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Dump QImage raw pixel data into `std::vector<char>`

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  • Christian EhrlicherC Christian Ehrlicher

    @abmyii malloc allocates memory - nothing more.
    The asan output is fine - no problems in your code. Only somwhere memory is allocated with new and deallocated with free() or allocated with malloc and deallocated with delete which is not allowed.

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    abmyii
    wrote on last edited by abmyii
    #25

    @Christian-Ehrlicher That's good to hear! I wonder what's causing the issue, then - as soon as I remove -fsanitize=address it starts failing again. Can I leave -fsanitize=address in for a release binary? Even if it was OK, I'd need to find a way to fix the above issue when running without the env variable. I'll have a look to see if I can find the offending lines of code.

    Edit: Didn't find any (especially with my untrained eyes...). The only malloc is the image one.

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    • Christian EhrlicherC Offline
      Christian EhrlicherC Offline
      Christian Ehrlicher
      Lifetime Qt Champion
      wrote on last edited by
      #26

      @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

      it starts failing again.

      This really sounds more like a threading issue.

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      • Christian EhrlicherC Christian Ehrlicher

        @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

        it starts failing again.

        This really sounds more like a threading issue.

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        abmyii
        wrote on last edited by abmyii
        #27

        @Christian-Ehrlicher How does having -fsantize=address cause it to stop failing? Also is there any way to debug it?

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        • Christian EhrlicherC Offline
          Christian EhrlicherC Offline
          Christian Ehrlicher
          Lifetime Qt Champion
          wrote on last edited by
          #28

          @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

          How does having -fsantize=address cause it to stop failing?

          When it's a thread issue then the timing is now different

          Also is there any way to debug it?

          With a debugger, yes. Examine your threads when it crashes

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          • Christian EhrlicherC Christian Ehrlicher

            @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

            How does having -fsantize=address cause it to stop failing?

            When it's a thread issue then the timing is now different

            Also is there any way to debug it?

            With a debugger, yes. Examine your threads when it crashes

            A Offline
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            abmyii
            wrote on last edited by abmyii
            #29

            @Christian-Ehrlicher

            (gdb) thread apply all bt
            
            Thread 6 (Thread 0xb0dfdbc0 (LWP 4458)):
            #0  0xb6e433a2 in ?? () from /usr/lib/arm-linux-gnueabihf/libvncserver.so.1
            #1  0x00000c00 in ?? ()
            Backtrace stopped: previous frame identical to this frame (corrupt stack?)
            
            Thread 5 (Thread 0xb15fdbc0 (LWP 4457)):
            #0  0xb69634e2 in select () at ../sysdeps/unix/syscall-template.S:84
            #1  0xb6e282c6 in ?? () from /usr/lib/arm-linux-gnueabihf/libvncserver.so.1
            Backtrace stopped: previous frame identical to this frame (corrupt stack?)
            
            Thread 4 (Thread 0xb20b0bc0 (LWP 4455)):
            #0  0xb69634e2 in select () at ../sysdeps/unix/syscall-template.S:84
            #1  0xb6e28868 in ?? () from /usr/lib/arm-linux-gnueabihf/libvncserver.so.1
            Backtrace stopped: previous frame identical to this frame (corrupt stack?)
            
            Thread 3 (Thread 0xb2f0abc0 (LWP 4454)):
            #0  __libc_do_syscall () at ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:46
            #1  0xb68badba in __pthread_cond_wait (cond=0x72f58, mutex=0x728f0) at pthread_cond_wait.c:186
            #2  0xb2f93ff4 in ?? ()
            Backtrace stopped: previous frame identical to this frame (corrupt stack?)
            
