Reading data from a socket

Question

I am having issues reading data from a socket. Supposedly, there is a server socket that is waiting for clients to connect. When I write a client to connect() to the server socket/port, it appears that I am connected. But when I try to read() data that the server is supposedly writing on the socket, the read() function hangs until the server app is stopped.

Why would a read() call ever hang if the socket is connected? I believe that I am not ever really connected to the socket/port but I can't prove it, b/c the connect() call did not return an error. The read() call is not returning an error either, it is just never returning at all.

Perhaps you have to send a message to the server before it will respond to you. — Jonathan Leffler
– Jonathan Leffler, Commented Apr 29, 2010 at 19:26

WhirlWind · Accepted Answer · 2010-04-29 19:07:10Z

3

Read is blocking until is receives some I/O (or an error).

answered Apr 29, 2010 at 19:07

WhirlWind

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Rakis · Accepted Answer · 2010-04-29 19:52:52Z

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As John & Whirl mentioned, the problem is almost certainly that the server hasn't sent any data for your read() call to return. Another easy thing to overlook when you're starting out with network programming is that the data transfered in a server's write() call is not always symmetrical with a client's read() call. Where the server may write("hello world"), your read() could easily return "hello world", "hello wo", "hel", or even just "h"

answered Apr 29, 2010 at 19:52

Rakis

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Mark B · Accepted Answer · 2010-04-29 19:24:16Z

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Unless you explicitly changed your reader's socket to non-blocking mode, a call to read will do exactly what you say until there is data available: It will block forever until some data is actually read.

You can also use netstat (I use it with -f inet) to figure out connections that have been made and see the status of your socket connection.

answered Apr 29, 2010 at 19:24

Mark B

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John Gordon · Accepted Answer · 2010-04-29 19:15:25Z

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Your server is probably not writing data to the socket, so your reader just blocks waiting for data to appear on the socket.

answered Apr 29, 2010 at 19:15

John Gordon

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