Thread: bidirectional pipes

Originally Posted by Elkvis

since you're redirecting stdin and stdout in both the parent and the child, try something like this:

Code:

std::string s;
std::getline(std::cin, s, '\0'); //could just say 0 but '\0' makes it clear that it's a char value

I tried that and works.The only problem appears when I send s in execl command(The problem appeared when I changed the type to string)
this is the message from the compiler:

cannot pass objects of non-POD type 'struct std::string' through '...'; call will abort at runtime

What does the error Broken Pipe means?
I send data through the pipe,the child accepts it then it writes back to the pipe and the parent accepts the message as well.But when I try to resend information to the child it returns that message.
In the parent's side I have:

Code:

for(int i=0;i<3;i++){
   cout<<"Send a message to the child"<<endl;
   //cin whatever the child sends back
}

It runs only for one loop and then that message appears...

I believe that error means that the pipe has been closed at one end.

Originally Posted by Elkvis

I believe that error means that the pipe has been closed at one end.

yes I figured it out..well,besides that error that doesn't appear any more,the parent only manages to write to the pipe and read from it only once.That means that while the parent does:

Code:

//parent sends s (message) to the bidirectional pipe
cout<<s<<endl;
//parent reads any data the child sends 
//cin is due to dup2 that has been used
getline(cin, str, '\0');
//parent writes everything that was received by the child through the
//bidirectional pipe to a file
myfile << str<<endl;

the child does:

Code:

//reads message s from the bidirectional pipe which has been sent //from the parent
cin.getline(s,LINESIZE);
//then the child reads data from an ifstream and writes them to the bidirectional pipe for the parent to receive that data
while(if.getline(line,LINESIZE))
    cout<<line<<endl;

everything works fine for one loop.That means
parent writes message to pipe--->child reads message from pipe--->child writes many data to pipe--->parent reads all data the child sent through the pipe


but when I try to send a new message from the parent process the whole thing seems to reach a deadlock..
The way I try to send a message through the pipe again is the following:

Code:

for(int j=0;j<3;j++){
   cout<<s<<endl;
   getline(cin, str, '\0'); 
   myfile << str<<endl;}

but as I said only goes on for one loop,for 3 loops as above it waits...

perhaps you're not sending a '\0' at the end of the string the second time

Originally Posted by brewbuck

THERE IS NO SUCH THING AS A BIDIRECTIONAL PIPE.
Not only is it non-standard, it doesn't even work. It's not surprising you're having problems while attempting the impossible.

What I have done is not correct then?
I didn't quite understand what you suggested to do

I didn't quite understand what you suggested to do

O_o

He says:

The pipe "FlowRight" communicates from parent to child.
The pipe "FlowLeft" communicates from child to parent.

The pipe "FlowRight" can not communicate from child to parent.
The pipe "FlowLeft" cant not communicate from parent to child.

If you want to communicate in both directions you need two pipes or one socket.

Soma

did you guys read this whole thread? he is using two pipes. unless his current code is radically different from what he posted in #7, he's calling pipe() twice, and dup2()-ing his stdin and stdout in both parent and child to the proper ends of each pipe.

I'll agree that it can cause confusion to refer to a pipe as bidirectional, because they generally are not, but calling a guy out only because of the comments in his code seems a little pedantic to me, and if his mental image of the situation is such that cin/cout together constitute a "bidirectional pipe," then I don't see it really being a confusion of ideas.

calling a guy out only because of the comments in his code seems a little pedantic to me

*shrug*

I don't think that's what happened here.

I don't see it really being a confusion of ideas

The ideas in his head isn't an issue; the ideas as he conveys them is an issue because he is trying to get help for a communication issue using pipes because people will naturally assume that he is trying to use a "bidirectional pipe" when he isn't.

Soma

all the disagreement aside...

what does your current code look like?

Originally Posted by Elkvis

all the disagreement aside...

what does your current code look like?

thank you guys for your help,actually it was very confusing the way I was trying to do it when it came to many child processes so I changed it and used 2 fifo pipes one to send and one to receive data,where no deadlock occured..
thanks again for the help