Why doesn't Symbian OS support writeable static data?

最新推荐文章于 2016-12-08 16:59:43 发布

原创最新推荐文章于 2016-12-08 16:59:43 发布 · 288 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#symbian #os #dll #allocation

本文探讨了Symbian OS为何不支持可写静态数据(WSD)。主要原因在于支持WSD会导致内存浪费，对于运行在内存受限设备上的Symbian OS来说，这将是一个不可接受的问题。文章详细解释了两个主要的内存浪费来源：需要为多个进程复制DLL，并且RAM页面使用效率低下。

Why doesn't Symbian OS support writeable static data?

The main reason writeable static data (WSD) is not supported on Symbian OS, is that it comes at the cost of significant memory wastage. Symbian OS runs in memory constrained devices, so Symbian have made the design decision not to support WSD.

The main sources of memory wastage are discussed below:

1. Requirement for multiple copies of DLLs in RAM
All processes that use a DLL must set the same address for the DLL's static data, because the address is referenced by the shared DLL code. If the required address is free when the process comes to use the DLL, then there is no problem. However it is perfectly possible that the address might not be free, so the copy of the DLL in memory cannot be used.

The workaround is to copy the DLL code and modify the copy to use a different address for static data. Symbian OS does not consider this duplication acceptable.

2. Inefficient use of RAM pages
The amount of memory needed for writable static data by any DLL is likely to be much smaller (<0x100 bytes) than the smallest possible RAM allocation (4K). The remaining memory is wasted. Since the memory is per-process-dll-static-data, the memory wastage on the machine is:
(4K-WSD bytes)*number-client-processes.