今天在cygwin下编译一个linux项目时报了类似下面的错误：

server.cpp:20: error: aggregate `addrinfo hints' has incomplete type and cannot be defined
server.cpp:25: error: `AI_PASSIVE' was not declared in this scope
server.cpp:27: error: `getaddrinfo' was not declared in this scope
server.cpp:31: error: invalid use of undefined type `struct addrinfo'
server.cpp:20: error: forward declaration of `struct addrinfo'
server.cpp:54:2: warning: no newline at end of file

显然从字面上看是没有找到addrinfo ,AI_PASSIVE等类型或符号的定义，
几经辗转在stackoverflow上找到下面这个讨论贴：

https://stackoverflow.com/questions/52157480/compiling-with-std-c11-on-cygwin-hides-available-system-calls

在最后发现了答案，原来我写的代码是c++11的所以我在编译选项中加了-std=c++11,而这个回答的意思是在cygwin上应该使用-std=gnu++11,修改后，果然编译通过

以下为进一步验证过程:
在/usr/include/netdb.h找到 addrinfo的定义，可以看到需要 __POSIX_VISIBLE >= 200112 才有效

#if __POSIX_VISIBLE >= 200112 && !defined(__INSIDE_CYGWIN_NET__)
struct addrinfo {
  int             ai_flags;		/* input flags */
  int             ai_family;		/* address family of socket */
  int             ai_socktype;		/* socket type */
  int             ai_protocol;		/* ai_protocol */
  socklen_t       ai_addrlen;		/* length of socket address */
  char            *ai_canonname;	/* canonical name of service location */
  struct sockaddr *ai_addr;		/* socket address of socket */
  struct addrinfo *ai_next;		/* pointer to next in list */
};
#endif

foo.cpp

#include <stdio.h>
#include <sys/socket.h>
#include <netdb.h>

int some_networking_code()
{
   addrinfo* addr = NULL;
   int flags = AI_NUMERICHOST;
   return 0;
}

用如下命令编译上面的代码foo.cpp,可以看到使用-std=c++11时 __POSIX_VISIBLE 定义为0，而不定义-std或-std=gnu++11时__POSIX_VISIBLE 定义为200809

$ g++ foo.cpp -c -dM -E | grep POSIX_VIS
#define __POSIX_VISIBLE 200809

$ g++ foo.cpp -c -std=c++11 -dM -E | grep POSIX_VIS
#define __POSIX_VISIBLE 0

$ g++ foo.cpp -c -std=gnu++11 -dM -E | grep POSIX_VIS
#define __POSIX_VISIBLE 200809

所以 cygwin下编译c++11代码使用-std=gnu++11代替-std=c++11可以解决类似addrinfo类型未定义问题