You only have a pointer to your struct (PANELMAP* mypanmap;) and not the actual object. So you want to fill a bitmap pointer than doesn't exists (the bitmap pointer only exists in an actual object, not in an empty pointer). Either use PANELMAP mypanmap;, or fill mypanmap with the object in your function (mypanmap = malloc( sizeof(PANELMAP) );)


Also, when referencing pointers, say:
PANELMAP* mypanmap;
mypanmap.bitmap = NULL;

Then always use the arrow notation:
PANELMAP* mypanmap;
mypanmap->bitmap = NULL;