Skip to content
  • Categories
  • Recent
  • Tags
  • Popular
  • Users
  • Groups
  • Search
  • Get Qt Extensions
  • Unsolved
Collapse
Brand Logo
  1. Home
  2. Qt Development
  3. General and Desktop
  4. Appending QStringList crashes
Forum Update on Monday, May 27th 2025

Appending QStringList crashes

Scheduled Pinned Locked Moved Solved General and Desktop
17 Posts 4 Posters 2.9k Views
  • Oldest to Newest
  • Newest to Oldest
  • Most Votes
Reply
  • Reply as topic
Log in to reply
This topic has been deleted. Only users with topic management privileges can see it.
  • H Hubbard

    I have a large program which does many things at once (including multiple threads)

    and a small addition to a particular part of my code crashes it, in the mainWindow class a pointer QStringList exists that stores a set of mac addresses, this QStringList is initialised initially with ownedMacAddresses = &tempStringList, and later on in the code prompted by the user a mac address can be found and passed by way of signal to a slot which processes it, which is fine (QString mac is passed through the connection), however, when I try to ownedMacAddresses->append(mac), it crashes when the user clicks the button at this point, why?

    jsulmJ Offline
    jsulmJ Offline
    jsulm
    Lifetime Qt Champion
    wrote on last edited by jsulm
    #2

    @Hubbard Can you please show the code? Else others can only guess what's wrong.
    Also - do you access this variable from several threads?

    https://forum.qt.io/topic/113070/qt-code-of-conduct

    1 Reply Last reply
    3
    • H Offline
      H Offline
      Hubbard
      wrote on last edited by
      #3

      No the program is pretty secure and tight in terms of things like this

      1 Reply Last reply
      0
      • H Offline
        H Offline
        Hubbard
        wrote on last edited by
        #4

        In fact it seems to crash using 'mac' for anything at all outside of using it to initialize a class and output it through qDebug() once, Its hard for me to show the code as it is on a different platform.

        O jsulmJ 2 Replies Last reply
        0
        • H Hubbard

          In fact it seems to crash using 'mac' for anything at all outside of using it to initialize a class and output it through qDebug() once, Its hard for me to show the code as it is on a different platform.

          O Offline
          O Offline
          ollarch
          wrote on last edited by
          #5

          @Hubbard Use the debugger. Maybe you are using an invalid pointer.

          H 1 Reply Last reply
          2
          • H Hubbard

            In fact it seems to crash using 'mac' for anything at all outside of using it to initialize a class and output it through qDebug() once, Its hard for me to show the code as it is on a different platform.

            jsulmJ Offline
            jsulmJ Offline
            jsulm
            Lifetime Qt Champion
            wrote on last edited by
            #6

            @Hubbard Well, then I have no idea.
            You should at least try to debug your app. Most probably the pointer is invalid at the time the app is crashing. What type of crash is it actually (SIGSEGV, ...)?

            https://forum.qt.io/topic/113070/qt-code-of-conduct

            1 Reply Last reply
            2
            • O ollarch

              @Hubbard Use the debugger. Maybe you are using an invalid pointer.

              H Offline
              H Offline
              Hubbard
              wrote on last edited by
              #7

              @ollarch You're right...

              none of it is the problem apart from trying to use ownedMacAddresses
              just

              qDebug() << ownedMacAddresses[0];

              in the slot

              crashes the program

              jsulmJ 1 Reply Last reply
              0
              • H Hubbard

                I have a large program which does many things at once (including multiple threads)

                and a small addition to a particular part of my code crashes it, in the mainWindow class a pointer QStringList exists that stores a set of mac addresses, this QStringList is initialised initially with ownedMacAddresses = &tempStringList, and later on in the code prompted by the user a mac address can be found and passed by way of signal to a slot which processes it, which is fine (QString mac is passed through the connection), however, when I try to ownedMacAddresses->append(mac), it crashes when the user clicks the button at this point, why?

                JonBJ Offline
                JonBJ Offline
                JonB
                wrote on last edited by
                #8

                @Hubbard said in Appending QStringList crashes:

                &tempStringList

                Has tempStringList gone out of scope?

