Hi, guys:


I am working on the shell using c language. I just would like to ask when I run some command in the backgrounds, how can I allow interleave out?

I want to the output like this:

shell> command1
shell> command2 &
shell> command3


This is the code to run the command:

pid=fork();

if (pid==0&&bg==0) // foreground
{


exec(command); // execute a command

exit(1);

}

else if (pid==0&&bg==1) // background
{

setpgid(pid,0);


exec(command); // execute a command

exit(1);
}

else if (bg!=1)
{
waitpid();

}




Thanks

There's no easy way to control what gets written when you share output to your /dev/tty between multiple (essentially independent) processes.

One alternative is to make the processes work in lockstep (instead of in parallel). Not a good solution

Another is to create some kind of semaphore (either a bona fide "ipc semaphore", or some global alternative) to regulate who does a "printf()" and who waits. Again - not a good solution.

Another choice is to redirect stdout to a disk file (e.g. "my_child_prog > $$.txt"), and then have the "master" process read the individual files at its leisure.

'Hope that helps .. PSM

I've used advisory file locking with success. See man fcntl and look for F_GETLK.
Kevin Barry

You could write something that opens a pipe reading from each command and does (pseudo code)

read(pipe1,s); print s;
read(pipe2,s); print s;
read(pipe3,s); print s;

Alternatively, if it's not for real-time, you could tag each line with a number and join them after.