4.4 Global & Module Variables

There is often confusion about what constitutes module variables versus global variables. Technically, both global and module variables are defined at file scope with external linkage. A module variable is intended to only be accessed across a small set of related routines that have strict rules for accessing the data; in effect, constrained to the set of files described within a single .inf file. A global variable is intended to be accessed throughout the firmware, usually indirectly through a protocol pointer or similar mechanism.

This is important when the time comes to maintain a module. A module variable should be fairly safe and easy to change because it is only accessed from a small number of routines. On the other hand, a global variable is accessed throughout the firmware and as the firmware evolves more code will tend to access the data resulting in a large number of uses to track down.

4.4.1 Recommendations for Global and Module Variables

4.4.1.1 The use of global and module data is strongly discouraged.

Global variables are appropriate for GUID, protocol, PPI definitions and other immutable objects. Attempting to create global variables can cause many problems, including: increased image size and variables actually residing in ROM.

The use of global and module variables may be appropriate for solving certain programming issues. A module is defined to be a set of data and routines that act on that data. Thus, in EFI a protocol could be thought of as a module. A complicated protocol may be built out of several smaller modules.

4.4.1.2 Use locking to protect access to global and module variables.

This protection is strongly encouraged and especially useful for data that is accessed at various task priority levels.