Thread - Shared Memory Access Violation

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I use a library to retrieve data from an Image (in unsigned char*), and use it to allocate to an Opengl Buffer (Texture).

Since the image can be large, Opengl by default creates another thread to perform this activity. But I do free this image right after the main thread, because the data is no longer needed. Then I get the access breach error at (Visual Studio).

  • C++ has some feature for checking that one memory is not being used by another Implicit Thread (i.e., I don’t have access)?

Details:

  • The function I use, which generates a new thread is glTexImage2D.
  • The error occurs randomly, it depends a lot if the main thread runs free before Opengl allocates in Buffer

Technical details:

Exception generated in 0x07B8F54F (ig8icd32.dll) in ...[nomedoprograma.exe]: 0xC0000005: access violation while reading the local 0x13E1B000.

The address 0x13E1B000 is null. Code snippet as requested

int width, height, nrChannels;

unsigned char *dataImg = stbi_load(path, &width, &height, &nrChannels, 0);
GLenum format;
if (nrChannels == 1)
    format = GL_RED;
else if (nrChannels == 3)
    format = GL_RGB;
else if (nrChannels == 4)
    format = GL_RGBA;

if (dataImg) {
    glTexImage2D(GL_TEXTURE_2D, 0, format, width, height, 0, format, GL_UNSIGNED_BYTE, dataImg);
    glGenerateMipmap(GL_TEXTURE_2D);
} else {
    log(StringBuilder::concat("STBI_LOAD::FAILED_TO_LOAD_IMAGE::", path));
}

stbi_image_free(dataImg);// <- ERRO AQUI
c++ visual-studio opengl competition

  • I do not know any language solutions to this competition problem that you put to the main point of the question. But there are competing libraries that help you avoid that. Semaphores and mutex are the ones I see most in C. It might be interesting to read what Visual Studio’s C++ compiler documentation has to say about competition.

  • The time is not conducive to draw attention, but I believe that giving a detail of the problematic code excerpt, presenting a [MCVE], can generate results when more users are active. And managing to simulate your problem too. Competition problems are heisenbugs. Helping to simulate the problem consistently helps in the solution

  • Code added, I don’t know if it’ll help much. I wanted to make the topic broader for shared memory problems, not just specific to Opengl.

  • 3

    Where you saw that the function glTexImage2D generates a new thread, as far as I know Opengl is entirely designed to work in single Thread. Try moving the code 'stbi_image_free(dataImg);' into if (dataImg), if there is no image it makes no sense to release resource.

1 answer

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Thanks for the tip in the comments. According to the opengl documentation, glTexImage2D can generate error during transfer if the image is 4-bit size.

You create Storage for a Texture and upload pixels to it with glTexImage2D (or similar functions, as appropriate to the type of Texture). If your program crashes During the upload, or diagonal Lines appear in the Resulting image, this is because the Alignment of each horizontal line of your pixel array is not Multiple of 4. This typically Happens to users loading an image that is of the RGB or BGR format (for example, 24 BPP images), Depending on the source of your image data.

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