The Elaboration Order Problem of Ada

A serious problem in huge Ada programs is finding a satisfactory order of elaboration for all of the program’s compilation units. Some elaboration sequence must he selected but access to a program entity before its elaboration will rake the exception (PROGRAM_ERROR) shortly after execution of the program begins. The rules of the 1983 Ada standard 1 fail to ensure that validated compilers produce a satisfactory elaboration order, or report an error if static analysis shows that such an order may not exist. The problem can make programs non-portable and sensitive to modifications. This paper presents an algorithm ada_elab which determines an order of elaboration, free of PROGRAM_ERROR, if link-time, static analysis can determine that there is one. Examples are used to illustrate the occurrence of elaboration errors and the inadequacy of pragma ELABORATE to resolve the problem. The paper closes with descriptions of elaboration and elaboration-related problems that cannot be handled by ada_elab, access before initialization and coding practices that should avoid elaboration and initialization errors. While Ada is the subject of this paper, the problem being studied could occur in any language that permits a complex initialization of individual program modules prior to the execution of the main program module.