971 lines
39 KiB
Plaintext
971 lines
39 KiB
Plaintext
This is bfd.info, produced by makeinfo version 4.1 from bfd.texinfo.
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START-INFO-DIR-ENTRY
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* Bfd: (bfd). The Binary File Descriptor library.
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END-INFO-DIR-ENTRY
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This file documents the BFD library.
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Copyright (C) 1991, 2000, 2001 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.1
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled "GNU Free Documentation License".
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File: bfd.info, Node: coff, Next: elf, Prev: aout, Up: BFD back ends
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coff backends
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=============
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BFD supports a number of different flavours of coff format. The
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major differences between formats are the sizes and alignments of
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fields in structures on disk, and the occasional extra field.
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Coff in all its varieties is implemented with a few common files and
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a number of implementation specific files. For example, The 88k bcs
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coff format is implemented in the file `coff-m88k.c'. This file
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`#include's `coff/m88k.h' which defines the external structure of the
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coff format for the 88k, and `coff/internal.h' which defines the
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internal structure. `coff-m88k.c' also defines the relocations used by
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the 88k format *Note Relocations::.
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The Intel i960 processor version of coff is implemented in
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`coff-i960.c'. This file has the same structure as `coff-m88k.c',
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except that it includes `coff/i960.h' rather than `coff-m88k.h'.
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Porting to a new version of coff
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--------------------------------
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The recommended method is to select from the existing
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implementations the version of coff which is most like the one you want
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to use. For example, we'll say that i386 coff is the one you select,
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and that your coff flavour is called foo. Copy `i386coff.c' to
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`foocoff.c', copy `../include/coff/i386.h' to `../include/coff/foo.h',
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and add the lines to `targets.c' and `Makefile.in' so that your new
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back end is used. Alter the shapes of the structures in
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`../include/coff/foo.h' so that they match what you need. You will
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probably also have to add `#ifdef's to the code in `coff/internal.h' and
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`coffcode.h' if your version of coff is too wild.
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You can verify that your new BFD backend works quite simply by
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building `objdump' from the `binutils' directory, and making sure that
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its version of what's going on and your host system's idea (assuming it
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has the pretty standard coff dump utility, usually called `att-dump' or
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just `dump') are the same. Then clean up your code, and send what
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you've done to Cygnus. Then your stuff will be in the next release, and
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you won't have to keep integrating it.
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How the coff backend works
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--------------------------
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File layout
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...........
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The Coff backend is split into generic routines that are applicable
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to any Coff target and routines that are specific to a particular
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target. The target-specific routines are further split into ones which
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are basically the same for all Coff targets except that they use the
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external symbol format or use different values for certain constants.
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The generic routines are in `coffgen.c'. These routines work for
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any Coff target. They use some hooks into the target specific code;
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the hooks are in a `bfd_coff_backend_data' structure, one of which
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exists for each target.
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The essentially similar target-specific routines are in
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`coffcode.h'. This header file includes executable C code. The
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various Coff targets first include the appropriate Coff header file,
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make any special defines that are needed, and then include `coffcode.h'.
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Some of the Coff targets then also have additional routines in the
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target source file itself.
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For example, `coff-i960.c' includes `coff/internal.h' and
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`coff/i960.h'. It then defines a few constants, such as `I960', and
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includes `coffcode.h'. Since the i960 has complex relocation types,
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`coff-i960.c' also includes some code to manipulate the i960 relocs.
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This code is not in `coffcode.h' because it would not be used by any
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other target.
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Bit twiddling
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.............
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Each flavour of coff supported in BFD has its own header file
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describing the external layout of the structures. There is also an
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internal description of the coff layout, in `coff/internal.h'. A major
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function of the coff backend is swapping the bytes and twiddling the
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bits to translate the external form of the structures into the normal
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internal form. This is all performed in the `bfd_swap'_thing_direction
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routines. Some elements are different sizes between different versions
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of coff; it is the duty of the coff version specific include file to
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override the definitions of various packing routines in `coffcode.h'.
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E.g., the size of line number entry in coff is sometimes 16 bits, and
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sometimes 32 bits. `#define'ing `PUT_LNSZ_LNNO' and `GET_LNSZ_LNNO'
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will select the correct one. No doubt, some day someone will find a
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version of coff which has a varying field size not catered to at the
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moment. To port BFD, that person will have to add more `#defines'.
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Three of the bit twiddling routines are exported to `gdb';
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`coff_swap_aux_in', `coff_swap_sym_in' and `coff_swap_lineno_in'. `GDB'
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reads the symbol table on its own, but uses BFD to fix things up. More
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of the bit twiddlers are exported for `gas'; `coff_swap_aux_out',
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`coff_swap_sym_out', `coff_swap_lineno_out', `coff_swap_reloc_out',
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`coff_swap_filehdr_out', `coff_swap_aouthdr_out',
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`coff_swap_scnhdr_out'. `Gas' currently keeps track of all the symbol
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table and reloc drudgery itself, thereby saving the internal BFD
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overhead, but uses BFD to swap things on the way out, making cross
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ports much safer. Doing so also allows BFD (and thus the linker) to
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use the same header files as `gas', which makes one avenue to disaster
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disappear.
