How to share the same variable between parent and child process?

zwylinux · 03-24-2007, 09:28 AM

There is a variable named "i", which equals 0
Now I want to do this:

for example ,i=0;
in the child process i=1;
Then I want to make i equals to 2 in the parent process.

How can i do that?

omnio · 03-24-2007, 09:57 AM

Are you reffering to shell scripting?
As you probably know, child processes can't pass variables to their parents directly. But there are some ways to have interprocess communication, even between shell scripts (processes).

1. sending signals. Have the child send a signal to its parent, and (inside the parent script or function or whatever) trap that signal and make it set the variable. Something like this:

Code:

#!/bin/bash
# parent script

var_change() {
    i=$(($i+1))
}

trap var_change INT
export parent=$$
i=0

./child
........
trap - INT

Code:

#!/bin/bash
# child script

kill -s SIGINT $parent
........

2. use files; make the child output to a file, and the parent read from that file.
3. If you launch a subshell (like in a loop, or using a command list in parentheses and don't launch in background (with "&")), echo that variable inside the subshell and use command substitution, setting a variable to it, in the main script. Like this:

Code:

# main script
i=0

some_fnc() {
    while true; do
        ...
        i=1
        echo $i
        ...
    done
}

i=$(some_fnc)

And probably there are some other ways of doing this.

zwylinux · 03-24-2007, 12:26 PM

I forgot to say that I want to make it in C Language

indienick · 03-24-2007, 12:38 PM

I don't know much about C (or C++ for that matter), but you might be able to pass variables back and forth between parent and child processes using memory pointers.

Going hand-in-hand with my lack of knowledge of C, I am not sure if the parents and children processes can directly interact with each other, but if they can't, you could always build up all the needed variables, and pass the memory pointers as arguments to the child process when it's spawned, that way, it's modifying the value, not the reference, and will affect the parent process' values it obtains from those memory locations as well.

kourama · 03-24-2007, 07:18 PM

There are a few of ways to do IPC (inter-process comunication), but what you're describing sounds like shared memory.

Here's a link to a page with some code and good descriptions:
http://www.cs.cf.ac.uk/Dave/C/node27.html

On a side note: I found that shared memory doesn't work for me in cygwin, but the last time I tried it was a while ago, maybe there has been an update since then.

good luck.

theNbomr · 03-25-2007, 06:22 PM

Or perhaps it would be appropriate to use threads, where all variables are 'shared' among peer threads.
--- rod.

bigearsbilly · 03-26-2007, 02:47 AM

in short, no you can't.
a fork produces an exact copy of the parent.
Like a clone, it looks the same, acts the same but if you poke it in
the eye the other won't feel it.

If you must then I would open a socket between them,
you can communicate all you like then.
The good thing about sockets is you can plug them in anywhere.
So they are a good thing to learn about.

I personally have never felt
the need to share memory or use threads.

maybe your design could be simpler?