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Forum Update on Tuesday, May 27th 2025

Save QImage from BYTE buffer segfaults ?

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  • SGaistS SGaist

    Hi,

    Are you sure you are putting valid data in there before making a QImage out of it ?
    Are you sure the buffer stays valid until the file is written ?

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    R-P-H
    wrote on last edited by
    #4

    @SGaist said in Save QImage from BYTE buffer segfaults ?:

    Hi,

    Are you sure you are putting valid data in there before making a QImage out of it ?
    Are you sure the buffer stays valid until the file is written ?

    I'm pretty sure that the buffer stays valid.

    The data is always written to the buffer the same way, but I guess it could be invalid. How would I check that after the buffer has been populated ?

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    • Kent-DorfmanK Kent-Dorfman

      FWIW, you cannot make the absolute assumption that buf will be 32bit word aligned. If I had to guess, that would be my guess as to why it crashes sometimes. The Qt docs ae specific about the need for buf to be 32bit word aligned.

      R Offline
      R Offline
      R-P-H
      wrote on last edited by
      #5

      @Kent-Dorfman said in Save QImage from BYTE buffer segfaults ?:

      FWIW, you cannot make the absolute assumption that buf will be 32bit word aligned. If I had to guess, that would be my guess as to why it crashes sometimes. The Qt docs ae specific about the need for buf to be 32bit word aligned.

      Yes I saw that in the documentation. I assumed that if buffer is being populated the same way each time and that if it works sometimes, then it should be fine.

      Does Qt have any other way to get around this besides using another image processing library to save the image ?

      Christian EhrlicherC 1 Reply Last reply
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      • SGaistS Offline
        SGaistS Offline
        SGaist
        Lifetime Qt Champion
        wrote on last edited by
        #6

        What are you using to fill that buffer ?
        Where do these data come from ?

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        • SGaistS SGaist

          What are you using to fill that buffer ?
          Where do these data come from ?

          R Offline
          R Offline
          R-P-H
          wrote on last edited by
          #7

          @SGaist said in Save QImage from BYTE buffer segfaults ?:

          What are you using to fill that buffer ?
          Where do these data come from ?

          It's populated using an API call which finishes successfully. So I can only interrogate the validity of the data after its completed.

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          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on last edited by
            #8

            That's a bit short on details. An API call can be a method from a library your are using or a remote call to a REST service.

            What kind of image are you getting ?

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            • R R-P-H

              @Kent-Dorfman said in Save QImage from BYTE buffer segfaults ?:

              FWIW, you cannot make the absolute assumption that buf will be 32bit word aligned. If I had to guess, that would be my guess as to why it crashes sometimes. The Qt docs ae specific about the need for buf to be 32bit word aligned.

              Yes I saw that in the documentation. I assumed that if buffer is being populated the same way each time and that if it works sometimes, then it should be fine.

              Does Qt have any other way to get around this besides using another image processing library to save the image ?

              Christian EhrlicherC Online
              Christian EhrlicherC Online
              Christian Ehrlicher
              Lifetime Qt Champion
              wrote on last edited by
              #9

              @R-P-H said in Save QImage from BYTE buffer segfaults ?:

              I assumed that if buffer is being populated the same way each time and that if it works sometimes, then it should be fine.

              Even if it does it would depend on your image size (or better: bytes per line). If it's a multiple of 4 it does not crash, if it's something else the alignment is wrong.

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              • SGaistS SGaist

                That's a bit short on details. An API call can be a method from a library your are using or a remote call to a REST service.

                What kind of image are you getting ?

                R Offline
                R Offline
                R-P-H
                wrote on last edited by
                #10

                @SGaist said in Save QImage from BYTE buffer segfaults ?:

                That's a bit short on details. An API call can be a method from a library your are using or a remote call to a REST service.

                What kind of image are you getting ?

                It's from a library. The API reference is very short on details. It's a 256 gray-level image.

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                • Christian EhrlicherC Christian Ehrlicher

                  @R-P-H said in Save QImage from BYTE buffer segfaults ?:

                  I assumed that if buffer is being populated the same way each time and that if it works sometimes, then it should be fine.

                  Even if it does it would depend on your image size (or better: bytes per line). If it's a multiple of 4 it does not crash, if it's something else the alignment is wrong.

                  R Offline
                  R Offline
                  R-P-H
                  wrote on last edited by
                  #11

                  @Christian-Ehrlicher said in Save QImage from BYTE buffer segfaults ?:

                  @R-P-H said in Save QImage from BYTE buffer segfaults ?:

                  I assumed that if buffer is being populated the same way each time and that if it works sometimes, then it should be fine.