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Symbol reading
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..............
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The simple canonical form for symbols used by BFD is not rich enough
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to keep all the information available in a coff symbol table. The back
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end gets around this problem by keeping the original symbol table
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around, "behind the scenes".
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When a symbol table is requested (through a call to
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`bfd_canonicalize_symtab'), a request gets through to
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`coff_get_normalized_symtab'. This reads the symbol table from the coff
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file and swaps all the structures inside into the internal form. It
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also fixes up all the pointers in the table (represented in the file by
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offsets from the first symbol in the table) into physical pointers to
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elements in the new internal table. This involves some work since the
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meanings of fields change depending upon context: a field that is a
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pointer to another structure in the symbol table at one moment may be
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the size in bytes of a structure at the next. Another pass is made
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over the table. All symbols which mark file names (`C_FILE' symbols)
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are modified so that the internal string points to the value in the
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auxent (the real filename) rather than the normal text associated with
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the symbol (`".file"').
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At this time the symbol names are moved around. Coff stores all
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symbols less than nine characters long physically within the symbol
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table; longer strings are kept at the end of the file in the string
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table. This pass moves all strings into memory and replaces them with
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pointers to the strings.
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The symbol table is massaged once again, this time to create the
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canonical table used by the BFD application. Each symbol is inspected
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in turn, and a decision made (using the `sclass' field) about the
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various flags to set in the `asymbol'. *Note Symbols::. The generated
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canonical table shares strings with the hidden internal symbol table.
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Any linenumbers are read from the coff file too, and attached to the
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symbols which own the functions the linenumbers belong to.
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Symbol writing
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..............
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Writing a symbol to a coff file which didn't come from a coff file
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will lose any debugging information. The `asymbol' structure remembers
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the BFD from which the symbol was taken, and on output the back end
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makes sure that the same destination target as source target is present.
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When the symbols have come from a coff file then all the debugging
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information is preserved.
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Symbol tables are provided for writing to the back end in a vector
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of pointers to pointers. This allows applications like the linker to
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accumulate and output large symbol tables without having to do too much
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byte copying.
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This function runs through the provided symbol table and patches
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each symbol marked as a file place holder (`C_FILE') to point to the
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next file place holder in the list. It also marks each `offset' field
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in the list with the offset from the first symbol of the current symbol.
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Another function of this procedure is to turn the canonical value
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form of BFD into the form used by coff. Internally, BFD expects symbol
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values to be offsets from a section base; so a symbol physically at
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0x120, but in a section starting at 0x100, would have the value 0x20.
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Coff expects symbols to contain their final value, so symbols have
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their values changed at this point to reflect their sum with their
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owning section. This transformation uses the `output_section' field of
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the `asymbol''s `asection' *Note Sections::.
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* `coff_mangle_symbols'
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This routine runs though the provided symbol table and uses the
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offsets generated by the previous pass and the pointers generated when
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the symbol table was read in to create the structured hierachy required
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by coff. It changes each pointer to a symbol into the index into the
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symbol table of the asymbol.
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* `coff_write_symbols'
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This routine runs through the symbol table and patches up the
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symbols from their internal form into the coff way, calls the bit
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twiddlers, and writes out the table to the file.
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`coff_symbol_type'
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..................
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*Description*
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The hidden information for an `asymbol' is described in a
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`combined_entry_type':
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typedef struct coff_ptr_struct
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{
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/* Remembers the offset from the first symbol in the file for
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this symbol. Generated by coff_renumber_symbols. */
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unsigned int offset;
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/* Should the value of this symbol be renumbered. Used for
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XCOFF C_BSTAT symbols. Set by coff_slurp_symbol_table. */
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unsigned int fix_value : 1;
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/* Should the tag field of this symbol be renumbered.
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Created by coff_pointerize_aux. */
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unsigned int fix_tag : 1;
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/* Should the endidx field of this symbol be renumbered.
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Created by coff_pointerize_aux. */
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unsigned int fix_end : 1;
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/* Should the x_csect.x_scnlen field be renumbered.
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Created by coff_pointerize_aux. */
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unsigned int fix_scnlen : 1;
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/* Fix up an XCOFF C_BINCL/C_EINCL symbol. The value is the
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index into the line number entries. Set by coff_slurp_symbol_table. */
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unsigned int fix_line : 1;
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/* The container for the symbol structure as read and translated
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from the file. */
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union
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{
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union internal_auxent auxent;
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struct internal_syment syment;
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} u;
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} combined_entry_type;
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/* Each canonical asymbol really looks like this: */
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typedef struct coff_symbol_struct
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{
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/* The actual symbol which the rest of BFD works with */
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asymbol symbol;
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/* A pointer to the hidden information for this symbol */
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combined_entry_type *native;
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/* A pointer to the linenumber information for this symbol */
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struct lineno_cache_entry *lineno;
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/* Have the line numbers been relocated yet ? */
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boolean done_lineno;
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} coff_symbol_type;
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`bfd_coff_backend_data'
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.......................
