Get logged in user in SharePoint Code behind

In Server Side Object Model

string username;
string sspURL = "URL where Sharepoint is deployed";

site = new SPSite(sspURL);
web = site.OpenWeb();
SPUser user = web.CurrentUser;
username = user.LoginName;

site.AllowUnsafeUpdates = true;

OR

SPContext.Current.Web.CurrentUser.Name;

OR

SPWeb theSite = SPControl.GetContextWeb(Context);
SPUser theUser = theSite.CurrentUser;
string strUserName = theUser.LoginName;
In Client Side Object Model
 function CallClientOM()
{
 var context = new SP.ClientContext.get_current();
 this.website = context.get_web();
 this.currentUser = website.get_currentUser();
 context.load(currentUser);
 context.executeQueryAsync(
  Function.createDelegate(this, this.onQuerySucceeded),
  Function.createDelegate(this, this.onQueryFailed)
 );
}

function onQuerySucceeded(sender, args)
{
 alert(currentUser.get_loginName());
}

function onQueryFailed(sender, args)
{
 alert('request failed'+args.get_message()+'\n'+args.get_stackTrace());
}