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C4H OR C5H LINSYM--SYMBOL LINE NUMBERS RECORD ============================================= Description ----------- This record will be used to output line numbers for functions specified through COMDAT records. Each LINSYM record is associated with a preceding COMDAT record. History ------- This record is a Microsoft extension for C 7.0. Record Format ------------- 1 2 1 1 or 2 2 2 or 4 1 C4 Record Flags Public Line Line Checksum or Length Name Number Number C5 Index Offset <---repeated---> Flags Field ----------- This field contains one defined bit: 01H Continuation bit. If clear, this COMDAT record establishes a new instance of the COMDAT variable; otherwise, the data is a continuation of the previous COMDAT of the symbol. Public Name Index Field ----------------------- A regular logical name index, indicating the name of the base of the LINSYM record. Line Number Field ----------------- An unsigned number in the range 0 to 65,535. Line Number Offset Field ------------------------ The offset relative to the base specified by the symbol name base. The size of this field depends on the record type. NOTES Record type C5H is identical to C4H except that the Line Number Offset field is 4 bytes instead of 2. This record is used to output line numbers for functions specified through COMDAT records. Often, the residing segment as well as the relative offsets of such functions is unknown at compile time, in that the linker is the final arbiter of such information. For such cases, the compiler will generate this record to specify the line number/offset pairs relative to a symbolic name. This record will also be used to discard duplicate LINNUM information. If the linker encounters two or more LINSYM records with matching symbolic names (corresponding to multiple COMDAT records with the same name), the linker will keep the one that corresponds to the retained COMDAT. LINSYM records must follow the COMDATs to which they refer. A LINSYM on a given symbol refers to the most recent COMDAT on the same symbol. LINSYMs inherit the "localness" of their COMDATs. C6H ALIAS--ALIAS DEFINITION RECORD ================================== Description ----------- This record has been introduced to support link-time aliasing, a method by which compilers or assemblers may direct the linker to substitute all references to one symbol for another. History ------- The record is a Microsoft extension for FORTRAN version 5.1 (LINK version 5.13). Record Format ------------- 1 2 <variable> <variable> 1 C6 Record Length Alias Name Substitute Name Checksum <-----------repeated---------> Alias Name Field ---------------- A regular length-preceded name of the alias symbol. Substitute Name Field --------------------- A regular length-preceded name of the substitute symbol. NOTES This record consists of two symbolic names: the alias symbol and the substitute symbol. The alias symbol behaves very much like a PUBDEF in that it must be unique. If a PUBDEF of an alias symbol is encountered later, the PUBDEF overrides the alias. If another ALIAS record with a different substitute symbol is encountered, a warning is emitted by the linker, and the second substitute symbol is used. When attempting to satisfy an external reference, if an ALIAS record whose alias symbol matches is found, the linker will halt the search for alias symbol definitions and will attempt to satisfy the reference with the substitute symbol. All ALIAS records must appear before the LINK Pass 2 record. C8H OR C9H NBKPAT--NAMED BACKPATCH RECORD ========================================= Description ----------- The Named Backpatch record is similar to a BAKPAT record, except that it refers to a COMDAT record, by logical name index, rather than an LIDATA or LEDATA record. NBKPAT records must immediately follow the COMDAT/FIXUPP block to which they refer. History ------- This record is a Microsoft extension for C 7.0. Record Format ------------- 1 2 1 1 or 2 2 or 4 2 or 4 1 C8 Record Location Public Offset Value Checksum or C9 Length Type Name <---repeated---> Location Type Field ------------------- Type of location to be patched; the only valid values are: 0 8-bit byte 1 16-bit word 2 32-bit double word, record type C9H only Public Name Index Field ----------------------- Regular logical name index of the COMDAT record to be back patched. Offset and Value Fields ----------------------- These fields are 32 bits for record type C8H, 16 bits for C9H. The Offset field specifies the location to be patched, as an offset into the COMDAT. The associated Value field is added to the location being patched (unsigned addition, ignoring overflow). The Value field is a fixed length (16 bits or 32 bits, depending on the record type) to make object module processing easier. CAH LLNAMES--LOCAL LOGICAL NAMES DEFINITION RECORD ================================================== Description ----------- The LLNAMES record is a list of local names that can be referenced by subsequent SEGDEF and GRPDEF records in the object module. The names are ordered by their occurrence, with the names in LNAMES records and referenced by index from subsequent records. More than one LNAMES and LLNAMES record may appear. The names themselves are used as segment, class, group, overlay, COMDAT, and selector names. History ------- This record is a Microsoft extension for C 7.0. Record Format ------------- 1 2 1 <--String Length--> 1 CA Record String Name Checksum Length Length String <----------repeated---------> Each name appears in <count, char> format, and a null name is valid. The character set is ASCII. Names can be up to 254 characters long. NOTE: Any LLNAMES records in an object module must appear before the records that refer to them. APPENDIX 1: CODEVIEW EXTENSIONS =============================== Calling the following information "CodeView's OMF" is a misnomer, since CodeView actually does very little to extend the OMF. Most CodeView information is stored within the confines of the existing OMF, except