crash from context shared QOpenGLWidget

Question

I have two QOpenGLWidgets(view1, view2)as children in a top-level widget. Qt document says 'When multiple QOpenGLWidgets are added as children to the same top-level widget, their contexts will share with each other'. So, view1 and view2 share OpenGL context. I have tried to render same scene which is initialized in view1's context and the application crashes in view2's paintGL(). What did I wrong?

Here's simplified code:

#include <QtGui/QtGui>
#include <QtWidgets/QOpenGLWidget>
#include <QtWidgets/QApplication>
#include <QtWidgets/QHBoxLayout>
static const char *vertexShaderSource =
 "attribute vec4 posAttr;\n"
 "void main() {\n"
 " gl_Position = posAttr;\n"
 "}\n";
static const char *fragmentShaderSource =
 "void main() {\n"
 " gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);\n"
 "}\n";
QOpenGLShaderProgram *program = nullptr;
QOpenGLBuffer arrayBuf;
int posAttr = -1;
class View3D : public QOpenGLWidget {
public:
 View3D()
 {
 setMinimumSize(300, 200);
 }
private:
 auto initializeGL() -> void override
 {
 if (program)
 return;
 program = new QOpenGLShaderProgram(this);
 program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource);
 program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource);
 program->link();
 posAttr = program->attributeLocation("posAttr");
 program->bind();
 GLfloat vertices[] = {
 0.0f, 0.707f, 0.f,
 -0.5f, -0.5f, 0.f,
 0.5f, -0.5f, 0.f,
 };
 arrayBuf.create();
 arrayBuf.bind();
 arrayBuf.allocate(vertices, sizeof(vertices));
 program->enableAttributeArray(posAttr);
 program->setAttributeBuffer(posAttr, GL_FLOAT, 0, 3, 0);
 program->release();
 }
 auto paintGL() -> void override
 {
 auto f = context()->functions();
 const auto dpr = devicePixelRatio();
 f->glViewport(0, 0, width() * dpr, height() * dpr);
 f->glClear(GL_COLOR_BUFFER_BIT);
 program->bind();
 arrayBuf.bind();
 program->enableAttributeArray(posAttr);
 f->glDrawArrays(GL_TRIANGLES, 0, 3);
 program->disableAttributeArray(posAttr);
 arrayBuf.release();
 program->release();
 }
};
auto main(int argc, char **argv) -> int
{
 QApplication app{argc, argv};
 QWidget w;
 auto hbox = new QHBoxLayout{&w};
 hbox->addWidget(new View3D); // view1
 hbox->addWidget(new View3D); // view2
 w.show();
 return app.exec();
}

Answer 1

It seems that vertex state shall not be shared between contexts even though those contexts are shared.

Here's fixed version:

#include <QtGui/QtGui>
#include <QtWidgets/QOpenGLWidget>
#include <QtWidgets/QApplication>
#include <QtWidgets/QHBoxLayout>
static const char *vertexShaderSource =
 "attribute vec4 posAttr;\n"
 "void main() {\n"
 " gl_Position = posAttr;\n"
 "}\n";
static const char *fragmentShaderSource =
 "void main() {\n"
 " gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);\n"
 "}\n";
QOpenGLBuffer *arrayBuf = nullptr;
QOpenGLShaderProgram *program = nullptr;
class View3D : public QOpenGLWidget {
public:
 View3D()
 {
 setMinimumSize(300, 200);
 }
private:
 int posAttr = -1;
 auto initializeGL() -> void override
 {
 if (!arrayBuf) {
 arrayBuf = new QOpenGLBuffer;
 arrayBuf->create();
 arrayBuf->bind();
 GLfloat vertices[] = {
 0.0f, 0.707f, 0.f,
 -0.5f, -0.5f, 0.f,
 0.5f, -0.5f, 0.f,
 };
 arrayBuf->allocate(vertices, sizeof(vertices));
 }
 if (!program) {
 program = new QOpenGLShaderProgram(this);
 program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource);
 program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource);
 program->link();
 }
 posAttr = program->attributeLocation("posAttr");
 program->bind();
 arrayBuf->bind();
 program->enableAttributeArray(posAttr);
 program->setAttributeBuffer(posAttr, GL_FLOAT, 0, 3, 0);
 program->release();
 }
 auto paintGL() -> void override
 {
 auto f = context()->functions();
 const auto dpr = devicePixelRatio();
 f->glViewport(0, 0, width() * dpr, height() * dpr);
 f->glClear(GL_COLOR_BUFFER_BIT);
 program->bind();
 arrayBuf->bind();
 program->enableAttributeArray(posAttr);
 f->glDrawArrays(GL_TRIANGLES, 0, 3);
 program->disableAttributeArray(posAttr);
 arrayBuf->release();
 program->release();
 }
};
auto main(int argc, char **argv) -> int
{
 QApplication app{argc, argv};
 QWidget w;
 auto hbox = new QHBoxLayout{&w};
 hbox->addWidget(new View3D); // view1
 hbox->addWidget(new View3D); // view2
 w.show();
 return app.exec();
}

From observation, sharing vertex attribute state also can cause problem. I knew that I cannot share VAO by OpenGL specification, but I never used VAO explictily here.

I am not sure that this is inhibited by specification or a bug of driver. However, at least on NVidia driver, it is obvious that you should not share vertex attribute state for any cases including shared contexts.