(linux)Socket returns 0 - c++

Question

Though i know that 0 is a valid file descriptor in linux & can be returned by the call to socket(), but the subsequent call to bind or setsockopt fails with errorcode:88 (operation on non-socket).
Also i am not able to understand where i might have closed my stdin file descriptor in order to get a socket value as 0.
Here is my code snippet.. Its a simple socket class..
bool serverHandler::startServer(string strAddress,int port)
{
//memset(&m_srvAddr,'\0',sizeof(m_srvAddr));
m_srvSock = -1;
if(m_srvSock = socket(AF_INET,SOCK_STREAM,0) < 0)
cout<<"serverHandler: Failed to create server socket."<<endl;
m_srvAddr.sin_family = AF_INET;
m_srvAddr.sin_addr.s_addr = inet_addr(strAddress.c_str());
m_srvAddr.sin_port = htons(port);
cout<<"Socket value:"<<m_srvSock<<" for "<<strAddress<<" "<<port<<endl;
if (bind(m_srvSock,(struct sockaddr *) &m_srvAddr,sizeof(m_srvAddr)) < 0)
{
cout<<"Failed with errno:"<<errno<<endl;
return false;
}
if (listen(m_srvSock, MAX_PENDING) < 0)
{
return false;
}
}
And the call to this function is a simple:
srvHandler.startServer("127.0.0.1",7878);
I have used the same way to write down many socket programs, not getting what is wrong in this one.

Answer 1

if(m_srvSock = socket(AF_INET,SOCK_STREAM,0) < 0)
Is not correct since < is evaluated before = (lower precedence), you need to add parenthesis:
if((m_srvSock = socket(AF_INET,SOCK_STREAM,0)) < 0)
If you're using GCC, you should set -W -Wall options to get warning for these kind of mistakes, with MVSC use at least /W4.

Answer 2

I am not able to understand where i might have closed my stdin file descriptor in order to get a socket value as 0.
I just discovered how this was happening in a project of mine. Consider this situation:
int sock;
void init()
{
sock = 0;
}
void start()
{
sock = socket(...);
}
void cancel()
{
close(sock);
}
This works fine so long as cancel is always called at most once per start, and you never have a call to init coming in-between.
In my case, an error handler was calling cancel after init had been called, which was causing close(0) and therefore closing stdin.