读取。 代码显然不检查这个标志,并尝试读取。

这可能是故意的。 即使poll没有说套接字已经准备好读取,但是它在编写的时候可能已经准备好了。 所以,它只是为了以防万一出现。 如果不是的话,它会重新poll ,再次等待。 这允许它更快速地处理传入的数据,因为它可以在一个呼叫而不是两个呼叫中得到它。