SegFault after calling glDrawArrays - c++

Question

I am attempting to write a sprite batcher.
Originally, I was uploading the following data to openGL in order to see if I could get anything to the screen:
static GLfloat const vertexData[] = {
//Position //Color //UV Coords
0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
-0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
-0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f,
-0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f
};
These are my Vertex and Fragment Shaders:
const char* vert = "#version 330 core\n"
"layout (location = 0) in vec2 vPos;\n"
"layout (location = 1) in vec4 vColor;\n"
"layout (location = 2) in vec2 vTexPos;\n"
"\n"
"out vec4 fColor;\n"
"out vec2 fTexPos;\n"
"\n"
"void main() {\n"
" gl_Position = vec4(vPos.xy, 0.0, 1.0);\n"
" fColor = vColor;\n"
" fTexPos = vec2(vTexPos.s, 1.0 - vTexPos.t);\n"
"}";
const char* frag = "#version 330 core\n"
"in vec4 fColor;\n"
"in vec2 fTexPos;\n"
"out vec4 finalColor;\n"
"\n"
"uniform sampler2D textureUniform;\n"
"\n"
"void main() {\n"
" vec4 textureColor = texture(textureUniform, fTexPos);"
" finalColor = fColor * textureColor;\n"
"}";
I set up the VAO and VBO like so:
auto vaoID_ = 0;
auto vboID_ = 0;
glGenVertexArrays(1, &vaoID_);
glGenBuffers(1, &vboID_);
glBindVertexArray(vaoID_);
glBindBuffer(GL_ARRAY_BUFFER, vboID_);
glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);
glEnableVertexAttribArray(2);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 8, (void*)0); // Screen Position
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 8, (void*)(2 *sizeof(GL_FLOAT)); //Color
glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8, (void*)(6 * sizeof(GL_FLOAT)); //UV
And finally, on each loop, I would orphan the buffer and then sub the data in.
glBufferData(GL_ARRAY_BUFFER, sizeof(vertexData), nullptr, GL_DYNAMIC_DRAW);
glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(vertexData), vertexData);
glDrawArrays(GL_TRIANGLES, 0, 6);
This worked fine!
So, I attempted to begin to write a basic sprite batch.
I grouped my vertex data into a POD struct:
struct VertexData {
struct {
GLfloat screenx;
GLfloat screeny;
} position;
struct {
GLfloat r;
GLfloat g;
GLfloat b;
GLfloat a;
} color;
struct {
GLfloat texu;
GLfloat texv;
} uv;
}
And used a vector to batch everything together:
std::vector<VertexData> spritebatch_
I changed the calls to glVertexAttribPointer to match this struct (although I think this change was unneeded?)
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(VertexData), (void*)offsetof(VertexData, position)); // Screen Position
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, sizeof(VertexData), (void*)offsetof(VertexData, color)); //Color
glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(VertexData), (void*)offsetof(VertexData, uv)); //UV
To test it, on every loop I would feed it the same data that was in the static GLfloat array:
spritebatch_.push_back({0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f});
spritebatch_.push_back({-0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f});
spritebatch_.push_back({0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f});
spritebatch_.push_back({-0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f});
spritebatch_.push_back({-0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f});
spritebatch_.push_back({0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f});
And then attempt to orphan the buffer and sub the data in:
glBufferData(GL_VERTEX_ARRAY, spritebatch_.size() * sizeof(VertexData), nullptr, GL_DYNAMIC_DRAW);
glBufferSubData(GL_VERTEX_ARRAY, 0, spritebatch_.size() * sizeof(VertexData), spritebatch_.data());
glDrawArrays(GL_TRIANGLES, 0, spritebatch_.size());
Finally, I would reset the spritebatch_: spritebatch_.clear();
However, this segfaults. Am I doing something incorrect when I go to orphan the buffer and sub the data in? The data formats still match (VertexData POD members are all GLfloats still), the size of spritebatch_ is still 6 (6 vertices to make a square), and the location of that data is still the same within each vertex.

Answer 1

The 5th parameter of glVertexAttribPointer is the "stride", the byte offset between consecutive generic vertex attributes.
So the change from
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 8, (void*)0); // Screen Position
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 8, (void*)(2 *sizeof(GL_FLOAT)); //Color
glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8, (void*)(6 * sizeof(GL_FLOAT)); //UV
to
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(VertexData), (void*)offsetof(VertexData, position)); // Screen Position
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, sizeof(VertexData), (void*)offsetof(VertexData, color)); //Color
glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(VertexData), (void*)offsetof(VertexData, uv)); //UV
is important, because sizeof(VertexData) is not 8, but it is 8*sizeof(GL_FLOAT) .
The first parameter of glBufferData respectively glBufferSubData has to be the enumerator constant for the target, which is one of GL_ARRAY_BUFFER, GL_ELEMENT_ARRAY_BUFFER, ... .
GL_VERTEX_ARRAY identifies the vertex array for the fixed function client-side capability - See glEnableClientState.
Change:
glBufferData(GL_VERTEX_ARRAY, ...);
glBufferData(GL_ARRAY_BUFFER, ...);
glBufferSubData(GL_VERTEX_ARRAY, ...);
glBufferSubData(GL_ARRAY_BUFFER, ...);
Note, if you would check for OpenGL errors (glGetError or Debug Output), you would get a GL_INVALID_ENUM error.
The data store for the array buffer is not created, this causes the segment fault.