Is it possible to "daemonize a command"?

Question

I have a command line utility which is able to receive an image via stdin and convert it to a different format and output it to stdout.

I use this from a program, but every time I run it, a subprocess needs to be invoked which is very slow.

So I wonder is there a way, ideally using systemd, to turn this command line program into a "server" that remains in memory and somehow is able to receive image data and output it?

Answer 1

1. you could of course start the program ahead of time, but not send anything to its stdin yet; then, when you need it, it's already started. Of course, after you've "used" it, the process would be dead, and you need to start another one. But assuming conversion and writing to a file isn't instantaneous either, you could already start the next process and hold it ready for a subsequent image. In fact, you could have a pool of waiting processes. Pretty standard technique from the early days of Linux webservers – "worker processes". (This all has nothing to do with systemd.)
2. This is a library. The right thing to do is not to hand of data to a separate process, if that handoff is costly (somehow I doubt that's the case – yes, process creation is costly, but Linux will cache your executable file…). Instead, simply use the cavif library from your program directly to convert the data. That's what libraries are for.