Note: This section applies mostly to C programs, so the explanation below uses examples from the C libraries. However, the feature may also be useful for assembler programs.
.INTERRUPTOR keywords it is possible to export functions in a
special way. The linker is able to generate tables with all functions of a
specific type. Such a table will only include symbols from object
files that are linked into a specific executable. This may be used to add
initialization and cleanup code for library modules, or a table of interrupt
The C heap functions are an example where module initialization code is used.
All heap functions (
free, ...) work with a few
variables that contain the start and the end of the heap, pointers to the free
list and so on. Since the end of the heap depends on the size and start of the
stack, it must be initialized at runtime. However, initializing these
variables for programs that do not use the heap are a waste of time and
So the central module defines a function that contains initialization code and
exports this function using the
.CONSTRUCTOR statement. If (and only if)
this module is added to an executable by the linker, the initialization
function will be placed into the table of constructors by the linker. The C
startup code will call all constructors before
main and all destructors
main, so without any further work, the heap initialization code is
called once the module is linked in.
While it would be possible to add explicit calls to initialization functions in the startup code, the new approach has several advantages:
The symbols are sorted in increasing priority order by the linker when using one of the builtin linker configurations, so the functions with lower priorities come first and are followed by those with higher priorities. The C library runtime subroutine that walks over the function tables calls the functions starting from the top of the table - which means that functions with a high priority are called first.
So when using the C runtime, functions are called with high priority functions first, followed by low priority functions.
When using these special symbols, please take care of the following:
callirqmodules in the C runtime for an example on how to do this.
FEATURE CONDESstatement in the linker config file. Each table has to be requested separately.
.INTERRUPTORstatements, there is also a more generic command:
.CONDES. This allows to specify an additional type. Predefined types are 0 (constructor), 1 (destructor) and 2 (interruptor). The linker generates a separate table for each type on request.