pipes for process communication

[foxcar]

Hi,

I am trying to write a program that can start a new process and communicate with it interactively. I want to be able to grab the output as soon as it is available and also be able to write to the newly created process's stdin. I am using Python, but I think the help I need is not relevant to the language but rather the lowlevel C functions, which are available in Python. Here is the pseudo code for what I have tried doing:

childr, parentw = pipe()
parentr, childw = pipe()

fork()
if child
     stdin = childr
     stdout = childw
     execvp(.....)
if parent
     while true
           readlines from parentr
           get input
           write input to parentw

I think my issue is getting the the stdout and stdin of the child process redirected to the file descriptors returned by the call to pipe(). From what I have read, execvp() will preserve open fd's so perhaps the way I am reassigning stdin and stdout is wrong?

I have looked at the pexpect module but it is not available on the production machines I need to do this on.

[maheshgupta024]

If i understand correct, are u directly assigning the childr and childw to stdin and stdout..
If you want so.. use the dup call. that will redirect the stdin to childr and stdout to childw.
Hope this helps.

[foxcar]

Ok here is what I have. I want to use this script to interact with another process, using the python script as an interface.

childr, parentw = pipe()
parentr, childw = pipe()

fork()
if child
     dup2(childr, 0)
     dup2(childw, 1)
     dup2(childw, 2)
     execvp('...t')
if parent
     while true
           readlines from parentr
           get input from stdin
           write input to parentw

But, I get an error message saying:
Warning: no access to tty (Bad file descriptor).
Thus no job control in this shell. I am trying to run ssh in my tests, btw.

[maheshgupta024]

Can you give me at what line u r getting the warning..
I dont understand y are u getting "no access to tty"

[foxcar]

There is no line number. Once execvp() runs, ssh throws the error and I read it in my while loop. It seems that calling dup() on stdin is what triggers it.

[foxcar]

I think I may have figured it out. I looked through the source code for a module called pexpect and communication is through a pseudo terminal, not a pipe. Seems to be working decently.