source: doc/working/exception/impl/lsda.h @ b0837e4

//This code was stolen from gcc to read exception tables


/* If using C++, references to abort have to be qualified with std::.  */
#if __cplusplus
#define __gxx_abort std::abort
#else
#define __gxx_abort abort
#endif

/* Pointer encodings, from dwarf2.h.  */
#define DW_EH_PE_absptr         0x00
#define DW_EH_PE_omit           0xff

#define DW_EH_PE_uleb128        0x01
#define DW_EH_PE_udata2         0x02
#define DW_EH_PE_udata4         0x03
#define DW_EH_PE_udata8         0x04
#define DW_EH_PE_sleb128        0x09
#define DW_EH_PE_sdata2         0x0A
#define DW_EH_PE_sdata4         0x0B
#define DW_EH_PE_sdata8         0x0C
#define DW_EH_PE_signed         0x08

#define DW_EH_PE_pcrel          0x10
#define DW_EH_PE_textrel        0x20
#define DW_EH_PE_datarel        0x30
#define DW_EH_PE_funcrel        0x40
#define DW_EH_PE_aligned        0x50

#define DW_EH_PE_indirect       0x80


int handler_found = 0;

/* Given an encoding, return the number of bytes the format occupies.
This is only defined for fixed-size encodings, and so does not
include leb128.  */
static unsigned int size_of_encoded_value (unsigned char encoding) __attribute__ ((unused));

static unsigned int size_of_encoded_value (unsigned char encoding)
{
        if (encoding == DW_EH_PE_omit) return 0;

        switch (encoding & 0x07) {
                case DW_EH_PE_absptr: return sizeof (void *);
                case DW_EH_PE_udata2: return 2;
                case DW_EH_PE_udata4: return 4;
                case DW_EH_PE_udata8: return 8;
        }
        __gxx_abort ();
}

/* Given an encoding and an _Unwind_Context, return the base to which
the encoding is relative.  This base may then be passed to
read_encoded_value_with_base for use when the _Unwind_Context is
not available.  */
static _Unwind_Ptr base_of_encoded_value (unsigned char encoding, struct _Unwind_Context *context)
{
        if (encoding == DW_EH_PE_omit) return 0;

        switch (encoding & 0x70) {
                case DW_EH_PE_absptr:
                case DW_EH_PE_pcrel:
                case DW_EH_PE_aligned:
                        return 0;
                case DW_EH_PE_textrel:
                        return _Unwind_GetTextRelBase (context);
                case DW_EH_PE_datarel:
                        return _Unwind_GetDataRelBase (context);
                case DW_EH_PE_funcrel:
                        return _Unwind_GetRegionStart (context);
        }
        __gxx_abort ();
}

/* Read an unsigned leb128 value from P, store the value in VAL, return
P incremented past the value.  We assume that a word is large enough to
hold any value so encoded; if it is smaller than a pointer on some target,
pointers should not be leb128 encoded on that target.  */
static const unsigned char * read_uleb128 (const unsigned char *p, _uleb128_t *val)
{
        unsigned int shift = 0;
        unsigned char byte;
        _uleb128_t result;

        result = 0;
        do
        {
                byte = *p++;
                result |= ((_uleb128_t)byte & 0x7f) << shift;
                shift += 7;
        }
        while (byte & 0x80);

        *val = result;
        return p;
}

/* Similar, but read a signed leb128 value.  */
static const unsigned char * read_sleb128 (const unsigned char *p, _sleb128_t *val)
{
        unsigned int shift = 0;
        unsigned char byte;
        _uleb128_t result;

        result = 0;
        do
        {
                byte = *p++;
                result |= ((_uleb128_t)byte & 0x7f) << shift;
                shift += 7;
        }
        while (byte & 0x80);

        /* Sign-extend a negative value.  */
        if (shift < 8 * sizeof(result) && (byte & 0x40) != 0) result |= -(((_uleb128_t)1L) << shift);

