将socket设置为非阻塞(non-blocking)

https://blog.csdn.net/SprintfWater/article/details/9232073 (opens new window)

有一个非常有迷惑性的做法是： u_long has = 1; ioctl(m_sock, FIONBIO , &has); 这个函数会非常无耻的返回你success，但是它实际上很可能什么也没做。

正确的做法应该是使用fcntl： int flags = fcntl(m_sock, F_GETFL, 0); fcntl(m_sock, F_SETFL, flags|O_NONBLOCK);

这真是一个隐蔽的问题，折腾了我两天。线程每每停留在send()调用那里，我始终没怀疑到：用ioctl设置FIONBIO成功之后，socket竟然还是阻塞的。

查看是否为non-blocking

#include <fcntl.h>

int flags = fcntl(s, F_GETFL, 0);

printf("flags&O_NONBLOCK=%x, O_NONBLOCK=%x, flags=%x\n", flags&O_NONBLOCK, O_NONBLOCK, flags);
//flags&O_NONBLOCK=800, O_NONBLOCK=800, flags=802

fcntl()

https://man7.org/linux/man-pages/man2/fcntl.2.html (opens new window)

FCNTL(2)                Linux Programmer's Manual               FCNTL(2)
NAME         top
       fcntl - manipulate file descriptor
SYNOPSIS         top
       #include <fcntl.h>

       int fcntl(int fd, int cmd, ... /* arg */ );
DESCRIPTION         top
       fcntl() performs one of the operations described below on the
       open file descriptor fd.  The operation is determined by cmd.

       fcntl() can take an optional third argument.  Whether or not this
       argument is required is determined by cmd.  The required argument
       type is indicated in parentheses after each cmd name (in most
       cases, the required type is int, and we identify the argument
       using the name arg), or void is specified if the argument is not
       required.

       Certain of the operations below are supported only since a
       particular Linux kernel version.  The preferred method of
       checking whether the host kernel supports a particular operation
       is to invoke fcntl() with the desired cmd value and then test
       whether the call failed with EINVAL, indicating that the kernel
       does not recognize this value.