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excpt(4)
NAME
excpt - Data structures and prototypes for exception handling support
SYNOPSIS
#include <excpt.h>
DESCRIPTION
The excpt.h include file brings together all of the data structures and
prototypes required to support the exception handling system. Calling
Standard for Alpha Systems for an overview of the system.
The excpt.h file also includes the following include files: signal.h,
pdsc.h, and c_excpt.h (which contains some C structured exception specific
definitions described in c_excpt(4)). The excpt.h include file defines the
following:
· Exception code encoding
· System exception record
· System context record
· System context pointers record
· Exception flags
· Exception dispositions
· Language handler definitions
· Run-time procedure type definition
· Macros to access runtime procedure descriptor fields
· Exception system prototypes
Exception Encoding
Existing exception code formats have been merged to represent them in a new
format. The existing codes are:
_____________________________________
LIBEXC NT VMS
_____________________________________
Code/NUMBER 32 bits 0:16 3:13
Facility -- 16:13 16:11
Customer -- 29:1 27:1
Severity -- 30:2 0:3
Control -- -- 28:4
_____________________________________
Libexc segments the address space into signals and other constants. There
is no compatibility with old LIBEXC constants. Instead, everything is a
case from the facility field.
The following example shows the typedef for a union for exception codes:
typedef union exception_code {
struct {
pdsc_uint_16 facility_dependent_1:16;
pdsc_uint_16 facility:12;
pdsc_uint_16 facility_dependent_2:4;
pdsc_uint_32 facility_dependent_3;
} exception_code_base;
struct {
pdsc_uint_32 osf_facility;
/* osf marker+signal,lang,etc */
pdsc_uint_32 code; /* subcode */
} exception_code_osf;
struct {
pdsc_uint_16 code:16; /* subcode */
pdsc_uint_16 facility:13; /* base distinguisher */
pdsc_uint_16 customer:1; /* nt versus customer */
pdsc_uint_16 severity:2; /* as it says */
pdsc_uint_32 reserved; /* sign extension of bit 31 */
} exception_code_nt;
struct {
pdsc_uint_16 severity:3; /* as it says */
pdsc_uint_16 message_number:13; /* subcode */
pdsc_uint_16 facility:11; /* base distinguisher */
pdsc_uint_16 customer:1; /* vms versus customer */
pdsc_uint_16 control:4; /* 1=>prnt,rest resrv */
pdsc_uint_32 reserved; /* sign extension of bit 31 */
} exception_code_vms;
} exception_code; /* exception_code */
A facility code for Tru64 UNIX is used as a base for all other Tru64 UNIX
codes. Constants chosen for the osf_facility let the program set the code
based on information it has (for example, the signal code for EXC_SIGNAL).
The possible values for the osf_facility field are in the excpt.h include
file.
An example of using EXC_VALUE is defining the codes for the EXC_INTERNAL
osf_facility (Note that some of the following definitions are shown on two
lines with the continuation character due to the space limitations of a
reference page):
#define EXC_STATUS_UNWIND EXC_VALUE(EXC_INTERNAL, 0)
#define EXC_STATUS_NONCONTINUABLE_EXCEPTION \
EXC_VALUE(EXC_INTERNAL, 1)
#define EXC_STATUS_INVALID_DISPOSITION \
EXC_VALUE(EXC_INTERNAL,2)
#define EXC_SIGNAL_EXPECTED EXC_VALUE(EXC_INTERNAL,3)
#define EXC_RUNTIME_FUNCTION_NOT_FOUND \
EXC_VALUE(EXC_INTERNAL,4)
#define EXC_INFINITE_LOOP_UNWIND \
EXC_VALUE(EXC_INTERNAL,5)
Typically, users either will print out the ascii values for the preceding
fields, or do final cleanup and call for operator assistance. There is
little in the way of recovery that can usually occur when these errors are
encountered. Usually, these errors indicate a programming error (for
example, attempted to continue a non-continuable exception) or some
corruption in the exception system data structures causes the exception
system to not be able to perform any useful task.
System Exception Record
The system exception record provides a handle to identify an exception.
This data structure communicates to routines that raise and dispatch
exceptions as well as the routine that unwinds and executes finally
handlers. The definition of the exception record follows:
typedef struct system_exrec *exrec_ptr;
/* UNIX Exception Record */
typedef struct system_exrec {
long ExceptionCode; /* reason for exception */
unsigned long ExceptionFlags; /* in progress, e.g. unwind */
exrec_ptr ExceptionRecord; /* rec chain, e.g.nested info */
void *ExceptionAddress; /* where error occurred */
unsigned long NumberParameters; /* # of ExceptionInformation's*/
unsigned long ExceptionInformation[1]; /* additional info */
} system_exrec_type;
The exception record pointer allows for nested exceptions to be chained.
