[Interest] QProcess vs popen: bug?

[Bob Hood]

By running two QtCreator processes, having the child process delay for 30 
seconds, and then attaching to it using the second QtCreator after the process 
in the first QtCreator start()'d it, I was able to determine that it was 
segfaulting on me during its start-up.  It was only doing this when being 
launched from the first process (launching it standalone didn't cause this).

So, figured it out. :)


On 11/12/2015 2:54 PM, Thiago Macieira wrote:
> On Thursday 12 November 2015 14:33:00 Bob Hood wrote:
>> I had a look at the code for QProcess, which seems to be doing standard
>> fork/exec launching of the process, whereas, if I'm not mistaken, popen()
>> uses  the shell to run it.  Is that the only difference here?  Should I be
>> running the QProcess version through the shell explicitly then to make it
>> function?
> No, it's not the only difference, but you can try to go through the shell to
> see if it solves your problem.
>
> The first thing you have to isolate when there's an I/O problem between
> multiple processes (pipes, sockets, etc.) is to isolate which side is at
> fault. Your symptom is that QProcess does not report anything ready to read.
> Why is that?
>
> Which side is at fault here? Did the child process even write anything to its
> pipes? If not, then QProcess is blameless.
>
> One more thing: the two snippets you pasted are not equivalent. For the
> QProcess case, you stopped at waitForStarted(), which only reports that the
> execve(2) call succeeded. It does not mean the main() function in the child
> process has been reached. On the popen case, you pasted code that does the
> I/O.
>
> So it's entirely possible there's a bug in your code but it's not in the parts
> you pasted.