        *val = (_sleb128_t) result;
        return p;
}

/* Load an encoded value from memory at P.  The value is returned in VAL;
The function returns P incremented past the value.  BASE is as given
by base_of_encoded_value for this encoding in the appropriate context.  */

static const unsigned char * read_encoded_value_with_base (unsigned char encoding, _Unwind_Ptr base, const unsigned char *p, _Unwind_Ptr *val)
{
        union unaligned
        {
                void *ptr;
                unsigned u2 __attribute__ ((mode (HI)));
                unsigned u4 __attribute__ ((mode (SI)));
                unsigned u8 __attribute__ ((mode (DI)));
                signed s2 __attribute__ ((mode (HI)));
                signed s4 __attribute__ ((mode (SI)));
                signed s8 __attribute__ ((mode (DI)));
        } __attribute__((__packed__));

        const union unaligned *u = (const union unaligned *) p;
        _Unwind_Internal_Ptr result;

        if (encoding == DW_EH_PE_aligned)
        {
                _Unwind_Internal_Ptr a = (_Unwind_Internal_Ptr) p;
                a = (a + sizeof (void *) - 1) & - sizeof(void *);
                result = *(_Unwind_Internal_Ptr *) a;
                p = (const unsigned char *) (_Unwind_Internal_Ptr) (a + sizeof (void *));
        }
        else
        {
                switch (encoding & 0x0f)
                {
                        case DW_EH_PE_absptr:
                                result = (_Unwind_Internal_Ptr) u->ptr;
                                p += sizeof (void *);
                                break;
                        case DW_EH_PE_uleb128:
                        {
                                _uleb128_t tmp;
                                p = read_uleb128 (p, &tmp);
                                result = (_Unwind_Internal_Ptr) tmp;
                        }
                        break;

                        case DW_EH_PE_sleb128:
                        {
                                _sleb128_t tmp;
                                p = read_sleb128 (p, &tmp);
                                result = (_Unwind_Internal_Ptr) tmp;
                        }
                        break;

                        case DW_EH_PE_udata2:
                                result = u->u2;
                                p += 2;
                                break;
                        case DW_EH_PE_udata4:
                                result = u->u4;
                                p += 4;
                                break;
                        case DW_EH_PE_udata8:
                                result = u->u8;
                                p += 8;
                                break;
                        case DW_EH_PE_sdata2:
                                result = u->s2;
                                p += 2;
                                break;
                        case DW_EH_PE_sdata4:
                                result = u->s4;
                                p += 4;
                                break;
                        case DW_EH_PE_sdata8:
                                result = u->s8;
                                p += 8;
                                break;
                        default:
                                __gxx_abort();
                }

                if (result != 0)
                {
                        result += ((encoding & 0x70) == DW_EH_PE_pcrel ? (_Unwind_Internal_Ptr) u : base);
                        
                        if (encoding & DW_EH_PE_indirect) result = *(_Unwind_Internal_Ptr *) result;
                }
        }

        *val = result;
        return p;
}

/* Like read_encoded_value_with_base, but get the base from the context
rather than providing it directly.  */
static inline const unsigned char * read_encoded_value (struct _Unwind_Context *context, unsigned char encoding, const unsigned char *p, _Unwind_Ptr *val)
{
        return read_encoded_value_with_base (encoding, base_of_encoded_value (encoding, context), p, val);
}

typedef struct
{
        _Unwind_Ptr Start;
        _Unwind_Ptr LPStart;
        _Unwind_Ptr ttype_base;
        const unsigned char *TType;
        const unsigned char *action_table;
        unsigned char ttype_encoding;
        unsigned char call_site_encoding;
} lsda_header_info;

static const unsigned char * parse_lsda_header (struct _Unwind_Context *context, const unsigned char *p, lsda_header_info *info)
{
        _uleb128_t tmp;
        unsigned char lpstart_encoding;

        info->Start = (context ? _Unwind_GetRegionStart (context) : 0);

        /* Find @LPStart, the base to which landing pad offsets are relative.  */
        lpstart_encoding = *p++;
        if (lpstart_encoding != DW_EH_PE_omit) p = read_encoded_value (context, lpstart_encoding, p, &info->LPStart);

        else info->LPStart = info->Start;

        /* Find @TType, the base of the handler and exception spec type data.  */
        info->ttype_encoding = *p++;
        if (info->ttype_encoding != DW_EH_PE_omit)
        {
                p = read_uleb128 (p, &tmp);
                info->TType = p + tmp;
        }
        else info->TType = 0;

        /* The encoding and length of the call-site table; the action table
        immediately follows.  */
        info->call_site_encoding = *p++;
        p = read_uleb128 (p, &tmp);
        info->action_table = p + tmp;

        return p;
}