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/* COFF symbol classifications. */
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enum coff_symbol_classification
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{
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/* Global symbol. */
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COFF_SYMBOL_GLOBAL,
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/* Common symbol. */
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COFF_SYMBOL_COMMON,
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/* Undefined symbol. */
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COFF_SYMBOL_UNDEFINED,
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/* Local symbol. */
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COFF_SYMBOL_LOCAL,
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/* PE section symbol. */
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COFF_SYMBOL_PE_SECTION
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};
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Special entry points for gdb to swap in coff symbol table parts:
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typedef struct
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{
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void (*_bfd_coff_swap_aux_in)
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PARAMS ((bfd *, PTR, int, int, int, int, PTR));
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void (*_bfd_coff_swap_sym_in)
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PARAMS ((bfd *, PTR, PTR));
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void (*_bfd_coff_swap_lineno_in)
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PARAMS ((bfd *, PTR, PTR));
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unsigned int (*_bfd_coff_swap_aux_out)
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PARAMS ((bfd *, PTR, int, int, int, int, PTR));
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unsigned int (*_bfd_coff_swap_sym_out)
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PARAMS ((bfd *, PTR, PTR));
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unsigned int (*_bfd_coff_swap_lineno_out)
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PARAMS ((bfd *, PTR, PTR));
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unsigned int (*_bfd_coff_swap_reloc_out)
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PARAMS ((bfd *, PTR, PTR));
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unsigned int (*_bfd_coff_swap_filehdr_out)
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PARAMS ((bfd *, PTR, PTR));
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unsigned int (*_bfd_coff_swap_aouthdr_out)
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PARAMS ((bfd *, PTR, PTR));
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unsigned int (*_bfd_coff_swap_scnhdr_out)
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PARAMS ((bfd *, PTR, PTR));
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unsigned int _bfd_filhsz;
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unsigned int _bfd_aoutsz;
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unsigned int _bfd_scnhsz;
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unsigned int _bfd_symesz;
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unsigned int _bfd_auxesz;
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unsigned int _bfd_relsz;
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unsigned int _bfd_linesz;
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unsigned int _bfd_filnmlen;
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boolean _bfd_coff_long_filenames;
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boolean _bfd_coff_long_section_names;
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unsigned int _bfd_coff_default_section_alignment_power;
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boolean _bfd_coff_force_symnames_in_strings;
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unsigned int _bfd_coff_debug_string_prefix_length;
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void (*_bfd_coff_swap_filehdr_in)
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PARAMS ((bfd *, PTR, PTR));
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void (*_bfd_coff_swap_aouthdr_in)
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PARAMS ((bfd *, PTR, PTR));
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void (*_bfd_coff_swap_scnhdr_in)
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PARAMS ((bfd *, PTR, PTR));
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void (*_bfd_coff_swap_reloc_in)
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PARAMS ((bfd *abfd, PTR, PTR));
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boolean (*_bfd_coff_bad_format_hook)
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PARAMS ((bfd *, PTR));
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boolean (*_bfd_coff_set_arch_mach_hook)
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PARAMS ((bfd *, PTR));
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PTR (*_bfd_coff_mkobject_hook)
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PARAMS ((bfd *, PTR, PTR));
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boolean (*_bfd_styp_to_sec_flags_hook)
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PARAMS ((bfd *, PTR, const char *, asection *, flagword *));
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void (*_bfd_set_alignment_hook)
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PARAMS ((bfd *, asection *, PTR));
