Linux内核错误？创建新的套接字默认为非阻塞

Question

对于简单的TCP客户端代码，connect调用返回EINPROGRESS而没有任何非阻塞安装指令，而send调用返回EAGAIN。这是该过程的strace记录：

// ...
getsockopt(4, SOL_SOCKET, SO_SNDTIMEO, "\1\0\0\0\0\0\0\0 \241\7\0\0\0\0\0", [16]) = 0
setsockopt(4, SOL_SOCKET, SO_LINGER, {l_onoff=1, l_linger=0}, 8) = 0
close(4)                                = 0
futex(0x16487b0, FUTEX_WAKE_PRIVATE, 1) = 1
socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 4
fcntl(4, F_SETFD, FD_CLOEXEC)           = 0
setsockopt(4, SOL_TCP, TCP_NODELAY, [1], 4) = 0
setsockopt(4, SOL_SOCKET, SO_RCVTIMEO, "\1\0\0\0\0\0\0\0 \241\7\0\0\0\0\0", 16) = 0
setsockopt(4, SOL_SOCKET, SO_SNDTIMEO, "\1\0\0\0\0\0\0\0 \241\7\0\0\0\0\0", 16) = 0
connect(4, {sa_family=AF_INET, sin_port=htons(798), sin_addr=inet_addr("192.168.143.1")}, 16) = -1 EINPROGRESS (Operation now in progress)
setsockopt(4, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
times(NULL)                             = 1718992611
sendto(4, "\f\0\0\0\206\0\0\0\0\213\1\203\1\0\0\0", 16, 0, NULL, 0) = -1 EAGAIN (Resource temporarily unavailable)
getsockopt(4, SOL_SOCKET, SO_SNDTIMEO, "\1\0\0\0\0\0\0\0 \241\7\0\0\0\0\0", [16]) = 0
setsockopt(4, SOL_SOCKET, SO_LINGER, {l_onoff=1, l_linger=0}, 8) = 0
close(4)                                = 0

我正在运行Ubuntu 18.04：Linux test 4.15.0-65-generic #74-Ubuntu SMP Tue Sep 17 17:06:04 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux。有什么我想念的东西还是一个错误？

Answer 1

从套接字（7）手册页

   SO_RCVTIMEO and SO_SNDTIMEO
          Specify  the  receiving or sending timeouts until reporting an error.  The argument
          is a struct timeval.  If an input or output function  blocks  for  this  period  of
          time, and data has been sent or received, the return value of that function will be
          the amount of data transferred; if no data has been transferred and the timeout has
          been  reached, then -1 is returned with errno set to EAGAIN or EWOULDBLOCK, or EIN‐
          PROGRESS (for connect(2)) just as if the socket was specified  to  be  nonblocking.
          If the timeout is set to zero (the default), then the operation will never timeout.
          Timeouts only have effect for system calls that perform socket I/O (e.g.,  read(2),
          recvmsg(2),  send(2),  sendmsg(2)); timeouts have no effect for select(2), poll(2),
          epoll_wait(2), and so on.

因此，通过设置超时时间，如果超时，您可能会得到EAGAIN / EINPROGRESS结果。