excpt(4)excpt(4)NAMEexcpt - 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 spe‐
cific 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, every‐
thing 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_INTER‐
NAL osf_facility (Note that some of the following definitions are shown
on two lines with the continuation character due to the space limita‐
tions 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 preced‐
ing 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, Num‐
berParameters is always zero.
Activation Context Record
This record defines the state of the machine registers and system soft‐
ware 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_vir‐
tual_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 Exception‐
Flags 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 /*Exe‐
cute 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 follow‐
ing:
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 col‐
lided 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 INFORMATIONc_excpt(4), exception_intro(3), longjmp(3), signal(2), signal(4),
pdsc(4), delim off
excpt(4)