            Thread 2 (Thread 0xb3dd9bc0 (LWP 4453)):
            #0  0xb6961c00 in poll () at ../sysdeps/unix/syscall-template.S:84
            #1  0xb684c7c8 in ?? () from /usr/lib/arm-linux-gnueabihf/libmircommon.so.7
            #2  0xb6851038 in ?? () from /usr/lib/arm-linux-gnueabihf/libmircommon.so.7
            #3  0xb6d9edc8 in ?? () from /usr/lib/arm-linux-gnueabihf/libstdc++.so.6
            #4  0xb68b65b4 in start_thread (arg=0x0) at pthread_create.c:335
            #5  0xb6967c5c in ?? () at ../sysdeps/unix/sysv/linux/arm/clone.S:89 from /lib/arm-linux-gnueabihf/libc.so.6
            Backtrace stopped: previous frame identical to this frame (corrupt stack?)
            
            Thread 1 (Thread 0xb40ea000 (LWP 4441)):
            #0  0xb69658d6 in munmap () at ../sysdeps/unix/syscall-template.S:84
            #1  0xb692480e in munmap_chunk (p=<optimized out>) at malloc.c:2860
            #2  0xb6ae6f12 in QImageData::~QImageData() () from /usr/lib/arm-linux-gnueabihf/libQt5Gui.so.5
            #3  0xb6ae7122 in QImage::~QImage() () from /usr/lib/arm-linux-gnueabihf/libQt5Gui.so.5
            #4  0x00019df6 in (anonymous namespace)::read_pixels(int, unsigned int, mir::geometry::Size const&, std::vector<char, std::allocator<char> >&) ()
            #5  0x0001b130 in (anonymous namespace)::do_screencast((anonymous namespace)::EGLSetup const&, mir::geometry::Size const&, int, double, std::ostream&) ()
            #6  0x0001c446 in main ()
            

            Quite indecipherable to me but the last (#1) thread's backtrace looks promising...

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            • Christian EhrlicherC Offline
              Christian EhrlicherC Offline
              Christian Ehrlicher
              Lifetime Qt Champion
              wrote on last edited by
              #30

              @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

              #4 0x00019df6 in (anonymous namespace)::read_pixels(int, unsigned int, mir::geometry::Size const&, std::vector<char, std::allocator<char> >&) ()

              What happens here?
              Also try to install the debug symbols of libvncserver

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              • Christian EhrlicherC Christian Ehrlicher

                @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                #4 0x00019df6 in (anonymous namespace)::read_pixels(int, unsigned int, mir::geometry::Size const&, std::vector<char, std::allocator<char> >&) ()

                What happens here?
                Also try to install the debug symbols of libvncserver

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                abmyii
                wrote on last edited by
                #31

                @Christian-Ehrlicher

                
                void read_pixels(int bpp, GLenum format, mir::geometry::Size const& size, std::vector<char> &fb)
                {
                    auto width = size.width.as_uint32_t();
                    auto height = size.height.as_uint32_t();
                
                    // Read pixels into image (https://community.khronos.org/t/render-to-texture-under-qt/68430/3)
                    QImage image(width, height, QImage::Format_RGBA8888);
                    glReadPixels(0, 0, width, height, format, GL_UNSIGNED_BYTE, image.bits());
                
                    if (mirror)
                	image = image.mirrored(true, true);
                
                    if (landscape) {
                        //QPoint center = image.rect().center();
                        QMatrix matrix;
                        matrix.translate(0, 0);
                        matrix.rotate(90);
                
                        image = image.transformed(matrix);
                    }
                
                    std::cout << "FB size: " << fb.size() << ", Image byteCount: " << image.byteCount() << std::endl;
                    std::memcpy(fb.data(), (const char *)image.constBits(), image.byteCount());
                #ifdef DEBUG
                    unsigned int sum = 0;
                    unsigned int *pui = (unsigned int *) buffer;
                    for (int i = 0; i < width * height; i++) {
                        sum += *(pui + i);
                    }
                    printf("read pixels sum 0x%x\n", sum);
                #endif
                }
                

                Sure, I'll try that.

