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exception_intro(3)
NAME
exception_intro, libexc - an overview of exception support supplied in
libexc.a
DESCRIPTION
The exception mechanism used on Tru64 UNIX requires no execution time in
user code. The only costs before an exception occurs are one-time
registering and deregistering of modules with the run-time exception
handling mechanism from the init and fini sections. This one-time cost is
a requirement to support shared objects.
After an exception, there are costs to find exception handling and unwind
information for procedure activations on the stack. This model assumes that
exceptions are exceptional occurrences and that they should not cost
anything until they occur. When an exception occurs, the exception system
makes every effort to be as efficient as possible by using mechanisms such
as binary searches and software caches where applicable.
The routines in this section come from four sources, although these
routines might not have the same names as the routines in the sources:
· ULTRIX libexc.a
· NT defined interfaces
· Calling Standard for Alpha Systems
· New routines required for this implementation
There may be multiple routines that do the same things but are supported
for backward compatibility. The data structures are defined in excpt(4),
pdsc(4) and the Calling Standard for Alpha Systems .
If you have used the run-time procedure descriptor from ULTRIX libexc as an
opaque pointer, your code will port easily. If not, you must modify field
names to match the new data structures.
The exception system supports three basic services:
· Unwinding
· Access to procedure-specific information
· Exception management
Unwinding support includes the ability to get a context, virtual and actual
unwinding of levels of procedure activations from the stack, and continuing
execution in a handler or other user code. Some of the unwind routines also
support invoking handlers as they unwind so that the language or user can
clean up items at particular procedure activations.
The routines that provide access to procedure-specific information can map
an address anywhere in a routine to the routine's information. This
information includes enough data to cause an unwind or determine if a
routine handles an exception. Typically these routines take an address and
then binary search a compiler system-created table for an entry covering
the address. These routines also contain support for multiple tables so
that shared objects and run-time generated code can participate in the
exception system.
The exception management routines provide ways for languages or users to
raise exceptions or cause signals to raise exceptions. The exception
management routines also provide the mechanism to dispatch the exceptions
to the appropriate handlers. Typically, languages provide handlers that
determine whether the user has specified a handler for a particular address
and exception. In the case of structured exception handling in C, the
language-specific handler invokes a routine containing user-supplied code
to determine what action to take (see c_excpt(4) for information on C-style
structured exception handling). Language-specific handlers can either
handle the exception or return for some other procedure activation to
handle it.
The following routines are described in Section 3 reference pages:
________________________________________________________________________
Routine Description
________________________________________________________________________
exc_continue Like longjmp(3)
exc_dispatch_exception
Deliver exceptions to language
handlers
exc_longjmp
Like longjmp(3) except calls
handlers for cleanup
exc_lookup_function_entry Map address to procedure information
exc_lookup_function_table_address
Map address to table containing
procedure information
exc_lookup_gp
Map address to GP value for a
procedure
exc_raise_exception
Raise exception given exception
structure
exc_raise_status_exception Raise exception given exception code
exc_set_last_chance_handler
Set user handler for unhandled
exceptions
exc_unwind
Actual unwind invoking cleanup
handlers
__exc_last_chance Unhandled exception handler
exc_resume Like longjmp(3)
exception_dispatcher
Take signal(2) arguments and raises
exceptions
exc_find_frame_ptr
Returns static link of current
activation
find_rpd
Maps address to procedure
information
exc_add_gp_range
Register range of address for a GP
value
exc_add_pc_range_table Register procedure information table
exc_remove_gp_range
Deregister range of addresses for a
GP value
exc_remove_pc_range_table
Deregister procedure information
table
exc_raise_signal_exception
Take signal(2) arguments and raises
exceptions
set_unhandled_exception
Set user handler for unhandled
exceptions
unwind
Virtual unwind updating activation
context
exc_virtual_unwind
Virtual unwind updating activation
context
________________________________________________________________________
See the IEEE routines in the SEE ALSO section for information on IEEE
floating-point exception support.
FILES
/usr/ccs/lib/cmplrs/cc/libexc.a -- exception handling library
/usr/include/excpt.h -- include file
/usr/include/pdsc.h -- include file
/usr/include/signal.h -- include file
/usr/include/machine/fpu.h -- include file
SEE ALSO
Functions: exception_dispatcher(3), exc_lookup_function_entry(3),
signal(2), sigaction(2), setjmp(3), exc_unwind(3), __exc_last_chance(3),
ieee(3).
Files: excpt(4), c_excpt(4), signal(4), pdsc(4).
Programmer's Guide
Assembly Language Programmer's Guide
Calling Standard for Alpha Systems
Object File/Symbol Table Format Specification
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