Hi,guys:

I want to use c to implement a pipe. For example:

ps auxwww | grep fred | more


I forked three child processes. Each is responsible for each command, and pipe to next one.



for(i=0;i<2;i++)
pipe(fd[i])

if(child==1) // child 1
{

close(1)
dup2(fd[0][1],1)
close(fd[0][0])
}
else if (child==1) // child 2
{
close(0)
close(1)
dup2(fd[0][0],0)
dup2(fd[1][1],1)
close(fd[0][1])
close(fd[1][0])
}

else if(child==2) //child3
{
close(0)
dup2(fd[1][0],0)
close(fd[1][1])
}




But it does not work, it seems like there is something wrong with child 2. Can anyone give me some suggestions?

This is only going to run once with i=1...

What do you mean? How can change that ?


Thanks

I'm not a programmer, but logically, this statement:

for(i=0;i<2;i++)

says, let i = 0, only run while less than 2, add one to i. So by the end of that line, i is already = to 1.

So, this will only run once, while i = 1.

Actually, it will run twice. Once for fd[0] and once for fd[1]. You can change it in either of the following to resolve it:

or

Not true. It will run with i==0 (i<2.) At the end of an iteration i++ and since i<2, it will run once more with i==1. After i++, however, i==2, so it won't run again. i++ happens at the end of the iteration; it wouldn't be logical to have it happen before.
Kevin Barry