在 Linux 中出现函数参数 (C++) 的未声明标识符错误，但在 Windows 中没有

Question

I have written these codes:

#include <iostream>
#include <limits>
#include <string>
#ifdef _WIN32
  #include "WinSock2.h"
  #include "WS2tcpip.h"

  #pragma comment(lib,"ws2_32.lib")

#elif __linux__
  #include <sys/socket.h>
  #include <arpa/inet.h>
  #define SOCKET socket
#else
  #error Compiler cannot interpret platform. Please compile this program in Windows 32/64-bit or Linux!
#endif

//...some codes here for initalization for _WIN32

int socket_create(SOCKET &socketHandler)
{
  socketHandler = socket(AF_INET, SOCK_STREAM, 0);

  if (socketHandler == INVALID_SOCKET)
  {
    #ifdef _WIN32
      return (WSAGetLastError());
    #else
      return -1;
    #endif
  }
  else
  {
    return 0;
  }
}

//...some codes for other functions
//...main function

When compiled using Clang 11.0.0 on Windows 10 x64 with arguments:

clang server.cpp -Wall -Wextra -std=c++17 -g -glldb -lws2_32 -fexceptions -O0 -o target\\debug\\win-amd64\\server.exe -fms-compatibility -m64

it was flawless. The program compiled without even a warning and was running perfectly. However, bringing it straight to linux (Ubuntu 20.04 x64, with clang-11 and libc++-dev & libc++abi-dev installed), and compiled using Clang 11.0.0 in there with arguments:

clang-11 server.cpp -Wall -Wextra -std=c++17 -g -glldb -fexceptions -O0 -o target\\debug\\deb-amd64\\server.exe -m64

It gives me this error:

error: use of undeclared identifier 'socketHandler'
int socket_create(SOCKET &socketHandler)
                          ^

and

error: expected ';' after top level declarator
int socket_create(SOCKET &socketHandler)
                                        ^

Question: Why is it different in Linux? Did I miss something in declaration? If so, how could the same version of clang in Windows would compile it fine, while straight up refused in Linux? TBH, this is my first time compiling things in Linux, so I don't know if I miss something that I should do in Linux but not in Windows for C++. Thank you.

Answer 1

What socket() in Linux returns is int , not socket nor SOCKET .

You are using #define SOCKET socket in Linux mode and it will have it put function name where type name is expected like this:

int hoge(){ return 0; }

int fuga(hoge& x) { // error
    return 0;
}

This error is not produced in Windows because the problematic line #define SOCKET socket is not used in Windows thanks to the #ifdef directives.

In conclusion, the line #define SOCKET socket should be typedef int SOCKET; .