source: doc/working/exception/except.c @ 43a284c

#include <stdlib.h>
#include <stdio.h>
#include <unwind.h>

#include "lsda.h"

//Global which defines the current exception
//Currently an int just to make matching easier
int this_exception;

//This is our personality routine
//For every stack frame anotated with ".cfi_personality 0x3,__gcfa_personality_v0"
//This function will be called twice when unwinding
//Once in the search phased and once in the cleanup phase
_Unwind_Reason_Code __gcfa_personality_v0 (
                     int version, _Unwind_Action actions, unsigned long long exceptionClass,
                     struct _Unwind_Exception* unwind_exception, struct _Unwind_Context* context)
{
        //DEBUG
        printf("Personality function (%d, %x, %llu, %p, %p):", version, actions, exceptionClass, unwind_exception, context);

        //If we've reached the end of the stack then there is nothing much we can do...
        if( actions & _UA_END_OF_STACK ) return _URC_END_OF_STACK;
        
        //DEBUG
        if (actions & _UA_SEARCH_PHASE) {
                printf(" lookup phase");
        } 
        //DEBUG
        else if (actions & _UA_CLEANUP_PHASE) {
                printf(" cleanup phase");
        }
        //Just in case, probably can't actually happen
        else {
                printf(" error\n");
                return _URC_FATAL_PHASE1_ERROR;
        }
        
        //Get a pointer to the language specific data from which we will read what we need
        const unsigned char * lsd = (const unsigned char*) _Unwind_GetLanguageSpecificData( context );

        if( !lsd ) {    //Nothing to do, keep unwinding
                printf(" no LSD");
                goto UNWIND;
        }

        //Get the instuction pointer and a reading pointer into the exception table
        lsda_header_info lsd_info;
        const unsigned char * cur_ptr = parse_lsda_header( context, lsd, &lsd_info);
        _Unwind_Ptr instruction_ptr = _Unwind_GetIP( context );

        //Linearly search the table for stuff to do
        while( cur_ptr < lsd_info.action_table ) {
                _Unwind_Ptr callsite_start;
                _Unwind_Ptr callsite_len;
                _Unwind_Ptr callsite_landing_pad;
                _uleb128_t  callsite_action;

                //Decode the common stuff we have in here
                cur_ptr = read_encoded_value (0, lsd_info.call_site_encoding, cur_ptr, &callsite_start);
                cur_ptr = read_encoded_value (0, lsd_info.call_site_encoding, cur_ptr, &callsite_len);
                cur_ptr = read_encoded_value (0, lsd_info.call_site_encoding, cur_ptr, &callsite_landing_pad);
                cur_ptr = read_uleb128 (cur_ptr, &callsite_action);

                //Have we reach the correct frame info yet?
                if( lsd_info.Start + callsite_start + callsite_len < instruction_ptr ) {
                        //DEBUG BEGIN
                        void * ls = (void*)lsd_info.Start;
                        void * cs = (void*)callsite_start;
                        void * cl = (void*)callsite_len;
                        void * bp = (void*)lsd_info.Start + callsite_start;
                        void * ep = (void*)lsd_info.Start + callsite_start + callsite_len;
                        void * ip = (void*)instruction_ptr;
                        printf("\nfound %p - %p (%p, %p, %p), looking for %p\n", bp, ep, ls, cs, cl, ip);
                        //DEBUG END
                        continue;
                }
                
                //Have we gone too far
                if( lsd_info.Start + callsite_start > instruction_ptr ) {
                        printf(" gone too far");
                        break;
                }

                //Something to do?
                if( callsite_landing_pad ) {
                        //Which phase are we in
                        if (actions & _UA_SEARCH_PHASE) {
                                //Search phase, this means we probably found a potential handler and must check if it is a match

                                //If we have arbitrarily decided that 0 means nothing to do and 1 means there is a potential handler
                                //This doesn't seem to conflict the gcc default behavior
                                if (callsite_action != 0) {
                                        //Now we want to run some code to see if the handler matches
                                        //This is the tricky part where we want to the power to run arbitrary code
                                        //However, generating a new exception table entry and try routine every time 
                                        //is way more expansive than we might like
                                        //The information we have is :
                                        //  - The GR (???)
                                        //  - The instruction pointer
                                        //  - The instruction pointer info (???)
                                        //  - The CFA (???)
                                        //  - The BSP (Probably the base stack pointer)


