A few HP-UX specific stubs have been designed to support the shared library
mechanism. Calls to external routines in HP-UX will return via an export stub.
The call itself will go through an import stub, similar to the calling and
called stubs of Chapter 5. The import stub stores the following registers: RP
(at SP - 24) and gr19 (at SP - 32).
In the HP-UX shared library implementation, gr19 plays the role of the shared
library link register. It points to a shared library descriptor. This
descriptor contains a pointer to the location where the unwind tables and stub
tables are located. Note that each shared library has its own tables.
When unwinding through the HP-UX-export stub, the. registers RP and gr19 are
restored from the stack. In most normal cases, the RP that is restored will
point to the caller. But in one particular case, the RP value has the address
of the export stub for _sigreturn(). This happens only when the stack
frame has an interrupt-marker on it, which means that a trap occurred and you
are unwinding from a trap handler.
Note that these stubs will not be encountered when unwinding in the absence of
shared libraries. In this case, gr19 will play the normal role of a caller-save
register only.
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