cygwin下编译报错 `addrinfo hints‘ has incomplete type and cannot be defined

系统管理员 2022-12-17 07:08 185阅读 0赞

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

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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

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