Compiler error C2275

Question

I've been programming a simple WinSock application in Visual Studio 2010. I have named my application entry point "main.c", then I came across this error while declaring a SOCKET object:

error C2275: 'SOCKET' : illegal use of this type as an expression

Oddly enough, I solved that problem by renaming the code file from main.c to main.cpp

Just out of curiosity, I want to know what is the meaning of this error, and what difference occurred by changing the extension.

Thanks in advance.

EDIT

Here is the relevant code:

#pragma comment(lib,"ws2_32")

#include <WinSock2.h>
#include <stdio.h>


int main()
{
// Startup the winsock
WORD wVersionRequested;
WSADATA wsaData;
int wsaerr;
wVersionRequested = MAKEWORD(2,2);
wsaerr = WSAStartup(wVersionRequested,&wsaData);
if(wsaerr != 0)
{
    printf("Winsock2 dll is not found!\n");
    WSACleanup();
    return 0;
}
else
{
    printf("Winsock2 dll is found!\n");
    printf("Current System Status: %s.\n",wsaData.szSystemStatus);
}

//Create a SOCKET object called socketobj.
SOCKET socketobj;
socketobj = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (socketobj == INVALID_SOCKET)
{
    printf("Socket Intialization Failed with error: %ld\n", WSAGetLastError());
    WSACleanup();
    return 0;
}
else

{
    printf("Socket Intialization Success\n");
}

Sleep(10000);
return 0;
}

Answer 1

Without seeing the code it's hard to tell.

But my guess is that you have some interleaved declarations and code. MSVC's C compiler is only C89 which does not support it. That would explain why the C++ compiler accepts it, but the C compiler doesn't.

Prior to C99, all declarations must be at the start of the function or a block.

EDIT : Your code doesn't show the whole function, but you probably have some (non-declaration) code before the SOCKET socketobj; declaration.

---

Now that the full function is shown, it confirms that you are interleaving declarations and code:

WORD wVersionRequested;            //  Declaration: ok
WSADATA wsaData;                   //  Declaration: ok
int wsaerr;                        //  Declaration: ok
wVersionRequested = MAKEWORD(2,2); //  Code: ok

...

SOCKET socketobj;                  //  Declaration: NOT ok
socketobj = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

The solution here is to move SOCKET socketobj; to the start of the function with the other declarations.

Answer 2

There are differences between C and C++. For example, in C89 you can't declare a variable in the middle of a block of code, but only at the beginning.

Answer 3

Have a look at the error description: http://msdn.microsoft.com/en-us/library/76c9k4ah%28v=vs.71%29.aspx

By changing the extension to .cpp Visual Studio uses the C++ compiler instead of the C compiler (which is quite a different language).

So you probably wrote C++ code and fed it into the C compiler, which (coincidently) resulted in that error beeing thrown.