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Pragma Propagate_Exceptions


     pragma Propagate_Exceptions (subprogram_LOCAL_NAME);

This pragma indicates that the given entity, which is the name of an imported foreign-language subprogram may receive an Ada exception, and that the exception should be propagated. It is relevant only if zero cost exception handling is in use, and is thus never needed if the alternative longjmp / setjmp implementation of exceptions is used (although it is harmless to use it in such cases).

The implementation of fast exceptions always properly propagates exceptions through Ada code, as described in the Ada Reference Manual. However, this manual is silent about the propagation of exceptions through foreign code. For example, consider the situation where P1 calls P2, and P2 calls P3, where P1 and P3 are in Ada, but P2 is in C. P3 raises an Ada exception. The question is whether or not it will be propagated through P2 and can be handled in P1.

For the longjmp / setjmp implementation of exceptions, the answer is always yes. For some targets on which zero cost exception handling is implemented, the answer is also always yes. However, there are some targets, notably in the current version all x86 architecture targets, in which the answer is that such propagation does not happen automatically. If such propagation is required on these targets, it is mandatory to use Propagate_Exceptions to name all foreign language routines through which Ada exceptions may be propagated.