#22 Exit status reporting seems to be unreliable

[Johannes Wienke]

I am running a subprocess call to the unix ps utility in a loop with a frequency of about 20 Hz. This works well for some time, but at some point in time, pstreams reports a weird exit status of the ps subprocess. This status does not make any sense at all to me and seems to be wrong, because I have put the same ps call into a while loop in my shell and it runs forever without errors.

This is a condensed form of what I am doing:

redi::ipstream in("ps --no-headers -eo ppid,pid",
        redi::pstreams::pstdout | redi::pstreams::pstderr);
string s;
while (getline(in, s)) {
    // here I do some output parsing.
}
int status = in.rdbuf()->status();
if (status != 0) {
    cout << "STATUS: " << status << endl;
    cout << "WIFEXITED: " << WIFEXITED(status) << endl;
    cout << "WEXITSTATUS: " << WEXITSTATUS(status) << endl;
    cout << "WIFSIGNALED: " << WIFSIGNALED(status) << endl;
    cout << "WTERMSIG: " << WTERMSIG(status) << endl;
    cout << "WIFSTOPPED: " << WIFSTOPPED(status) << endl;
    cout << "WSTOPSIG: " << WSTOPSIG(status) << endl;
    cout << "WIFCONTINUED: " << WIFCONTINUED(status) << endl;

    // print output from stderr
    in.clear();
    string s;
    cout << "Error:" << endl;
    while (getline(in.err(), s)) {
        cout << s << endl;
    }
    cout << "Error end" << endl;
}

As said, this works well for thousands of iterations, but with a high probability, it fails at some point in time. The output I see for the different interpretations of the status code is the following:

STATUS: -1
WIFEXITED: 0
WEXITSTATUS: 255
WIFSIGNALED: 0
WTERMSIG: 127
WIFSTOPPED: 0
WSTOPSIG: 255
WIFCONTINUED: 0
Error:
Error end

How can it be that neither there is error output, nor there was a return code from the process or anything like a TERM signal?

[Johannes Wienke]

I just noticed that it might be necessary to call close in the ipstream instance before having the status set correctly. I implemented this strategy, but still i encounter the exact same error.

[Jonathan Wakely]

http://pstreams.sourceforge.net/doc/classredi_1_1basic__pstreambuf.html#a6b8782afacf7c7f92617798499f46702

Returns: The exit status of the child process, or -1 if wait() has not yet been called to wait for the child to exit.

Are you calling wait() to ensure the process has exited?

[Johannes Wienke]

How can I call this? It is a protected method.

[Jonathan Wakely]

By calling one of the functions that call wait(), e.g. close()

I see in your second comment you say you added that. Without that your code was definitely wrong. I don't know if it's still wrong without seeing the code.

If status() returns -1 then it means you haven't waited for the process to exit.

[Johannes Wienke]

I think I found the issue. I have created a situation where too many files were open, so starting the subprocess in the first place failed because no more pipes could be created. I probably should also check whether the stream is good before starting to read from it.

[Johannes Wienke]

Ok, this issue can be closed. When checking the good flag of the stream, the error becomes evident.

[Jonathan Wakely]

OK, thanks for the follow-up