Communicating with a C program without altering stdin/stdout/stderr

[Mike Aubury]

Hi,

I want to be able to communicate with a C program while maintaining stdio streams as-is - sort of like the way NodeJS can send messages to child nodejs programs.

I can see in the documentation - i can use something like : 

   spawn('prg', [], { stdio: ['pipe', 'pipe', 'pipe', 'pipe' /* extra one here */] });

But - what do i need to do on the C side to link back to that new pipe ? 

Do I need 2 pipes (one for read, one for write)

Am I better trying to do something with IPC ?

Any ideas, and/or code samples would be greatly appreciated !

[Dave Benson]

you can access the new file-descriptor from C simply as 3, the index of extra 'pipe'.  for example:

   read(3, buf, sizeof(buf))

   write(3, buf, sizeof(buf))

   fcntl(3, ...)

i usually give my programs command-line arguments for the file descriptor, as it feels a bit ugly to hard-code the relationship.  But, there's plenty of precedence for hard-coding as well.

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[Mikkel Wilson]

This would be a good case for using Native Addons for Node (nan). https://nodejs.org/api/n-api.html . You may be tempted to use the new C++ addons for node (https://nodejs.org/api/addons.html), but it's not API complete and spits annoying warnings (even with flags) so I'd suggest avoiding it.

The raw docs on nodejs.org are a bit hard to get started with, but I found this article helpful. I think it's still relevant. https://community.risingstack.com/using-buffers-node-js-c-plus-plus/

Also, the libsodium library has a bunch of macros that teach you how to do data conversion to/from node/C: https://github.com/paixaop/node-sodium/blob/master/src/include/node_sodium.h

Best,

Mikkel