where noted below. CodeView information is stored on a per-module basis in specially- named logical segments. These segments are defined in the usual way (SEGDEF records), but LINK handles them specially, and they do not end up as segments in the .EXE file. These segment names are reserved: Segment Name Class Name Combine Type ------------ ---------- ------------ $$TYPES DEBTYP Private $$SYMBOLS DEBSYM Private The segment $$IMPORT should also be considered a reserved name, although it is not used anymore. This segment was not part of any object files but was emitted by the linker to get the loader to automatically do fixups for CodeView information. The linker emitted a standard set of imports, not just ones referenced by the program being linked. Use of this segment may be revisited in the future. CodeView-specific data is stored in LEDATA records for the $$TYPES and $$SYMBOLS segments, in various proprietary formats. The $$TYPES segment contains information on user-defined variable types; $$SYMBOLS contains information about nonpublic symbols: stack, local, procedure, block start, constant, and register symbols and code labels. For instantiated functions in C 7.0, symbol information for CodeView will be output in COMDAT records that refer to segment $$SYMBOLS and have decorated names based on the function names. Type information will still go into the $$TYPES segment in LEDATA records. All OMF records that specify a Type Index field, including EXTDEF, PUBDEF, and COMDEF records, use proprietary CodeView values. Since many types are common, Type Index values in the range 0 through 511 (1FFH) are reserved for a set of predefined primitive types. Indexes in the range 512 through 32767 (200H-7FFFH) index into the set of type definitions in the module's $$TYPES segment, offset by 512. Thus 512 is the first new type, 513 the second, and so on. APPENDIX 2: MICROSOFT MS-DOS LIBRARY FORMAT =========================================== Libraries under MS-DOS are always multiples of 512-byte blocks. The first record in the library is a header. This first record looks very much like any other Microsoft object module format record. Library Header Record (<n> bytes) --------------------------------- 1 2 4 2 1 <n - 10> Type Record Length Dictionary Dictionary Flags Padding Offest Size (F0H) (Page Size Minus 3) in Blocks The first byte of the record identifies the record's type, and the next two bytes specify the number of bytes remaining in the record. Note that this word field is byte-swapped (that is, the low-order byte precedes the high-order byte). The record type for this library header is F0H (240 decimal). The Record Length field specifies the page size within the library. Modules in a library always start at the beginning of a page. Page size is determined by adding three to the value in the Record Length field (thus the header record always occupies exactly one page). Legal values for the page size are given by 2 to the <n>, where <n> is greater than or equal to 4 and less than or equal to 15. The four bytes immediately following the Record Length field are a byte-swapped long integer specifying the byte offset within the library of the first byte of the first block of the dictionary. The next two bytes are a byte-swapped word field that specifies the number of blocks in the dictionary. (The MS-DOS Library Manager cannot create a library whose dictionary would require more than 251 512-byte pages.) The next byte contains flags describing the library. The current flag definition is: 0x01 = case sensitive (applies to both regular and extended dictionaries) All other values are reserved for future use and should be 0. The remaining bytes in the library header record are not significant. This record deviates from the typical Microsoft OMF record in that the last byte is not used as a checksum on the rest of the record. Immediately following the header is the first object module in the library. It, in turn, is succeeded by all other object modules in the library. Each module is in Microsoft OMF (that is, it starts with a LHEADR record and ends with a MODEND record). Individual modules are aligned so as to begin at the beginning of a new page. If, as is commonly the case, a module does not occupy a number of bytes that is exactly a multiple of the page size, then its last block will be padded with as many null bytes as are required to fill it. Following the last object module in the library is a record that serves as a marker between the object modules and the dictionary. It also resembles a Microsoft OMF record. Library End Record (marks end of objects and beginning of dictionary) 1 2 <n> Type (F1H) Record Length Padding The record's Type field contains F1H (241 decimal), and its Record Length field is set so that the dictionary begins on a 512-byte boundary. The record contains no further useful information; the remaining bytes are insignificant. As with the library header, the last byte is not a checksum. Dictionary ---------- The remaining blocks in the library compose the dictionary. The number of blocks in the dictionary is given in the library header. Note that there should always be a prime number of blocks in the dictionary. The dictionary is a hashed index to the library. Public symbols are essentially hashed twice, though in fact, both hash indexes are produced simultaneously. The first hash index, the block index, is used to determine a block within the dictionary in which to place the symbol. The second hash index, the bucket index, is used to choose a bucket within the block for the symbol. Blocks always have 37 buckets; they are the first 37 bytes of each block. If a bucket is full, it contains a nonzero value that points to the text of the symbol. To actually find the symbol, take the bucket value, multiply it by two, and use the resulting number as a byte offset from the beginning of the block. Collisions (that is, two or more distinct symbols hashing to the same block and bucket in the dictionary) are resolved by a technique known as