Why eglGetDisplay(EGL_DEFAULT_DISPLAY) returns EGL_NO_DISPLAY?

Hi, I'm new to dri-devel and sorry if this question doesn't fit here. In my code, the case 1 of getting eglDisplay using EGL_DEFAULT_DISPLAY failed, but, the case 2 succeeded.

// Case 1 egl_.display = eglGetDisplay (EGL_DEFAULT_DISPLAY);

// Case 2 struct gbm_device * gbm_device_; gbm_device_ = gbm_create_device (drm _-> GetFD ()); egl_.display = eglGetDisplay ((EGLNativeDisplayType) gbm_device_);

I don't know if EGL_DEFAULT_DISPLAY is missing or it's a problem with my machine. The output of lspci is :

VGA compatible controller: Intel Corporation Device 5906 (rev 02)

Thank you for any comments or links to related documents.