                H 1 Reply Last reply
                1
                • JonBJ JonB

                  @Hubbard said in Appending QStringList crashes:

                  &tempStringList

                  Has tempStringList gone out of scope?

                  H Offline
                  H Offline
                  Hubbard
                  wrote on last edited by
                  #9

                  @JonB No but when I qDebug() << it from somewhere else it doesn't crash but outputs the element in brackets

                  ("2C:3A:E8:37:D8:6A") as opposed to "2C:3A:E8:37:D8:6A"

                  1 Reply Last reply
                  0
                  • H Offline
                    H Offline
                    Hubbard
                    wrote on last edited by
                    #10

                    Never ->at(0) instead fixes that

                    1 Reply Last reply
                    0
                    • H Offline
                      H Offline
                      Hubbard
                      wrote on last edited by
                      #11

                      Still crashes inside the slot

                      qDebug() << ownedMacAddresses->at(0);

                      anywhere else in mainWindow is fine, in this slot, nah

                      1 Reply Last reply
                      0
                      • H Hubbard

                        @ollarch You're right...

                        none of it is the problem apart from trying to use ownedMacAddresses
                        just

                        qDebug() << ownedMacAddresses[0];

                        in the slot

                        crashes the program

                        jsulmJ Offline
                        jsulmJ Offline
                        jsulm
                        Lifetime Qt Champion
                        wrote on last edited by jsulm
                        #12

                        @Hubbard said in Appending QStringList crashes:

                        qDebug() << ownedMacAddresses[0];

                        This can't work the way you expect it to work, because ownedMacAddresses is a pointer. If you want to do it this way then so:

                        qDebug() << (*ownedMacAddresses)[0];
                        

                        "Still crashes inside the slot
                        qDebug() << ownedMacAddresses->at(0);" - did you try to place a breakpoint at that line and see whether ownedMacAddresses is a valid pointer? Also - what kind of crash is it (SIGSEGV?)?

                        https://forum.qt.io/topic/113070/qt-code-of-conduct

                        1 Reply Last reply
                        1
                        • H Offline
                          H Offline
                          Hubbard
                          wrote on last edited by
                          #13

                          What do you mean by 'valid pointer'?

                          jsulmJ 1 Reply Last reply
                          0
                          • H Offline
                            H Offline
                            Hubbard
                            wrote on last edited by
                            #14

                            If its related I have begun to pass the pointer through the signal

                            now the crash error gives me an 'index out of range' error at element 0, implying something is definitely not right with the pointer to the array

                            1 Reply Last reply
                            0
                            • H Offline
                              H Offline
                              Hubbard
                              wrote on last edited by
                              #15

                              Like 'emit signal(..., QStringList* currentMacs);'

                              1 Reply Last reply
                              0
                              • H Hubbard

                                What do you mean by 'valid pointer'?

                                jsulmJ Offline
                                jsulmJ Offline
                                jsulm
                                Lifetime Qt Champion
                                wrote on last edited by jsulm
                                #16

                                @Hubbard Mit "valid pointer" I mean a pointer pointing to reserved memory and not to nirvana.

                                QStringList* p1; // Invalid pointer
                                QStringList* p2 = new QStringList(); // Valid pointer
                                

                                "Index out of range at element 0" means that you do not have any elements in the list. Check that with:

                                qDebug() << currentMacs->size();
                                

                                https://forum.qt.io/topic/113070/qt-code-of-conduct

                                1 Reply Last reply
                                1
                                • H Offline
                                  H Offline
                                  Hubbard
                                  wrote on last edited by
                                  #17

                                  I see now

                                  QStringList declared in the header file makes a tonne more sense and works better than QStringList*.

                                  Thinking about it QStringList and arrays and things come with their own addresses anyway.

                                  Thank you guys

                                  1 Reply Last reply
                                  0

                                  • Login

                                  • Login or register to search.
                                  • First post
                                    Last post
                                  0
                                  • Categories
                                  • Recent
                                  • Tags
                                  • Popular
                                  • Users
                                  • Groups
                                  • Search
                                  • Get Qt Extensions
                                  • Unsolved