Check whether a process PID exists or not

pid_exists(pid, debug = getOption2("parallelly.debug", FALSE))

## Arguments

pid

A positive integer.

## Value

Returns TRUE if a process with the given PID exists, FALSE if a process with the given PID does not exists, and NA if it is not possible to check PIDs on the current system.

## Details

There is no single go-to function in R for testing whether a PID exists or not. Instead, this function tries to identify a working one among multiple possible alternatives. A method is considered working if the PID of the current process is successfully identified as being existing such that pid_exists(Sys.getpid()) is TRUE. If no working approach is found, pid_exists() will always return NA regardless of PID tested. On Unix, including macOS, alternatives tools::pskill(pid, signal = 0L) and system2("ps", args = pid) are used. On Windows, various alternatives of system2("tasklist", ...) are used.

## References

1. The Open Group Base Specifications Issue 7, 2018 edition, IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008) https://pubs.opengroup.org/onlinepubs/9699919799/functions/kill.html

pskill() and system2().