                  Even if it does it would depend on your image size (or better: bytes per line). If it's a multiple of 4 it does not crash, if it's something else the alignment is wrong.

                  The image size is always the same (260 x 300 px).

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                  • SGaistS Offline
                    SGaistS Offline
                    SGaist
                    Lifetime Qt Champion
                    wrote on last edited by
                    #12

                    And one byte per pixel ?

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                    • SGaistS SGaist

                      And one byte per pixel ?

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                      R-P-H
                      wrote on last edited by
                      #13

                      @SGaist said in Save QImage from BYTE buffer segfaults ?:

                      And one byte per pixel ?

                      That is correct yes.

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                      • SGaistS Offline
                        SGaistS Offline
                        SGaist
                        Lifetime Qt Champion
                        wrote on last edited by
                        #14

                        Can you get the stack trace of the crash ?

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                        • SGaistS SGaist

                          Can you get the stack trace of the crash ?

                          R Offline
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                          R-P-H
                          wrote on last edited by
                          #15

                          @SGaist said in Save QImage from BYTE buffer segfaults ?:

                          Can you get the stack trace of the crash ?

                          I'm testing the application on another machine using the executable. I don't think I'd be able to get it without QT or another IDE right ?

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                          • SGaistS Offline
                            SGaistS Offline
                            SGaist
                            Lifetime Qt Champion
                            wrote on last edited by
                            #16

                            Windows machine ?

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                            • SGaistS SGaist

                              Windows machine ?

                              R Offline
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                              R-P-H
                              wrote on last edited by
                              #17

                              @SGaist said in Save QImage from BYTE buffer segfaults ?:

                              Windows machine ?

                              Yes it is. Windows 7 and also Windows 10.

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                              • mranger90M Offline
                                mranger90M Offline
                                mranger90
                                wrote on last edited by
                                #18

                                try:
                                BYTE *buf = new BYTE[imWidth * imHeight];

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                                • mranger90M mranger90

                                  try:
                                  BYTE *buf = new BYTE[imWidth * imHeight];

                                  R Offline
                                  R Offline
                                  R-P-H
                                  wrote on last edited by
                                  #19

                                  @mranger90 said in Save QImage from BYTE buffer segfaults ?:

                                  try:
                                  BYTE *buf = new BYTE[imWidth * imHeight];

                                  Hi, it is not crashing anymore, however the image seems to be coming out completely black. I will confirm tomorrow if the image is supposed to be completely black or not...

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                                  • J Offline
                                    J Offline
                                    Jarek B
                                    wrote on last edited by
                                    #20

                                    From the documentation
                                    https://doc.qt.io/qt-5/qimage.html#QImage-4
                                    data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

                                    So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

                                    R 1 Reply Last reply
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                                    • J Jarek B

                                      From the documentation
                                      https://doc.qt.io/qt-5/qimage.html#QImage-4
                                      data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

                                      So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

                                      R Offline
                                      R Offline
                                      R-P-H
                                      wrote on last edited by
                                      #21

                                      @Jarek-B said in Save QImage from BYTE buffer segfaults ?:

                                      From the documentation
                                      https://doc.qt.io/qt-5/qimage.html#QImage-4
                                      data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

                                      So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

                                      Hi, interesting...I have not seen any examples like that. How are you arriving at 288 bytes ? Are you suggesting I make the buffer 288x300 bytes instead ?

                                      mranger90M J 2 Replies Last reply
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                                      • R R-P-H

                                        @Jarek-B said in Save QImage from BYTE buffer segfaults ?:

                                        From the documentation
                                        https://doc.qt.io/qt-5/qimage.html#QImage-4
                                        data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

                                        So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

                                        Hi, interesting...I have not seen any examples like that. How are you arriving at 288 bytes ? Are you suggesting I make the buffer 288x300 bytes instead ?

                                        mranger90M Offline
                                        mranger90M Offline
                                        mranger90
                                        wrote on last edited by mranger90
                                        #22

                                        @R-P-H Of course it's black, the data never gets initialized.
                                        Also, the reason why it doesn't crash is because:
                                        BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
                                        BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

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                                        • mranger90M mranger90

                                          @R-P-H Of course it's black, the data never gets initialized.
                                          Also, the reason why it doesn't crash is because:
                                          BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
                                          BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

                                          R Offline
                                          R Offline
                                          R-P-H
                                          wrote on last edited by
                                          #23

                                          @mranger90 said in Save QImage from BYTE buffer segfaults ?:

                                          @R-P-H Of course it's black, the data never gets initialized.
                                          Also, the reason why it doesn't crash is because:
                                          BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
                                          BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

                                          I don't quite understand what you're saying to be honest. Where are you saying the issue is ?

                                          mranger90M 1 Reply Last reply
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