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boolean (*_bfd_coff_slurp_symbol_table)
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PARAMS ((bfd *));
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boolean (*_bfd_coff_symname_in_debug)
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PARAMS ((bfd *, struct internal_syment *));
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boolean (*_bfd_coff_pointerize_aux_hook)
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PARAMS ((bfd *, combined_entry_type *, combined_entry_type *,
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unsigned int, combined_entry_type *));
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boolean (*_bfd_coff_print_aux)
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PARAMS ((bfd *, FILE *, combined_entry_type *, combined_entry_type *,
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combined_entry_type *, unsigned int));
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void (*_bfd_coff_reloc16_extra_cases)
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PARAMS ((bfd *, struct bfd_link_info *, struct bfd_link_order *, arelent *,
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bfd_byte *, unsigned int *, unsigned int *));
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int (*_bfd_coff_reloc16_estimate)
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PARAMS ((bfd *, asection *, arelent *, unsigned int,
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struct bfd_link_info *));
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enum coff_symbol_classification (*_bfd_coff_classify_symbol)
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PARAMS ((bfd *, struct internal_syment *));
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boolean (*_bfd_coff_compute_section_file_positions)
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PARAMS ((bfd *));
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boolean (*_bfd_coff_start_final_link)
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PARAMS ((bfd *, struct bfd_link_info *));
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boolean (*_bfd_coff_relocate_section)
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PARAMS ((bfd *, struct bfd_link_info *, bfd *, asection *, bfd_byte *,
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struct internal_reloc *, struct internal_syment *, asection **));
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reloc_howto_type *(*_bfd_coff_rtype_to_howto)
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PARAMS ((bfd *, asection *, struct internal_reloc *,
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struct coff_link_hash_entry *, struct internal_syment *,
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bfd_vma *));
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boolean (*_bfd_coff_adjust_symndx)\
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PARAMS ((bfd *, struct bfd_link_info *, bfd *, asection *,
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struct internal_reloc *, boolean *));
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boolean (*_bfd_coff_link_add_one_symbol)
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PARAMS ((struct bfd_link_info *, bfd *, const char *, flagword,
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asection *, bfd_vma, const char *, boolean, boolean,
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struct bfd_link_hash_entry **));
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boolean (*_bfd_coff_link_output_has_begun)
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PARAMS ((bfd *, struct coff_final_link_info *));
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boolean (*_bfd_coff_final_link_postscript)
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PARAMS ((bfd *, struct coff_final_link_info *));
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} bfd_coff_backend_data;
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#define coff_backend_info(abfd) \
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((bfd_coff_backend_data *) (abfd)->xvec->backend_data)
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#define bfd_coff_swap_aux_in(a,e,t,c,ind,num,i) \
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((coff_backend_info (a)->_bfd_coff_swap_aux_in) (a,e,t,c,ind,num,i))
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#define bfd_coff_swap_sym_in(a,e,i) \
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((coff_backend_info (a)->_bfd_coff_swap_sym_in) (a,e,i))
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#define bfd_coff_swap_lineno_in(a,e,i) \
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((coff_backend_info ( a)->_bfd_coff_swap_lineno_in) (a,e,i))
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#define bfd_coff_swap_reloc_out(abfd, i, o) \
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((coff_backend_info (abfd)->_bfd_coff_swap_reloc_out) (abfd, i, o))
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#define bfd_coff_swap_lineno_out(abfd, i, o) \
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((coff_backend_info (abfd)->_bfd_coff_swap_lineno_out) (abfd, i, o))
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#define bfd_coff_swap_aux_out(a,i,t,c,ind,num,o) \
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((coff_backend_info (a)->_bfd_coff_swap_aux_out) (a,i,t,c,ind,num,o))
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#define bfd_coff_swap_sym_out(abfd, i,o) \
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((coff_backend_info (abfd)->_bfd_coff_swap_sym_out) (abfd, i, o))
