Question

I found a bunch of scripts in the project I have been newly assigned to that are the ‘shutdown’ scripts. They just do some basic searches and run the Unix kill command. Is there any reason they shouldn’t shutdown the process this way? Does this ensure that dynamically allocated memory will return properly? Are there any other negative effects? I’ve operated under an intuition that this is a last resort way of terminating a process.

Answer 1

The kill command sends a signal to a Unix process. That signal defaults to SIGTERM, which is a polite request for the program to exit.

When a process exits for any reason, the Unix OS does clean up its memory allocations, file handles and other resources. The only resources that do not get cleaned up are those that are supposed to be shared, like the contents of files and of shared memory (like System V IPC).

Many programs do not need to do any special cleanup on exit and use the default SIGTERM behavior, which is to let the OS stop the process.

If a program does need special behavior, it can install a signal handler, and it can then run a function to handle the signal.

Now the SIGKILL signal, which is number 9, is evil, but also necessary. This signal never gets to the process itself, the OS simple stops the process. This should only be used when really, really necessary. It often becomes necessary in multithreaded programs that get into deadlocks or programs that have installed a TERM signal handler, but screwed up during their exit process.