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#include <excpt.h>
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
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.
/* 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.
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;
#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)
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;
c_excpt(4), exception_intro(3), longjmp(3), signal(2), signal(4), pdsc(4), delim off