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                • Christian EhrlicherC Christian Ehrlicher

                  @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                  #4 0x00019df6 in (anonymous namespace)::read_pixels(int, unsigned int, mir::geometry::Size const&, std::vector<char, std::allocator<char> >&) ()

                  What happens here?
                  Also try to install the debug symbols of libvncserver

                  A Offline
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                  abmyii
                  wrote on last edited by
                  #32

                  @Christian-Ehrlicher

                  Thread 6 "mirvncserver" received signal SIGBUS, Bus error.
                  [Switching to Thread 0xb0db8bc0 (LWP 3495)]
                  0xb6e433b2 in CheckSolidTile32 (cl=0xb1700470, needSameColor=0 '\000', colorPtr=0xb0db8438, h=16, w=16, y=-1231936575, x=20) at tight.c:611
                  611	tight.c: No such file or directory.
                  
                  (gdb) thread apply all bt
                  
                  Thread 6 (Thread 0xb0db8bc0 (LWP 3495)):
                  #0  0xb6e433b2 in CheckSolidTile32 (cl=0xb1700470, needSameColor=0 '\000', colorPtr=0xb0db8438, h=16, w=16, y=-1231936575, x=20) at tight.c:611
                  #1  CheckSolidTile (cl=cl@entry=0xb1700470, x=x@entry=0, y=y@entry=465, w=w@entry=16, h=h@entry=16, colorPtr=colorPtr@entry=0xb0db8438, 
                      needSameColor=needSameColor@entry=0 '\000') at tight.c:570
                  #2  0xb6e45aae in SendRectEncodingTight (cl=cl@entry=0xb1700470, x=x@entry=0, y=433, w=w@entry=432, h=h@entry=335) at tight.c:388
                  #3  0xb6e45fba in SendRectEncodingTight (cl=cl@entry=0xb1700470, x=0, y=0, w=432, h=768) at tight.c:450
                  #4  0xb6e460f6 in rfbSendRectEncodingTight (cl=cl@entry=0xb1700470, x=<optimized out>, y=<optimized out>, w=<optimized out>, h=h@entry=768) at tight.c:275
                  #5  0xb6e2cf7c in rfbSendFramebufferUpdate (cl=cl@entry=0xb1700470, givenUpdateRegion=givenUpdateRegion@entry=0xb0200470) at rfbserver.c:3165
                  #6  0xb6e27e0c in clientOutput (data=0xb1700470) at main.c:497
                  #7  0xb68b65b4 in start_thread (arg=0x0) at pthread_create.c:335
                  #8  0xb6967c5c in ?? () at ../sysdeps/unix/sysv/linux/arm/clone.S:89 from /lib/arm-linux-gnueabihf/libc.so.6
                  Backtrace stopped: previous frame identical to this frame (corrupt stack?)
                  
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                  • Christian EhrlicherC Offline
                    Christian EhrlicherC Offline
                    Christian Ehrlicher
                    Lifetime Qt Champion
                    wrote on last edited by
                    #33

                    @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                    std::vector<char> &fb

                    Is this used in another thread?

                    Otherwise I don't see anything obvious.

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                    • Christian EhrlicherC Christian Ehrlicher

                      @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                      std::vector<char> &fb

                      Is this used in another thread?

                      Otherwise I don't see anything obvious.

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                      abmyii
                      wrote on last edited by
                      #34

                      @Christian-Ehrlicher I think the while loop is in another thread and VNC is accessing fb.data() throughout execution.

                      Christian EhrlicherC 1 Reply Last reply
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                      • A abmyii

                        @Christian-Ehrlicher I think the while loop is in another thread and VNC is accessing fb.data() throughout execution.

                        Christian EhrlicherC Offline
                        Christian EhrlicherC Offline
                        Christian Ehrlicher
                        Lifetime Qt Champion
                        wrote on last edited by
                        #35

                        @abmyii So you have your answer. Concurrent access to a memory region from two different threads.

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                        • Christian EhrlicherC Christian Ehrlicher

                          @abmyii So you have your answer. Concurrent access to a memory region from two different threads.

