Perl and server-client sockets

Question

Background/Context

I have a two scripts, a server-side script that can handle multiple clients, and a client-side script that connects to the server. Any of the clients that send a message to the server have that message copied/echoed to all the other connected clients.

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Where I'm stuck.

This afternoon, I have been grasping at thin air searching for a thorough explanation with examples covering all that there is for Perl and TCP sockets. A surprising large number of results from google still list articles from 2007-2012 . It appears there originally there was the 'Socket' module , and over time IO::Socket was added , then IO::Select. But the Perldoc pages don't cover or reference everything in one place, or provide sufficent cross referencing links. I gather that most of the raw calls in Socket have an equivalent in IO::Socket. And its possible (recommended ? yes/no?) to do a functional call on the socket if something isn't available via the OO modules...

Problem 1. The far-side/peer has disconnected / the socket is no longer ESTABLISHED?

I have been trying everything I ran across today, including IO::Select with calls to can_read, has_exception, but the outputs from these show no differences regardless if the socket is up or down - I confirmed from netstat output that the non-blocking socket is torn down instantly by the OS (MacOS).

Problem 2. Is there data available to read?

For my previous perl client scripts, I have rolled my own method of using sysread (https://perldoc.perl.org/functions/sysread.html) , but today I noticed that recv is listed within the synopsis on this page near the top https://perldoc.perl.org/IO/Socket.html , but there is no mention of the recv method in the detailed info below...

From other C and Java doco pages, I gather there is a convention of returning undef, 0, >0, and on some implementations -1 when doing the equivalent of sysread. Is there an official perl spec someone can link me to that describes what Perl has implemented? Is sysread or recv the 'right' way to be reading from TCP sockets in the first place?

I haven't provided my code here because I'm asking from a 'best-practices' point of view, what is the 'right' way to do client-server communication? Is polling even the right way to begin with? Is there an event-driven method that I've somehow missed..

My sincere apologies if what I've asked for is already available, but google keeps giving me the same old result pages and derivative blogs/articles that I've already read.

Many thanks in advance.

Answer 1

And its possible (recommended ? yes/no?) to do a functional call on the socket if something isn't available via the OO modules...

I'm not sure which functional calls you refer to which are not available in IO::Socket. But in general IO::Socket objects are also normal file handles. This means you can do things like $server->accept but also accept($server).

Problem 1. The far-side/peer has disconnected / the socket is no longer ESTABLISHED?

This problem is not specific to Perl but how select and the socket API work in general. Perl does not add its own behavior in this regard. In general: If the peer has closed the connection then select will show that the socket is available for read and if one does a read on the socket it will return no data and no error - which means that no more data are available to read from the peer since the peer has properly closed its side of the connection (connection close is not considered an error but normal behavior). Note that it is possible within TCP to still send data to the peer even if the peer has indicated that it will not send any more data.

Problem 2. Is there data available to read?

sysread and recv are different the same as read and recv/recvmsg or different in the underlying libc. Specifically recv can have flags which for example allow peeking into data available in the systems socket buffer without reading the data. See the the documentation for more information.

I would recommend to use sysread instead of recv since the behavior of sysread can be redefined when tying a file handle while the behavior of recv cannot. And tying the file handle is for example done by IO::Socket::SSL so that not the data from the underlying OS socket are returned but the decrypted data from the SSL socket.

From other C and Java doco pages, I gather there is a convention of returning undef, 0, >0, and on some implementations -1 when doing the equivalent of sysread. Is there an official perl spec someone can link me to that describes what Perl has implemented?

The behavior of sysread is well documented. To cite from what you get when using perldoc -f sysread:

... Returns the number of bytes actually read, 0 at end of file, or undef if there was an error (in the latter case $! is also set).

Apart from that, you state your problem as Is there data available to read? but then you only talk about sysread and recv and not how to check if data is available before calling these functions. I assume that you are using select (or IO::Select, which is just a wrapper) to do this. While can_read of IO::Select can be used to get the information in most cases it will return the information only from the underlying OS socket. With plain sockets this is enough but for example when using SSL there is some internal buffering done in the SSL stack and can_read might return false even though there are still data available to read in the buffer. See Common Usage Errors: Polling of SSL sockets on how to handle this properly.