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#define bfd_coff_swap_scnhdr_out(abfd, i,o) \
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((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_out) (abfd, i, o))
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#define bfd_coff_swap_filehdr_out(abfd, i,o) \
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((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_out) (abfd, i, o))
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#define bfd_coff_swap_aouthdr_out(abfd, i,o) \
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((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_out) (abfd, i, o))
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|
||
#define bfd_coff_filhsz(abfd) (coff_backend_info (abfd)->_bfd_filhsz)
|
||
#define bfd_coff_aoutsz(abfd) (coff_backend_info (abfd)->_bfd_aoutsz)
|
||
#define bfd_coff_scnhsz(abfd) (coff_backend_info (abfd)->_bfd_scnhsz)
|
||
#define bfd_coff_symesz(abfd) (coff_backend_info (abfd)->_bfd_symesz)
|
||
#define bfd_coff_auxesz(abfd) (coff_backend_info (abfd)->_bfd_auxesz)
|
||
#define bfd_coff_relsz(abfd) (coff_backend_info (abfd)->_bfd_relsz)
|
||
#define bfd_coff_linesz(abfd) (coff_backend_info (abfd)->_bfd_linesz)
|
||
#define bfd_coff_filnmlen(abfd) (coff_backend_info (abfd)->_bfd_filnmlen)
|
||
#define bfd_coff_long_filenames(abfd) \
|
||
(coff_backend_info (abfd)->_bfd_coff_long_filenames)
|
||
#define bfd_coff_long_section_names(abfd) \
|
||
(coff_backend_info (abfd)->_bfd_coff_long_section_names)
|
||
#define bfd_coff_default_section_alignment_power(abfd) \
|
||
(coff_backend_info (abfd)->_bfd_coff_default_section_alignment_power)
|
||
#define bfd_coff_swap_filehdr_in(abfd, i,o) \
|
||
((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_in) (abfd, i, o))
|
||
|
||
#define bfd_coff_swap_aouthdr_in(abfd, i,o) \
|
||
((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_in) (abfd, i, o))
|
||
|
||
#define bfd_coff_swap_scnhdr_in(abfd, i,o) \
|
||
((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_in) (abfd, i, o))
|
||
|
||
#define bfd_coff_swap_reloc_in(abfd, i, o) \
|
||
((coff_backend_info (abfd)->_bfd_coff_swap_reloc_in) (abfd, i, o))
|
||
|
||
#define bfd_coff_bad_format_hook(abfd, filehdr) \
|
||
((coff_backend_info (abfd)->_bfd_coff_bad_format_hook) (abfd, filehdr))
|
||
|
||
#define bfd_coff_set_arch_mach_hook(abfd, filehdr)\
|
||
((coff_backend_info (abfd)->_bfd_coff_set_arch_mach_hook) (abfd, filehdr))
|
||
#define bfd_coff_mkobject_hook(abfd, filehdr, aouthdr)\
|
||
((coff_backend_info (abfd)->_bfd_coff_mkobject_hook) (abfd, filehdr, aouthdr))
|
||
|
||
#define bfd_coff_styp_to_sec_flags_hook(abfd, scnhdr, name, section, flags_ptr)\
|
||
((coff_backend_info (abfd)->_bfd_styp_to_sec_flags_hook)\
|
||
(abfd, scnhdr, name, section, flags_ptr))
|
||
|
||
#define bfd_coff_set_alignment_hook(abfd, sec, scnhdr)\
|
||
((coff_backend_info (abfd)->_bfd_set_alignment_hook) (abfd, sec, scnhdr))
|
||
|
||
#define bfd_coff_slurp_symbol_table(abfd)\
|
||
((coff_backend_info (abfd)->_bfd_coff_slurp_symbol_table) (abfd))
|
||
|
||
#define bfd_coff_symname_in_debug(abfd, sym)\
|
||
((coff_backend_info (abfd)->_bfd_coff_symname_in_debug) (abfd, sym))
|
||
|
||
#define bfd_coff_force_symnames_in_strings(abfd)\
|
||
(coff_backend_info (abfd)->_bfd_coff_force_symnames_in_strings)
|
||
|
||
#define bfd_coff_debug_string_prefix_length(abfd)\
|
||
(coff_backend_info (abfd)->_bfd_coff_debug_string_prefix_length)
|
||
|
||
#define bfd_coff_print_aux(abfd, file, base, symbol, aux, indaux)\
|
||
((coff_backend_info (abfd)->_bfd_coff_print_aux)\
|
||
(abfd, file, base, symbol, aux, indaux))
|
||
|
||
#define bfd_coff_reloc16_extra_cases(abfd, link_info, link_order, reloc, data, src_ptr, dst_ptr)\
|
||
((coff_backend_info (abfd)->_bfd_coff_reloc16_extra_cases)\
|
||
(abfd, link_info, link_order, reloc, data, src_ptr, dst_ptr))
|
||
|
||
#define bfd_coff_reloc16_estimate(abfd, section, reloc, shrink, link_info)\
|
||
((coff_backend_info (abfd)->_bfd_coff_reloc16_estimate)\
|
||
(abfd, section, reloc, shrink, link_info))
|
||
|
||
#define bfd_coff_classify_symbol(abfd, sym)\
|
||
((coff_backend_info (abfd)->_bfd_coff_classify_symbol)\
|
||
(abfd, sym))
|
||
|
||
#define bfd_coff_compute_section_file_positions(abfd)\
|
||
((coff_backend_info (abfd)->_bfd_coff_compute_section_file_positions)\
|
||
(abfd))
|
||
|
||
#define bfd_coff_start_final_link(obfd, info)\
|
||
((coff_backend_info (obfd)->_bfd_coff_start_final_link)\
|
||
(obfd, info))
|
||
#define bfd_coff_relocate_section(obfd,info,ibfd,o,con,rel,isyms,secs)\
|
||
((coff_backend_info (ibfd)->_bfd_coff_relocate_section)\
|
||
(obfd, info, ibfd, o, con, rel, isyms, secs))
|
||
#define bfd_coff_rtype_to_howto(abfd, sec, rel, h, sym, addendp)\
|
||
((coff_backend_info (abfd)->_bfd_coff_rtype_to_howto)\
|
||
(abfd, sec, rel, h, sym, addendp))
|
||
#define bfd_coff_adjust_symndx(obfd, info, ibfd, sec, rel, adjustedp)\
|
||
((coff_backend_info (abfd)->_bfd_coff_adjust_symndx)\
|
||
(obfd, info, ibfd, sec, rel, adjustedp))
|
||
#define bfd_coff_link_add_one_symbol(info,abfd,name,flags,section,value,string,cp,coll,hashp)\
|
||
((coff_backend_info (abfd)->_bfd_coff_link_add_one_symbol)\
|
||
(info, abfd, name, flags, section, value, string, cp, coll, hashp))
|
||
|
||
#define bfd_coff_link_output_has_begun(a,p) \
|
||
((coff_backend_info (a)->_bfd_coff_link_output_has_begun) (a,p))
|
||
#define bfd_coff_final_link_postscript(a,p) \
|
||
((coff_backend_info (a)->_bfd_coff_final_link_postscript) (a,p))
|
||
|
||
Writing relocations
|
||
...................