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                          abmyii
                          wrote on last edited by abmyii
                          #36

                          @Christian-Ehrlicher Ah I see, that makes perfect sense now! Is there a simple solution to this? I tried the following which didn't work:

                          std::vector<char> temp_fb(frame_size_bytes, 0);
                          std::memcpy(temp_fb.data(), (const char *)image.constBits(), image.byteCount());
                          fb = temp_fb
                          
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                          • Christian EhrlicherC Offline
                            Christian EhrlicherC Offline
                            Christian Ehrlicher
                            Lifetime Qt Champion
                            wrote on last edited by
                            #37

                            Use mutexes or copy the data - I would suggest reading a little bit on how threading works. E.g. https://doc.qt.io/qt-5/threads-synchronizing.html - c++ uses the same mechanisms

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                              abmyii
                              wrote on last edited by
                              #38

                              Whoops, I've been saying malloc (which is apparently thread-safe) all along when in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead?

                              JonBJ 1 Reply Last reply
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                              • A abmyii

                                Whoops, I've been saying malloc (which is apparently thread-safe) all along when in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead?

                                JonBJ Offline
                                JonBJ Offline
                                JonB
                                wrote on last edited by
                                #39

                                @abmyii
                                Hence what @Christian-Ehrlicher has been saying above. malloc and memcpy do totally different things, it makes no sense to ask whether one can be used in place of the other....

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                                • JonBJ JonB

                                  @abmyii
                                  Hence what @Christian-Ehrlicher has been saying above. malloc and memcpy do totally different things, it makes no sense to ask whether one can be used in place of the other....

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                                  abmyii
                                  wrote on last edited by abmyii
                                  #40

                                  @JonB I understand - I got confused due to my unfamiliarity with the functions but I never asked if I could use malloc instead of memcpy - I asked if I could use push_back instead of malloc (sic).

                                  JonBJ 1 Reply Last reply
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                                  • A abmyii

                                    @JonB I understand - I got confused due to my unfamiliarity with the functions but I never asked if I could use malloc instead of memcpy - I asked if I could use push_back instead of malloc (sic).

                                    JonBJ Offline
                                    JonBJ Offline
                                    JonB
                                    wrote on last edited by JonB
                                    #41

                                    @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                                    I never asked if I could use malloc instead of memcpy [sic.]

                                    in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead? [*sic.]

                                    If you say so! I'll leave others to untangle, who clearly understand what you are saying better than I :) Don't worry about explaining to me.

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                                    • JonBJ JonB

                                      @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                                      I never asked if I could use malloc instead of memcpy [sic.]

                                      in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead? [*sic.]

                                      If you say so! I'll leave others to untangle, who clearly understand what you are saying better than I :) Don't worry about explaining to me.

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                                      abmyii
                                      wrote on last edited by abmyii
                                      #42

                                      @JonB OK, you got me! Too much to keep track of...

                                      I tried this and it worked, but is quite a lot slower than memcpy:

                                      std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
                                      fb = vectorBuffer;
                                      
                                      JonBJ Christian EhrlicherC 2 Replies Last reply
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                                      • A abmyii

                                        @JonB OK, you got me! Too much to keep track of...

                                        I tried this and it worked, but is quite a lot slower than memcpy:

                                        std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
                                        fb = vectorBuffer;
                                        
                                        JonBJ Offline
                                        JonBJ Offline
                                        JonB
                                        wrote on last edited by
                                        #43

                                        @abmyii
                                        I am lost in the complexities of your questions :) But I would suggest heeding @Christian-Ehrlicher's advice, he is usually right! He last offered:

                                        Use mutexes or copy the data

                                        So why not pursue that?

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                                        • A abmyii

                                          @JonB OK, you got me! Too much to keep track of...

                                          I tried this and it worked, but is quite a lot slower than memcpy:

                                          std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
                                          fb = vectorBuffer;
                                          
                                          Christian EhrlicherC Offline
                                          Christian EhrlicherC Offline
                                          Christian Ehrlicher
                                          Lifetime Qt Champion
                                          wrote on last edited by
                                          #44

                                          @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                                          std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
                                          fb = vectorBuffer;

                                          This does two memcpy for no good reason. Since I don't know what libvncserver function you use you're on your own. Read the documentation of them and also read what's needed for threadsafe access.

                                          Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                                          Visit the Qt Academy at https://academy.qt.io/catalog

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