UDP unable to bind socket

Question

I get the error "unable to bind socket" , Error : 10038, An operation was attempted on something that was not a socket. the socket gets created, but I cannot bind it.

struct sockaddr_in  serverAddress ;   // declared as global
struct sockaddr_in  clientAddress ; // declared as global
int len=sizeof(struct sockaddr); // declared as global
SOCKET s = NULL ; // declared as global

memset (& serverAddress , 0 , sizeof ( serverAddress ));
serverAddress.sin_family = AF_UNSPEC ;
serverAddress.sin_addr.s_addr =INADDR_ANY;
serverAddress.sin_port = htons(12345);

if( s = socket(PF_INET,SOCK_DGRAM, IPPROTO_UDP) == INVALID_SOCKET)
    {
        printf (" Unable to create a socket \n");
        printf (" Failed with error : %d\n%s\n", WSAGetLastError () ,
            gai_strerror ( WSAGetLastError ()));
        exit (1);
    }
    else
    {
        std::cout<<"CREATED"<<std::endl;
    }

if( bind(s,(struct sockaddr *)&serverAddress,sizeof(serverAddress)) < 0)
    {
                printf (" Unable to bind socket \n");
        printf (" Failed with error : %d\n%s\n", WSAGetLastError () ,
        gai_strerror ( WSAGetLastError ()));
    }
    else
    {
        printf (" Bound to socket .\n");
    }

Answer 1

Parenthesis problem.

if( s = socket(PF_INET,SOCK_DGRAM, IPPROTO_UDP) == INVALID_SOCKET)

should be

if((s = socket(PF_INET,SOCK_DGRAM, IPPROTO_UDP)) == INVALID_SOCKET)

In your code 's' is either 0 or 1 after your line.

Answer 2

You need to fix this line:

if( s = socket(PF_INET,SOCK_DGRAM, IPPROTO_UDP) == INVALID_SOCKET)

The equality operator (==) is getting executed first, so it creates a socket and checks if it is invalid. However, it never assigns the socket information to s. Rather, it assigns the boolean result of the equality test, meaning s is probably set to 0.

You either need to put parentheses in to correct the order of execution, or simply move the assignment into a separate statement. I'd recommend the latter, for sake of readability:

s = socket(PF_INET,SOCK_DGRAM, IPPROTO_UDP);
if (s == INVALID_SOCKET)
{
   ...

It's worth noting that your compiler is probably giving you a warning about that line. It might be saying something like "assignment within conditional expression". You generally shouldn't ignore warnings, because they can help you catch subtle mistakes like this.

Answer 3

this does not assigns to s , therefore I never get a socket number

if( s = socket(PF_INET,SOCK_DGRAM, IPPROTO_UDP) == INVALID_SOCKET)
{

}

this is the correct way

s = socket(PF_INET,SOCK_DGRAM, IPPROTO_UDP);
if( s == INVALID_SOCKET )
{
     echo "error";
}