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Problem with running application after updating QT

  • Hello,

    A friend of mine created a program using QT that runs on Ubuntu 12.04. This program has been running for a few years now without any problems but since I updated Ubuntu the program crashes when someone tries to login in (it is a server and people can login using a client).

    I'm not a professional coder so I googled how I could fix this and most topics linked to valgrind or gdb. I ran both debuggers but I still can't find the problem.

    The output valgrind gives is:

    bq. ==18300== Thread 2:
    ==18300== Use of uninitialised value of size 4
    ==18300== at 0x455E814: QString::operator=(QString const&) (in /usr/lib/i386-linux-gnu/
    ==18300== by 0x4D9B657: ???
    ==18300== Invalid read of size 4
    ==18300== at 0x455E814: QString::operator=(QString const&) (in /usr/lib/i386-linux-gnu/
    ==18300== by 0x4D9B657: ???
    ==18300== Address 0x0 is not stack'd, malloc'd or (recently) free'd
    ==18300== Process terminating with default action of signal 11 (SIGSEGV)
    ==18300== Access not within mapped region at address 0x0
    ==18300== at 0x455E814: QString::operator=(QString const&) (in /usr/lib/i386-linux-gnu/
    ==18300== by 0x4D9B657: ???
    ==18300== If you believe this happened as a result of a stack
    ==18300== overflow in your program's main thread (unlikely but
    ==18300== possible), you can try to increase the size of the
    ==18300== main thread stack using the --main-stacksize= flag.
    ==18300== The main thread stack size used in this run was 8388608.
    ==18300== HEAP SUMMARY:
    ==18300== in use at exit: 256,719 bytes in 823 blocks
    ==18300== total heap usage: 5,321 allocs, 4,498 frees, 1,622,113 bytes allocated
    ==18300== LEAK SUMMARY:
    ==18300== definitely lost: 0 bytes in 0 blocks
    ==18300== indirectly lost: 0 bytes in 0 blocks
    ==18300== possibly lost: 5,764 bytes in 50 blocks
    ==18300== still reachable: 250,955 bytes in 773 blocks
    ==18300== suppressed: 0 bytes in 0 blocks
    ==18300== Rerun with --leak-check=full to see details of leaked memory
    ==18300== For counts of detected and suppressed errors, rerun with: -v
    ==18300== Use --track-origins=yes to see where uninitialised values come from
    ==18300== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)

    I only see links to the qt libs but I don't think there is a problem with QT. Do you have any idea how I can fix this?

  • Lifetime Qt Champion

    Hi and welcome to devnet,

    Valgrind is a memory analyzer not a debugger. In your case you should use gdb to find the crash source

  • bq. Program received signal SIGSEGV, Segmentation fault.
    [Switching to Thread 0xb5d82b40 (LWP 18368)]
    0xb7946814 in QString::operator=(QString const&) () from /usr/lib/i386-linux-gnu/

    I ran gdb and i got this message back from the system. Because of the Segmentation Fault I used valgrind

  • Lifetime Qt Champion

    You should have more information about what lead to that crash, e.g. all functions called before this one

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