Erlang Port Data Transfer Length

Question

I am trying to evaluate php code through erlang using erlang ports. The problem is when Data to be evaluated is bigger then I am getting parse error from php. But if data is smaller then I am getting the correct output. I think when the Data length is bigger erlang is truncating the data before it is being sent to php for evaluation. Is there any limit on data length which can be sent or received on erlang port. Or is this error due to some other reason ?

I am using open_port(PortName, PortSettings) to open a new port and in PortSettings I am setting [{packet,4},exit_status] as my port options.

Answer 1

The {packet, 4} tuple says the program launched to handle the other end of the port expects data in a 4-byte length-prefixed form. I don't see anything in the docs for the php(1) program that says it knows how to deal with such data. Probably the only reason it works for short inputs is that the length prefix looks kinda like ASCII if you squint, as long as the data you're sending is under 127 bytes. As soon as you go over that, PHP is probably running into a UTF-8 decoding error.

I'm pretty sure you want to say spawn here instead. This gets you standard Unix-like pipe interaction: data sent down the port goes to stdin on the launched process, and anything it sends to stdout comes back to your Erlang process.

The only problem doing it this way is that it re-launches php(1) on each transaction. This may seem expensive, but it's not too bad on any Unix type system, due to the relative efficiency of the fork(2) system call. If you're on Windows or you've benchmarked this and found that you really do need to build a FastCGI like system, you may be out of luck. There seems to be no libphp to embed PHP into a program you write to deal with packetized input, and no way to run php(1) in a way that lets it stay active on the other end of a port. You might be better off switching to a native Erlang templating system.

Also, note that the exit_status atom passed to open_port() does nothing unless you use spawn .