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 C/C++ 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. Check out this project for a GO verion.
Simply run
make
This will create two binaries, bin/runner and bin/checker
- 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