                                        //The current apprach uses one exception table entry per try block
                                        _uleb128_t imatcher;
                                        //Get the relative offset to the 
                                        cur_ptr = read_uleb128 (cur_ptr, &imatcher);

                                        //Get a function pointer from the relative offset and call it
                                        _Unwind_Reason_Code (*matcher)() = (_Unwind_Reason_Code (*)())lsd_info.LPStart + imatcher;                                      
                                        _Unwind_Reason_Code ret = matcher();

                                        //Based on the return value, check if we matched the exception
                                        if( ret == _URC_HANDLER_FOUND) printf(" handler found\n");
                                        else printf(" no handler\n");
                                        return ret;
                                }

                                //This is only a cleanup handler, ignore it
                                printf(" no action");
                        } 
                        else if (actions & _UA_CLEANUP_PHASE) {

                                if( (callsite_action != 0) && !(actions & _UA_HANDLER_FRAME) ){
                                        //If this is a potential exception handler 
                                        //but not the one that matched the exception in the seach phase,
                                        //just ignore it
                                        goto UNWIND;
                                }

                                //We need to run some clean-up or a handler
                                //These statment do the right thing but I don't know any specifics at all
                                _Unwind_SetGR( context, __builtin_eh_return_data_regno(0), (_Unwind_Ptr) unwind_exception );
                                _Unwind_SetGR( context, __builtin_eh_return_data_regno(1), 0 );

                                //I assume this sets the instruction pointer to the adress of the landing pad
                                //It doesn't actually set it, it only state the value that needs to be set once we return _URC_INSTALL_CONTEXT
                                _Unwind_SetIP( context, lsd_info.LPStart + callsite_landing_pad );

                                //DEBUG
                                printf(" action\n");

                                //Return have some action to run
                                return _URC_INSTALL_CONTEXT;
                        }
                }

                //Nothing to do, move along
                printf(" no landing pad");
        }
        //No handling found
        printf(" table end reached\n");

        //DEBUG
        UNWIND:
        printf(" unwind\n");

        //Keep unwinding the stack
        return _URC_CONTINUE_UNWIND;
}

//We need a piece of storage to raise the exception
struct _Unwind_Exception this_exception_storage;

//Function needed by force unwind
//It basically says to unwind the whole stack and then exit when we reach the end of the stack
static _Unwind_Reason_Code _Stop_Fn(    
        int version, 
        _Unwind_Action actions, 
        _Unwind_Exception_Class exceptionClass, 
        struct _Unwind_Exception * unwind_exception, 
        struct _Unwind_Context * context, 
        void * some_param
) {
        if( actions & _UA_END_OF_STACK  ) exit(1);
        if( actions & _UA_CLEANUP_PHASE ) return _URC_NO_REASON;

        return _URC_FATAL_PHASE2_ERROR;
}

//Example throw routine
void throw( int val ) {
        //Store the current exception
        this_exception = val;

        //DEBUG
        printf("Throwing exception %d\n", this_exception);

        //Call stdlibc to raise the exception
        _Unwind_Reason_Code ret = _Unwind_RaiseException( &this_exception_storage );

        //If we reach here it means something happened
        //For resumption to work we need to find a way to return back to here
        //Most of them will probably boil down to setting a global flag and making the phase 1 either stop or fail.
        //Causing an error on purpose may help avoiding unnecessary work but it might have some weird side effects.
        //If we just pretend no handler was found that would work but may be expensive for no reason since we will always
        //search the whole stack

        if( ret == _URC_END_OF_STACK ) {
                //No proper handler was found
                //This can be handled in several way
                //C++ calls std::terminate
                //Here we force unwind the stack, basically raising a cancellation
                printf("Uncaught exception %p\n", &this_exception_storage);
                
                ret = _Unwind_ForcedUnwind( &this_exception_storage, _Stop_Fn, (void*)0x22 );
                printf("UNWIND ERROR %d after force unwind\n", ret);
                abort();
        }

        //We did not simply reach the end of the stack without finding a handler,
        //Something wen't wrong
        printf("UNWIND ERROR %d after raise exception\n", ret);
        abort();
}