source: doc/working/exception/main.c @ 03bb816

#include <stdio.h>
#include "except.h"

#define EXCEPTION 2

struct type_raii_t {
        char * msg;
};

//Dtor function to test clean up routines
void dtor( struct type_raii_t * this ) {
        printf("%s\n", this->msg);
}

//Type macro use to make scope unwinding easier to see.
#define raii_t __attribute__(( cleanup(dtor) )) struct type_raii_t

//Leaf functions that raises exception
void bar() {
        raii_t a = { "Bar dtor" };

        throw( EXCEPTION );
}

//Matcher function which will check if the exception was correctly caught
extern int this_exception;
_Unwind_Reason_Code foo_try_match() {
        return this_exception == 2 ? _URC_HANDLER_FOUND : _URC_CONTINUE_UNWIND;
}

//Try statements are hoisted out see comments for details
//With this could probably be unique and simply linked from
//libcfa but there is one problem left, see the exception table 
//for details
void try( void (*try_block)(), void (*catch_block)() )
{
        //Setup statments
        //These 2 statments won't actually result in any code,
        //they only setup global tables.
        //However, they clobber gcc cancellation support from gcc.
        //We can replace the personality routine but replacing the exception
        //table gcc generates is not really doable, it generates labels based
        //on how the assembly works.
        //Setup the personality routine
        asm volatile (".cfi_personality 0x3,__gcfa_personality_v0");
        //Setup the exception table
        asm volatile (".cfi_lsda 0x3, .LLSDACFA2");

        //Label which defines the start of the area for which the handler is setup
        asm volatile (".TRYSTART:");

        //The actual statements of the try blocks
        try_block();

        //asm statement to prevent deadcode removal
        asm volatile goto ("" : : : : CATCH );

        //Normal return
        return;

        //Exceptionnal path
        CATCH : __attribute__(( unused ));
        //Label which defines the end of the area for which the handler is setup
        asm volatile (".TRYEND:");
        //Label which defines the start of the exception landing pad
        //basically what will be called when the exception is caught
        //Note, if multiple handlers are given, the multiplexing should be done
        //by the generated code, not the exception runtime
        asm volatile (".CATCH:");

        //Exception handler
        catch_block();
}

//Exception table data we need to generate
//While this is almost generic, the custom data refers to
//foo_try_match try match, which is no way generic
//Some more works need to be done if we want to have a single 
//call to the try routine
asm (
        //HEADER
        ".LFECFA1:\n"
        "       .globl  __gcfa_personality_v0\n"
        "       .section        .gcc_except_table,\"a\",@progbits\n"
        ".LLSDACFA2:\n"                                                 //TABLE header
        "       .byte   0xff\n"
        "       .byte   0xff\n"
        "       .byte   0x1\n"
        "       .uleb128 .LLSDACSECFA2-.LLSDACSBCFA2\n"         //BODY length
        //Body uses language specific data and therefore could be modified arbitrarily
        ".LLSDACSBCFA2:\n"                                              //BODY start
        "       .uleb128 .TRYSTART-try\n"                               //Handled area start  (relative to start of function)
        "       .uleb128 .TRYEND-.TRYSTART\n"                           //Handled area length
        "       .uleb128 .CATCH-try\n"                          //Hanlder landing pad adress  (relative to start of function)
        "       .uleb128 1\n"                                           //Action code, gcc seems to use always 0
        //Beyond this point we don't match gcc data'
        "       .uleb128 foo_try_match-try\n"                   //Handler routine to check if the exception is matched
        ".LLSDACSECFA2:\n"                                              //BODY end
        "       .text\n"                                                        //TABLE footer
        "       .size   try, .-try\n"
        "       .ident  \"GCC: (Ubuntu 6.2.0-3ubuntu11~16.04) 6.2.0 20160901\"\n"
        "       .section        .note.GNU-stack,\"x\",@progbits\n"
);

void foo() {
        raii_t a = { "Foo dtor" };

        //Since try will clobber the gcc exception table assembly,
        //We need to nest this to have gcc regenerate the data
        void foo_try_block() {
                raii_t b = { "Foo try dtor" };

                bar();

                printf("Called bar successfully\n");
        }

        void foo_catch_block() {
                printf("Exception caught\n");
        }

        //Actual call to the try block
        try( foo_try_block, foo_catch_block );

        printf( "Foo exited normally\n" );
}

int main() {
        raii_t a = { "Main dtor" };

        foo();

        printf("End of program reached\n");
}