[译]C++异常的幕后17：异常类型的发射以及读.gcc_except_table

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struct LSDA_Header {

    uint8_t start_encoding;

    uint8_t type_encoding;

    // This is the offset, from the end of the header, to the types table

    uint8_t type_table_offset;

};

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struct LSDA_CS {

    // Offset into function from which we could handle a throw

    uint8_t start;

    // Length of the block that might throw

    uint8_t len;

    // Landing pad

    uint8_t lp;

    // Offset into action table + 1 (0 means no action)

    uint8_t action;

};

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// Pointer to the beginning of the raw LSDA

LSDA_ptr lsda = (uint8_t*)_Unwind_GetLanguageSpecificData(context);

// Read LSDA headerfor the LSDA

LSDA_Header header(&lsda);

const LSDA_ptr types_table_start = lsda + header.type_table_offset;

// Read the LSDA CS header

LSDA_CS_Header cs_header(&lsda);

// Calculate where the end of the LSDA CS table is

const LSDA_ptr lsda_cs_table_end = lsda + cs_header.length;

// Get the start of action tables

const LSDA_ptr action_tbl_start = lsda_cs_table_end;

// Get the first call site

LSDA_CS cs(&lsda);

// cs.action is the offset + 1; that way cs.action == 0

// means there is no associated entry in the action table

const size_t action_offset = cs.action - 1;

const LSDA_ptr action = action_tbl_start + action_offset;

// For a landing pad with a catch the action table will

// hold an index to a list of types

int type_index = action[0];

// types_table_start actually points to the end of the table, so

// we need to invert the type_index. There we'll find a ptr to

// the std::type_info for the specification in our catch

const void* catch_type_info = types_table_start[ -1 * type_index ];

const std::type_info *catch_ti = (const std::type_info *) catch_type_info;

// If everything went OK, this should print something like Fake_Exception

printf("%s\n", catch_ti->name());