special_invoke.c

typedef void (*dispatch_f)(void*, ...);
dispatch_f dispatch_block_special_invoke()
{
    static dispatch_once_t onceToken;
    static dispatch_f f;
    dispatch_once(&onceToken, ^{
        f = (__bridge struct Block_layout *)dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, ^{})->invoke;
    });
    return f;
}