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    <p>The problem is here:</p> <pre><code>cudaMalloc((void**)&amp;nL,sizeof(NLayer)); cudaMalloc((void**)&amp;nL-&gt;neurons,6*sizeof(Neuron)); </code></pre> <p>In first line, <code>nL</code> is pointing to structure in global memory on device. Therefore, in second line the first argument to <code>cudaMalloc</code> is address residing on GPU, which is undefined behaviour (on my test system, it causes segfault; in your case, though, there is something more subtle).</p> <p>The correct way to do what you want is first to create structure in host memory, fill it with data, and then copy it to device, like this:</p> <pre><code>NLayer* nL; NLayer h_nL; int i; int tmp=9; // Allocate data on device cudaMalloc((void**)&amp;nL, sizeof(NLayer)); cudaMalloc((void**)&amp;h_nL.neurons, 6*sizeof(Neuron)); // Copy nlayer with pointers to device cudaMemcpy(nL, &amp;h_nL, sizeof(NLayer), cudaMemcpyHostToDevice); </code></pre> <p>Also, don't forget to always check for any errors from CUDA routines.</p> <p><strong>UPDATE</strong></p> <p>In second version of your code:</p> <p><code>cudaMemcpy(&amp;d_layer-&gt;neurons[i].weights,&amp;d_weights,...)</code> --- again, you are dereferencing device pointer (<code>d_layer</code>) on host. Instead, you should use</p> <pre><code>cudaMemcpy(&amp;h_layer.neurons[i].weights,&amp;d_weights,sizeof(float*),cudaMemcpyHostToDevice </code></pre> <p>Here you take <code>h_layer</code> (host structure), read its element (<code>h_layer.neurons</code>), which is pointer to device memory. Then you do some pointer arithmetics on it (<code>&amp;h_layer.neurons[i].weights</code>). No access to device memory is needed to compute this address.</p>
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    1. COI've modified my code, but it's doesn't work, could you take a look? The new code is in my initial post...Thanks!
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      1. USAndrea Sylar Solla
    2. COOh! Thank you it works!! I've only one more question: If I want to access from the host to the data contened into the integer variable **d_layer->neurons[0].n_weights** I must previously copy **d_layer** on the host, then I've to copy the **d_layer->neurons[0]** on the host, then, finally, I can take the "d_layer->neurons[0].n_weights variable?? I'm asking it just because I've tried to copy directly the "d_layer->neurons[0].n_weights with cudaMemcpy(...), but it return always an "invalid argument" error.
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      1. USAndrea Sylar Solla
    3. CO@AndreaSylarSolla You could simply use `int t; cudaMemcpy(&t, &h_layer.neurons[0].n_weights,....)` or `Neuron t; cudaMemcpy(&t, &h_layer.neurons[0],....)`. There is no need to copy `d_layer`, since you only need the value of `neurons` pointer from it, but the very same value is alredy in `h_layer`.
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