Technical Q&A IC03

Sharing Globals Between Apps & Code Fragment Manager

Q: I have a shared library (PowerPC native CFM) that is used by several applications. The library includes a few global variables that are used in all of the applications (they declare them as "extern"s). My problem is that each application seems to have its own copy of the globals, but when I check the address of the same global in several applications, they are different. Aren't globals supposed to be sharable between applications?

A: The Code Fragment Manager allows shared libraries to allocate and use global variables in two different ways: Global instantiation or Per-context instantiation. At link time, you determine whether your library will support either global or per-context instantiation. The default PPCLink option is to support per-context instantiation, which is probably why you are seeing different addresses for the same global across applications.

These are the differences between the two types of instantiation:

1. Global instantiation: CFM allocates a single copy of the library's staticmdata, regardless of how many clients use the data.
2. Per-context instantiation: CFM allocates one copy of the static data for each separate process that uses that data.

If you want the CFM to allocate only a single copy of your library's static data, and if you are using PPCLink 1.2a1 from ETO 16, your link line must include the option (-share global), which sets the sharing mode for the static data to global instantiation. If you are using version 1.0 of the PPCLink tool, you must specify the global instantiation option (-s 4) on your MakePef command line. For MakePef, the default option is per-context (-s 1).

To learn more about how the Code Fragment Manager deals with global data by reading pages 1-50 through 1-52 of Inside Macintosh:PowerPC System Software. "Building for PowerPC" on ETO 16 has additional information on building a shared library.