linear open addressing. At the same time the hash indexes are produced, two hash deltas are also produced. If a symbol collides with a symbol already installed in the dictionary, the librarian attempts to find an empty bucket for it by adding the bucket delta to the bucket index and using the result mod 37 as a new bucket index. If this new bucket index points to a bucket that is empty, the librarian will install the symbol in that bucket. If the bucket is not empty, the delta is applied repeatedly until an empty bucket is found or until it is determined that there are no empty buckets on the block. If the latter is the case, the block delta is added to the block index, and the result mod the number of blocks in the dictionary is used as a new block index. With the new block index and the original bucket index, the sequence is repeated until an empty bucket on some block is found. The number of blocks and the number of buckets are prime so that no matter what values of hash indexes and deltas are produced for a symbol, in the worst case, all possible block-bucket combinations will be tried. Once a free block-bucket pair has been found for a symbol, the pair and information concerning its place of definition must be installed. Since a bucket is a single byte pointing into a 512-byte block, the bucket can give at best a word offset within that block. Thus, symbol entries within a dictionary must start on word boundaries. Since bytes 0 through 36 of each dictionary block make up the hash table, the first symbol entry will begin at byte 38 (decimal). Dictionary Record (length is the dictionary size in 512-byte blocks) 37 1 <variable> 2 <conditional> HTAB FFLAG Name Block Number Align Byte <-------------------repeated-------------> Entries consist of the following: the first byte is the length of the symbol to follow, the following bytes are the text of the symbol, and the last two bytes are a byte-swapped word field that specifies the page number (counting the library header as the 0th page) at which the module defining the symbol begins. All entries may have at most one trailing null byte in order to align the next entry on a word boundary. It is possible for a dictionary block to be full without all its buckets being used. Such will be the case, for example, if symbol names are longer on average than nine characters each. Therefore, there must be some way to mark a block as full so that empty buckets will be ignored. Byte 37 decimal (counting from 0) is reserved for this purpose. If the block is not full, byte 37 will contain the word offset of the beginning of free space in the block, but if the block is full, byte 37 will contain the special value 255 decimal (FFH). Module names are stored in the LHEADR record of each module. Extended Dictionary ------------------- The extended dictionary is optional and indicates dependencies between modules in the library. Versions of LIB earlier than 3.09 do not create an extended dictionary. The extended dictionary is placed at the end of the library. The dictionary is preceded by these values: BYTE =0xF2 Extended Dictionary header WORD length of extended dictionary in bytes excluding first three bytes Start of extended dictionary: WORD number of modules in library = N Module table, indexed by module number, with N + 1 fixed-length entries: WORD module page number WORD offset from start of extended dictionary to list of required modules Last entry is null. Dictionary Hashing Algorithm ---------------------------- The last part of each library file is a dictionary, which contains indexes to all symbols in the library. The dictionary is divided into 512-byte pages. Each page consists of a 37-byte bucket table and a 475- byte table of symbol entries. To find the right spot in the dictionary for a given symbol, the hashing algorithm is used. The hashing algorithm analyzes the name of the symbol and produces two indexes: a page and a bucket index, which together point to a single entry in the dictionary. The only problem is that more than one symbol name can generate exactly the same address. Because of this, the name found in the dictionary entry must be compared with the symbol's name, and if they are not identical, a correction to the address must be made. To make this correction possible, the hashing algorithm, in addition to the base address (page, bucket), also produces the correction values (delta-page, delta- bucket), which are added to the base address if the symbol's name does not match the name in an entry. The number of pages in the dictionary must always be prime, so if the symbol is not found, the consecutive adding of deltas produces the starting address again. Below is psueudocode illustrating the hashing algorithm used by the LIB (librarian) utility. extern char *symbol; /* Symbol to find */ extern int dictlength; /* Dictionary length in pages */ extern int buckets; /* Number of buckets on one page */ char *pb; /* A pointer to the beginning of the symbol */ char *pe; /* " end " */ int slength; /* Length of the symbol's name */ int page_index; /* Page index */ int page_index_delta;/* Page index delta */ int page_offset; /* Page offset (bucket #) */ int page_offset_delta;/* Page offset delta */ unsigned c; slength = strlen(symbol); pb = symbol; pe = symbol + slength; page_index = 0; page_index_delta = 0; page_offset = 0; page_offset_delta = 0; while( slength-) { c = *(pf++) | 32; /* Convert character to lowercase */ page_index = (page_index<<2) XOR c; /* Hash */ page_offset_delta = (page_offset_delta>>2)XORc; c = *(pe++) | 32; page_offset = (page_offset>>2) XOR c; /* Hash */ pageiindexdelta = (page_index_delta<<2) XOR c; } /* Calculate page index */ page_index = page_index MODULO dictlength; /* Calculate page index delta */ if((page_index_delta = page_index_delta MODULO dictlength) == 0) page_index_delta = 1; /* Calculate page offset */ page_offset = page_offset MODULO buckets; /* Calculate page offset delta */ if( (page_offset_delta = page_offset_delta MODULO buckets) == 0) page_offset_delta = 1;