QProcess::start and GDB leading to missing symbols / ?? in stack

Uriel Elias

I am currently having some trouble debugging a program that starts processes via QProcess. Simply executing the binary without dbg works just fine but when I try to debug the executable with gdb I am getting a SIGTRAP when the process has started. After that the stack always shows '??' instead of function names. When continuing I get a SIGILL.

In my project I also get following output:

Probes-based dynamic linker interface failed.

I am not sure whether this is related to loaded plugins or libraries.
The problem can be reproduced (except from the " Probes-based dynamic linker interface failed." output) with the following code:

#include <QCoreApplication>

#include <QProcess>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QProcess proc;
    QString exe = "echo";
    QStringList arguments;
    arguments << "Test";

    proc.start(exe, arguments);
    if (!proc.waitForFinished()) {
        qDebug() << "failed to start" << proc.exitCode() << proc.errorString();
    }

    qDebug() << "process finished";
    qDebug() << proc.readAllStandardOutput();

    return a.exec();
}

My OS is arch linux (up to date)

Qt5: qt5-base 5.15.1-1
GDB: gdb 9.2-1
clang++: clang 10.0.1-1

Does anybody have an idea what is causing the SIGTRAP when debugging with dbg?

(This is a copy of my question on stackoverflow: https://stackoverflow.com/questions/63968813/why-do-qprocess-qt-5-15-1-and-gdb-lead-to-missing-symbols)

Uriel Elias

I found out that the trap is not caused when no breakpoint is set.

JonB

@Uriel-Elias
I don't know what the cause/solution is, but for Probes-based dynamic linker interface failed you probably need to read https://dustri.org/b/solving-warning-probes-based-dynamic-linker-interface-failed-in-gdb.html and https://github.com/matrix-construct/construct/issues/58 to see where others are reporting it.

Uriel Elias

@JonB thank you for your answer!
I found these two posts too but I am not sure whether my issue is related to my main problem: Crashing when starting a subprocess.
Maybe this might explain my problems with "Probes-based dynamic linker interface failed" but I am afraid that the subprocess issue might be a little more tricky...

Luca Ottaviano

@Uriel-Elias I'm pretty sure I'm hitting a similar issue. Did you find a solution?

Uriel Elias

Yes and no - it seems to have been solved in an update.
I checked it right now and the issue does not persist.

I did not really find out what caused the problems.