Why cant I draw to a texture in SDL2? - c++

Question

So my main goal is to read the pixel data of a given texture, so to test this, i've created a new texture with target access then drawn it red.
The problem is that when I access the pixel data, SDL_RenderReadPixels() says the pitch is 0, and all the pixel data is 0. Also when I try to draw the new texture, it comes out black.
SDL_Texture* tempChar = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, 3, 5);
if (SDL_SetRenderTarget(renderer, tempChar) != 0) {
cout << "SDL_SetRenderTarget() Error! " << SDL_GetError() << endl;
} else {
cout << "SDL_SetRenderTarget() success" << endl;
}
if (SDL_RenderClear(renderer) != 0) {
cout << "SDL_RenderClear() Error! " << SDL_GetError() << endl;
} else {
cout << "SDL_RenderClear() success" << endl;
}
if (SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255) != 0) {
cout << "SDL_SetRenderDrawColor() Error! " << SDL_GetError() << endl;
} else {
cout << "SDL_SetRenderDrawColor() success" << endl;
}
if (SDL_RenderFillRect(renderer, NULL) != 0) {
cout << "SDL_RenderClear() Error! " << SDL_GetError() << endl;
} else {
cout << "SDL_RenderClear() success" << endl;
}
int w;
int h;
Uint32 format;
int access;
if (SDL_QueryTexture(tempChar, &format, &access, &w, &h) != 0) {
cout << "SDL_QueryTexture() ERROR : " << SDL_GetError() << endl;
} else {
cout << "SDL_QueryTexture() no error" << endl;
}
cout << "width is : " << w << " height is : " << h << endl;
if (access == SDL_TEXTUREACCESS_TARGET) {
cout << "TARGET" << endl;
}
if (format == SDL_PIXELFORMAT_RGBA8888) {
cout << "format is : SDL_PIXELFORMAT_RGBA8888" << endl;
}
void* readPixels = NULL;
int pitch;
if (SDL_RenderReadPixels(renderer, NULL, 0, readPixels, pitch) != 0) {//12 pitch, 3x4bytes
//an error occurred
cout << "SDL_RenderReadPixels() Error, text probably wont work... :-/\n" << SDL_GetError() << endl;
} else {
cout << "SDL_RenderReadPixels() success" << endl;
}
cout << "pitch returned : " << pitch << endl;
menuMouse = tempChar;
for (int i = 0; i < 5; i++) {
cout << "i is : " << i << endl;
char* rowStart = ((char*) readPixels) + i * pitch;
cout << "pix data is : " << int(rowStart) << endl;
}

Answer 1

For some reason you assume that SDL_RenderReadPixels allocates pixels array and returns resulting pitch; it does neither. Calling side passes pixels array big enough to hold data, and its correct pitch, e.g.:
Uint32 pixels[3*5];
int pitch = sizeof(Uint32)*3;
In C/C++, functions does not modify input parameters directly, unless parameter is a reference type (in C++), or value is modified indirectly through pointer, so if function takes int - it is clearly input parameter, not output. E.g. take a look at SDL_CreateWindowAndRenderer, which outputs window and renderer pointers and hence its parameter types are pointer-to-pointer.