|
||
|
||
To write relocations, the back end steps though the canonical
|
||
relocation table and create an `internal_reloc'. The symbol index to
|
||
use is removed from the `offset' field in the symbol table supplied.
|
||
The address comes directly from the sum of the section base address and
|
||
the relocation offset; the type is dug directly from the howto field.
|
||
Then the `internal_reloc' is swapped into the shape of an
|
||
`external_reloc' and written out to disk.
|
||
|
||
Reading linenumbers
|
||
...................
|
||
|
||
Creating the linenumber table is done by reading in the entire coff
|
||
linenumber table, and creating another table for internal use.
|
||
|
||
A coff linenumber table is structured so that each function is
|
||
marked as having a line number of 0. Each line within the function is
|
||
an offset from the first line in the function. The base of the line
|
||
number information for the table is stored in the symbol associated
|
||
with the function.
|
||
|
||
Note: The PE format uses line number 0 for a flag indicating a new
|
||
source file.
|
||
|
||
The information is copied from the external to the internal table,
|
||
and each symbol which marks a function is marked by pointing its...
|
||
|
||
How does this work ?
|
||
|
||
Reading relocations
|
||
...................
|
||
|
||
Coff relocations are easily transformed into the internal BFD form
|
||
(`arelent').
|
||
|
||
Reading a coff relocation table is done in the following stages:
|
||
|
||
* Read the entire coff relocation table into memory.
|
||
|
||
* Process each relocation in turn; first swap it from the external
|
||
to the internal form.
|
||
|
||
* Turn the symbol referenced in the relocation's symbol index into a
|
||
pointer into the canonical symbol table. This table is the same
|
||
as the one returned by a call to `bfd_canonicalize_symtab'. The
|
||
back end will call that routine and save the result if a
|
||
canonicalization hasn't been done.
|
||
|
||
* The reloc index is turned into a pointer to a howto structure, in
|
||
a back end specific way. For instance, the 386 and 960 use the
|
||
`r_type' to directly produce an index into a howto table vector;
|
||
the 88k subtracts a number from the `r_type' field and creates an
|
||
addend field.
|
||
|
||
|
||
File: bfd.info, Node: elf, Next: mmo, Prev: coff, Up: BFD back ends
|
||
|
||
|
||
|
||
ELF backends
|
||
|
||
BFD support for ELF formats is being worked on. Currently, the best
|
||
supported back ends are for sparc and i386 (running svr4 or Solaris 2).
|
||
|
||
Documentation of the internals of the support code still needs to be
|
||
written. The code is changing quickly enough that we haven't bothered
|
||
yet.
|
||
|
||
`bfd_elf_find_section'
|
||
......................
|
||
|
||
*Synopsis*
|
||
struct elf_internal_shdr *bfd_elf_find_section (bfd *abfd, char *name);
|
||
*Description*
|
||
Helper functions for GDB to locate the string tables. Since BFD hides
|
||
string tables from callers, GDB needs to use an internal hook to find
|
||
them. Sun's .stabstr, in particular, isn't even pointed to by the
|
||
.stab section, so ordinary mechanisms wouldn't work to find it, even if
|
||
we had some.
|
||
|
||
|
||
File: bfd.info, Node: mmo, Prev: elf, Up: BFD back ends
|
||
|
||
mmo backend
|
||
===========
|
||
|
||
The mmo object format is used exclusively together with Professor
|
||
Donald E. Knuth's educational 64-bit processor MMIX. The simulator
|
||
`mmix' which is available at
|
||
<http://www-cs-faculty.stanford.edu/~knuth/programs/mmix.tar.gz>
|
||
understands this format. That package also includes a combined
|
||
assembler and linker called `mmixal'. The mmo format has no advantages
|
||
feature-wise compared to e.g. ELF. It is a simple non-relocatable
|
||
object format with no support for archives or debugging information,
|
||
except for symbol value information and line numbers (which is not yet
|
||
implemented in BFD). See
|
||
<http://www-cs-faculty.stanford.edu/~knuth/mmix.html> for more
|
||
information about MMIX. The ELF format is used for intermediate object
|
||
files in the BFD implementation.
