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Socket Descriptor return value

Posted on 2004-10-10
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Last Modified: 2008-03-04
Hi All,

I have a question that, Socket function may return a value 0 when it creates socket successful?

socket function:
SOCKET socket(  int af,   int type,   int protocol );

Thanks in advandce.

Regards,
MGlobal
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Question by:MGlobal
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Expert Comment

by:bass20
ID: 12275912
Yes and no; using C in Unix/Linux the socket() system call returns a file descriptor for that socket. Therefore, the return value must be greather than or equal (>=) than 0 for the call to be successfull. In case of error, socket() will return -1.
You can test it's result like this:
   int mySock=0;
   if( (mySock=socket(...))<0 ) //there's an error
      perror("socket");
   //if it doesn't produce an error, mySock will have the value of the recently created file descriptor


Hope I helped :)
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by:MGlobal
ID: 12294063
Hi  bass20,

I am using Windows. How about the return value?  All documenst I have read, they only check error when it is -1. But  by testing, I found that in Windows, it never returns 0 (if successful). But I am not sure socket() realy always returns > 0 (if successful). This is importance for me, because I have a program (written by other person), there will be a bug in that program, if socket() can return 0 in case successful.

Thanks,
MGlobal

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Accepted Solution

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bass20 earned 300 total points
ID: 12296483
The socket() function (both in Windows and UNIX) returns a file descriptor, in an incremented fashion (for example, the first call returns 19, the second returns 20, the third returns 21, etc...); it'll never return 0 (to your app) because when the OS is running there are already three file descriptors assigned: "stdin", "stdout" and "stderr" (input, output, error output), and they take descriptors number 0, 1 and 2. Socket CAN RETURN 0 if there is no file descriptor with that number (for example, if you change stdin's descriptor), but it'll generally return > 2 because of those 3 file descriptors I mentioned.
When Windows' socket() produces an error it returns an INVALID_SOCKET value; if you want to catch them or require detailed info about the API, read this: http://msdn.microsoft.com/library/en-us/winsock/winsock/socket_2.asp?frame=true

To see the error produced by the socket call upon create you can do something like:
If myRecentlyCreatedSocket = INVALID_SOCKET Then
     Call ShowErrorMessage(Err.LastDllError)"
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by:MGlobal
ID: 12314536
Thank  bass20.
MGlobal
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