proc-alive
This is a sample project (supporting this blog post) demonstrating how to check whether a process (usually a daemon) is alive or not as cheaply as possible, ie a single syscall. The idea relies on the behavior of named pipes (aka FIFO) - specifically the following part:
A process can open a FIFO in nonblocking mode. In this case, opening for read-only succeeds even if no one has opened on the write side yet and opening for write-only fails with ENXIO (no such device or address) unless the other end has already been opened.
Let's define the following two processes:
- runner - the process whose status we're interested in knowing, this process may not get a chance to exit gracefully
- checker - the process which will be responsible for checkin the status of runner
runner and checker first need to establish some shared randevu point, such as a path - usually this would be a path to a .pid file or the like, in our case the runner and checker will be able to create the path to a named pipe. The pipe will be used as follows:
- runner - at startup, creates and/or opens the pipe for read-only in non-blocking mode (O_RDONLY | O_NONBLOCK). Cleanup is not strictly necessary as the OS will detach the process from the pipe on exit (clean or not)
- checker - attempt to open the file in write-only in non-blocking mode (O_WRONLY | O_NONBLOCK) - this will only succeed if the pipe exists and there is at least one reader on the pipe
I've built this in Go but the concept is useful in other languages as well, as long as they provide you access to pipes, e.g. go, python, node etc
How to
Build
Simply run
make
This will create two binaries, runner and checker in bin.
To test
- First, to emulate a daemon execute
runnerwith a path to a location you have read-write access to and an amount of time for therunnerto sleep
bin/runner /tmp/mah-pipe 120
- (in another window) Check the status of the
runnerby using the following command - feel free to kill the runner, reboot etc
bin/checker /tmp/mah-pipe