The ExceptionAddress is the address at which the error occurred. The
parameters may be arguments which qualify an exception. The
NumberParameters is dictated by the ExceptionCode. Currently,
NumberParameters is always zero.
Activation Context Record
This record defines the state of the machine registers and system software
flags (for signals and traps) for a procedure's activation on the stack.
The struct sigcontext found in signal.h (which is also used by
setjmp/longjmp) represents a procedure's context.
#include <signal.h>
typedef struct sigcontext CONTEXT, *PCONTEXT;
The following code example defines context pointers to support
exc_virtual_unwind(3). These pointers can also provide a set of addresses
from which the registers in the CONTEXT are filled:
typedef exc_address CONTEXT_POINTERS[64];
typedef CONTEXT_POINTERS *PCONTEXT_POINTERS;
Exception Disposition
An exception disposition is returned by a language exception handler. The
handler may also choose not to return and call a routine such as
exc_unwind(3) directly.
typedef enum _EXCEPTION_DISPOSITION {
ExceptionContinueExecution,
ExceptionContinueSearch,
ExceptionNestedException,
ExceptionCollidedUnwind
} EXCEPTION_DISPOSITION;
Exception Record Flags
The exception record flags are used to communicate what is going on with a
particular exception. Both the exception system and user code may set these
flags. Macros are provided to easily test the ExceptionFlags field of the
exception record (system_exrec_type):
#define EXCEPTION_NONCONTINUABLE 0x1 /* Noncontinuable exception */
#define EXCEPTION_UNWINDING 0x2 /* Unwind is in progress */
#define EXCEPTION_EXIT_UNWIND 0x4 /* Exit unwind is in progress */
#define EXCEPTION_STACK_INVALID 0x08 /* Stack out of limits
or unaligned */
#define EXCEPTION_NESTED_CALL 0x10 /* Nested exception handler call */
#define EXCEPTION_TARGET_UNWIND 0x20 /*Execute termination handler
for it*/
#define EXCEPTION_COLLIDED_UNWIND 0x40 /*unwind through unwind
dispatcher*/
#define EXCEPTION_UNWIND (EXCEPTION_UNWINDING | \
EXCEPTION_EXIT_UNWIND | \
EXCEPTION_TARGET_UNWIND | \
EXCEPTION_COLLIDED_UNWIND)
#define IS_UNWINDING(flag) (((flag) & EXCEPTION_UNWIND) != 0)
#define IS_DISPATCHING(flag) (((flag) & EXCEPTION_UNWIND) == 0)
#define IS_TARGET_UNWIND(flag) ((flag) \
& EXCEPTION_TARGET_UNWIND)
Run-time Function Type
The run-time function is used to access information regarding how to unwind
a procedure's activation and where it has a handler. On Tru64 UNIX this
structure is defined to be a code range descriptor (see pdsc(4)). Although
it does not provide direct access to most of the information required, the
run-time function indirectly does the following:
typedef union pdsc_crd RUNTIME_FUNCTION, *PRUNTIME_FUNCTION, \
*PRUNTIME_FUNCTION;
The macros in the following table facilitate accessing the information
related to the procedures the preceding structures represent. The argument
for each macro is PRUNTIME_FUNCTION.
________________________________________________________
Macro Comment
________________________________________________________
EXCEPT_PD return pointer to pdsc_rpd
EXCPT_BEGIN_ADDRESS first address in code range
EXCPT_END_ADDRESS last address in code range +4
EXCPT_LANG_HANDLER handler address
EXCPT_LANG_HANDLER_DATA handler data address
EXCPT_PROLOG_END_ADDRESS end address of prolog +4
________________________________________________________
The excpt.h include file also provides direct access to the code range
descriptor table and its number of elements with function_table and
function_table_size variables (actually macros) respectively.
The dispatcher context contains enough information for the dispatcher to
deliver pertinent information to language handlers about what it was asked
to do and by whom and enough information for them to detect collided
unwinds.
typedef struct {
unsigned long pc; /* current pc in backup */
pRUNTIME_FUNCTION functionTable; /* entry matching pc */
PCONTEXT originating_context; /* disp called with this */
} DISPATCHER_CONTEXT;
Function prototypes
This include file includes function prototypes for all of the external
routines listed in exception_intro(3).
RELATED INFORMATION
c_excpt(4), exception_intro(3), longjmp(3), signal(2), signal(4), pdsc(4),
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Index for
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Alphabetical
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Top of
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