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excpt(4) Kernel Interfaces Manual 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 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: tab(@); lfHB lfHB lfHB lfHB l l l l . _ @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 DIGITAL UNIX is used as a base for all other DIGI- TAL 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 in- clude 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) #de- fine 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 er- rors are encountered. Usually, these errors indicate a programming er- ror (for example, attempted to continue a non-continuable exception) or some corruption in the exception system data structures causes the ex- ception 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 oc- curred. The parameters may be arguments which qualify an exception. The NumberParameters is dictated by the ExceptionCode. Currently, NumberPa- rameters 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 ex- ception handler call */ #define EXCEPTION_TARGET_UNWIND 0x20 /*Execute termination handler for it*/ #define EXCEPTION_COLLIDED_UNWIND 0x40 /*unwind through un- wind dispatcher*/ #define EXCEPTION_UNWIND (EXCEPTION_UNWINDING | \ EXCEPTION_EXIT_UNWIND | \ EXCEPTION_TARGET_UNWIND | \ EXCEPTION_COLLIDED_UNWIND) #define IS_UNWINDING(flag) (((flag) & EXCEPTION_UNWIND) != 0) #de- fine 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 DIGITAL 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 in- formation 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 ar- gument for each macro is PRUNTIME_FUNCTION. tab(@); lfHB lfHB ll. _ 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 INFORMATION c_excpt(4), exception_intro(3), longjmp(3), signal(2), signal(4), pdsc(4), delim off excpt(4)