Chapter 03 - Child inherits PID of parent with exec()?
I am a bit confused with this part of the chapter 03 - Creating Processes (about Fork and Exec):
Often, rather than just a fork, one follows it with an exec, where the parent process terminates, and the child process inherits the process ID of the parent.
When a process forks, the child gets a new PID. But if the child does an exec()
, it terminates the parent and inherits its PID?
What if a process creates multiple forks and each child runs the exec()
? Would that create multiple processes with the same PID?
Doesn't terminal emulators use this fork-exec pattern to run the commands that we type? Running a command foo
(that I would expect to fork and exec(foo)
) does not kill the terminal (the parent process). So, terminals don't use this pattern?
Comments
-
Hi @heitorpb ,
I'd suggest starting by reading the man pages for exec(3), execve(2), and fork(2). Fork(2) will provide you with valuable information about how it works.
I also found a couple of URLs that can be of help:
https://stackoverflow.com/questions/1653340/differences-between-fork-and-exec
https://programmerbay.com/difference-between-fork-and-exec/
Regards,
Luis.1 -
It's actually pretty simple. We are not talking about doing a fork() and then an exec*(). Note the text says:
Often, rather than just a fork, one follows it with an exec, where the parent process terminates, and the child process inherits the process ID of the parent.
Note the rather. an exec creates a new process with the same ID, one doesn't do both.
1 -
Thanks for the answers! Those man pages were very helpful.
Often, rather than just a fork, one follows it with an exec, where the parent process terminates, and the child process inherits the process ID of the parent.
I think my main source of confusion was English here (English is not my first language)... what I understood of this sentence is that if a child does an
exec()
, the parent dies and the child changes its PID to the parent PID.Is it possible to suggest to rewrite this paragraph in the course material? I would be glad to help
Just to be sure that I understood this:
- if a process does a
fork()
and it succeeds, it clones itself. The child is another process with a different PID. - if a process does an
exec()
, regardless of being a child or not, it changes its own program to a different one. This does not affect the parent of the process doing theexec
. This does not change the PID of the process, only the program running. - if a process has multiple threads, they are all terminated when one of them calls
exec
. The thread that calledexec
will survive, but with a different program.
Are those items correct?
In the case that a process has multiple threads: what happens to the other threads if the
execve
fails? Do they terminate as well?0 - if a process does a
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