Question abt read(fd, buffer, 1024)

tesla · 09-11-2009, 11:18 PM

This is some of my code, what I want to do is keep trying to read
information from my server until it gives me a response. But is
this a correct way of doing it? or will it cause me problems? Cos if the
server gives me the wrong message then i'm going to exit otherwise I'll
process the response. So after this code I process "buffer" to determine
my course of action.

Code:

//
	write(nsFD,lookUp, strlen(lookUp));
	
	//Iterate until a response is returned
	while((numBytesRead = read(nsFD, buffer, 1024) < 0)) {
	
                 }

Cheers

EVOEx · 09-12-2009, 03:52 AM

No. read() will return:
- -1 on error (look at the available errno's; some you might want to ignore and loop until you get a different result. For instance EINTR is quite useless there).
- 0 on disconnect
- Otherwise the number of bytes received.
Note that read, when the socket blocks, will not return at all until it has data (or an error or end of file). So there's no reason to loop until you get anything: it only returns when you get anything.

Your program might contain another bug that a lot of programs have and actually is ignored by most coders. If the socket is TCP, this protocol is stream based. That means that the server, if it wants to send 2 bytes, might only send 1 byte first (theoretically, although in practice I've never seen it happen). So read() might return only one byte even though the server sent two. So if you expect more bytes, keep reading until you have as much as you expect.

09-11-2009, 11:18 PM	#1
tesla Registered User Join Date: Sep 2009 Posts: 11	Question abt read(fd, buffer, 1024) This is some of my code, what I want to do is keep trying to read information from my server until it gives me a response. But is this a correct way of doing it? or will it cause me problems? Cos if the server gives me the wrong message then i'm going to exit otherwise I'll process the response. So after this code I process "buffer" to determine my course of action. Code: // write(nsFD,lookUp, strlen(lookUp)); //Iterate until a response is returned while((numBytesRead = read(nsFD, buffer, 1024) < 0)) { } Cheers
tesla is offline

09-12-2009, 03:52 AM	#2
EVOEx Registered User Join Date: Oct 2008 Posts: 452	No. read() will return: - -1 on error (look at the available errno's; some you might want to ignore and loop until you get a different result. For instance EINTR is quite useless there). - 0 on disconnect - Otherwise the number of bytes received. Note that read, when the socket blocks, will not return at all until it has data (or an error or end of file). So there's no reason to loop until you get anything: it only returns when you get anything. Your program might contain another bug that a lot of programs have and actually is ignored by most coders. If the socket is TCP, this protocol is stream based. That means that the server, if it wants to send 2 bytes, might only send 1 byte first (theoretically, although in practice I've never seen it happen). So read() might return only one byte even though the server sent two. So if you expect more bytes, keep reading until you have as much as you expect.
EVOEx is offline

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