|
||
|
||
* Menu:
|
||
|
||
* File layout::
|
||
* Symbol-table::
|
||
* mmo section mapping::
|
||
|
||
|
||
File: bfd.info, Node: File layout, Next: Symbol-table, Prev: mmo, Up: mmo
|
||
|
||
File layout
|
||
-----------
|
||
|
||
The mmo file contents is not partitioned into named sections as with
|
||
e.g. ELF. Memory areas is formed by specifying the location of the
|
||
data that follows. Only the memory area `0x0000...00' to `0x01ff...ff'
|
||
is executable, so it is used for code (and constants) and the area
|
||
`0x2000...00' to `0x20ff...ff' is used for writable data. *Note mmo
|
||
section mapping::.
|
||
|
||
Contents is entered as 32-bit words, xor:ed over previous contents,
|
||
always zero-initialized. A word that starts with the byte `0x98' forms
|
||
a command called a `lopcode', where the next byte distinguished between
|
||
the thirteen lopcodes. The two remaining bytes, called the `Y' and `Z'
|
||
fields, or the `YZ' field (a 16-bit big-endian number), are used for
|
||
various purposes different for each lopcode. As documented in
|
||
<http://www-cs-faculty.stanford.edu/~knuth/mmixal-intro.ps.gz>, the
|
||
lopcodes are:
|
||
|
||
There is provision for specifying "special data" of 65536 different
|
||
types. We use type 80 (decimal), arbitrarily chosen the same as the
|
||
ELF `e_machine' number for MMIX, filling it with section information
|
||
normally found in ELF objects. *Note mmo section mapping::.
|
||
|
||
`lop_quote'
|
||
0x98000001. The next word is contents, regardless of whether it
|
||
starts with 0x98 or not.
|
||
|
||
`lop_loc'
|
||
0x9801YYZZ, where `Z' is 1 or 2. This is a location directive,
|
||
setting the location for the next data to the next 32-bit word
|
||
(for Z = 1) or 64-bit word (for Z = 2), plus Y * 2^56. Normally
|
||
`Y' is 0 for the text segment and 2 for the data segment.
|
||
|
||
`lop_skip'
|
||
0x9802YYZZ. Increase the current location by `YZ' bytes.
|
||
|
||
`lop_fixo'
|
||
0x9803YYZZ, where `Z' is 1 or 2. Store the current location as 64
|
||
bits into the location pointed to by the next 32-bit (Z = 1) or
|
||
64-bit (Z = 2) word, plus Y * 2^56.
|
||
|
||
`lop_fixr'
|
||
0x9804YYZZ. `YZ' is stored into the current location plus 2 - 4 *
|
||
YZ.
|
||
|
||
`lop_fixrx'
|
||
0x980500ZZ. `Z' is 16 or 24. A value `L' derived from the
|
||
following 32-bit word are used in a manner similar to `YZ' in
|
||
lop_fixr: it is xor:ed into the current location minus 4 * L. The
|
||
first byte of the word is 0 or 1. If it is 1, then L = (LOWEST 24
|
||
BITS OF WORD) - 2^Z, if 0, then L = (LOWEST 24 BITS OF WORD).
|
||
|
||
`lop_file'
|
||
0x9806YYZZ. `Y' is the file number, `Z' is count of 32-bit words.
|
||
Set the file number to `Y' and the line counter to 0. The next Z
|
||
* 4 bytes contain the file name, padded with zeros if the count is
|
||
not a multiple of four. The same `Y' may occur multiple times,
|
||
but `Z' must be 0 for all but the first occurrence.
|
||
|
||
`lop_line'
|
||
0x9807YYZZ. `YZ' is the line number. Together with lop_file, it
|
||
forms the source location for the next 32-bit word. Note that for
|
||
each non-lopcode 32-bit word, line numbers are assumed incremented
|
||
by one.
|
||
|
||
`lop_spec'
|
||
0x9808YYZZ. `YZ' is the type number. Data until the next lopcode
|
||
other than lop_quote forms special data of type `YZ'. *Note mmo
|
||
section mapping::.
|
||
|
||
Other types than 80, (or type 80 with a content that does not
|
||
parse) is stored in sections named `.MMIX.spec_data.N' where N is
|
||
the `YZ'-type. The flags for such a sections say not to allocate
|
||
or load the data. The vma is 0. Contents of multiple occurrences
|
||
of special data N is concatenated to the data of the previous
|
||
lop_spec Ns. The location in data or code at which the lop_spec
|
||
occurred is lost.
|
||
|
||
`lop_pre'
|
||
0x980901ZZ. The first lopcode in a file. The `Z' field forms the
|
||
length of header information in 32-bit words, where the first word
|
||
tells the time in seconds since `00:00:00 GMT Jan 1 1970'.
|
||
|
||
`lop_post'
|
||
0x980a00ZZ. Z > 32. This lopcode follows after all
|
||
content-generating lopcodes in a program. The `Z' field denotes
|
||
the value of `rG' at the beginning of the program. The following
|
||
256 - Z big-endian 64-bit words are loaded into global registers
|
||
`$G' ... `$255'.
|
||
|
||
`lop_stab'
|
||
0x980b0000. The next-to-last lopcode in a program. Must follow
|
||
immediately after the lop_post lopcode and its data. After this
|
||
lopcode follows all symbols in a compressed format (*note
|
||
Symbol-table::).
|
||
|
||
`lop_end'
|
||
0x980cYYZZ. The last lopcode in a program. It must follow the
|
||
lop_stab lopcode and its data. The `YZ' field contains the number
|
||
of 32-bit words of symbol table information after the preceding
|
||
lop_stab lopcode.
|
||
|
||
Note that the lopcode "fixups"; `lop_fixr', `lop_fixrx' and
|
||
`lop_fixo' are not generated by BFD, but are handled. They are
|
||
generated by `mmixal'.
|
||
|
||
This trivial one-label, one-instruction file:
|
||
|
||
:Main TRAP 1,2,3
|
||
|
||
can be represented this way in mmo:
|
||
|
||
0x98090101 - lop_pre, one 32-bit word with timestamp.
|
||
<timestamp>
|
||
0x98010002 - lop_loc, text segment, using a 64-bit address.
|
||
Note that mmixal does not emit this for the file above.
|
||
0x00000000 - Address, high 32 bits.
|
||
0x00000000 - Address, low 32 bits.
|
||
0x98060002 - lop_file, 2 32-bit words for file-name.
|
||
0x74657374 - "test"
|
||
0x2e730000 - ".s\0\0"
|
||
0x98070001 - lop_line, line 1.
|
||
0x00010203 - TRAP 1,2,3
|
||
0x980a00ff - lop_post, setting $255 to 0.
|
||
0x00000000
|
||
0x00000000
|
||
0x980b0000 - lop_stab for ":Main" = 0, serial 1.
|
||
0x203a4040 *Note Symbol-table::.
|
||
0x10404020
|
||
0x4d206120
|
||
0x69016e00
|
||
0x81000000
|
||
0x980c0005 - lop_end; symbol table contained five 32-bit words.
|
||
|
||
|
||
File: bfd.info, Node: Symbol-table, Next: mmo section mapping, Prev: File layout, Up: mmo
|
||
|
||
Symbol table format
|
||
-------------------
|
||
|
||
From mmixal.w (or really, the generated mmixal.tex) in
|
||
<http://www-cs-faculty.stanford.edu/~knuth/programs/mmix.tar.gz>):
|
||
"Symbols are stored and retrieved by means of a `ternary search trie',
|
||
following ideas of Bentley and Sedgewick. (See ACM-SIAM Symp. on
|
||
Discrete Algorithms `8' (1997), 360-369; R.Sedgewick, `Algorithms in C'
|
||
(Reading, Mass. Addison-Wesley, 1998), `15.4'.) Each trie node stores
|
||
a character, and there are branches to subtries for the cases where a
|
||
given character is less than, equal to, or greater than the character
|
||
in the trie. There also is a pointer to a symbol table entry if a
|
||
symbol ends at the current node."
|
||
|
||
So it's a tree encoded as a stream of bytes. The stream of bytes
|
||
acts on a single virtual global symbol, adding and removing characters
|
||
and signalling complete symbol points. Here, we read the stream and
|
||
create symbols at the completion points.
|
||
|
||
First, there's a control byte `m'. If any of the listed bits in `m'
|
||
is nonzero, we execute what stands at the right, in the listed order:
|
||
|
||
(MMO3_LEFT)
|
||
0x40 - Traverse left trie.
|
||
(Read a new command byte and recurse.)
|
||
|
||
(MMO3_SYMBITS)
|
||
0x2f - Read the next byte as a character and store it in the
|
||
current character position; increment character position.
|
||
Test the bits of `m':
|
||
|
||
(MMO3_WCHAR)
|
||
0x80 - The character is 16-bit (so read another byte,
|
||
merge into current character.
|
||
|
||
(MMO3_TYPEBITS)
|
||
0xf - We have a complete symbol; parse the type, value
|
||
and serial number and do what should be done
|
||
with a symbol. The type and length information
|
||
is in j = (m & 0xf).
|
||
|
||
(MMO3_REGQUAL_BITS)
|
||
j == 0xf: A register variable. The following
|
||
byte tells which register.
|
||
j <= 8: An absolute symbol. Read j bytes as the
|
||
big-endian number the symbol equals.
|
||
A j = 2 with two zero bytes denotes an
|
||
unknown symbol.
|
||
j > 8: As with j <= 8, but add (0x20 << 56)
|
||
to the value in the following j - 8
|
||
bytes.
|
||
|
||
Then comes the serial number, as a variant of
|
||
uleb128, but better named ubeb128:
|
||
Read bytes and shift the previous value left 7
|
||
(multiply by 128). Add in the new byte, repeat
|
||
until a byte has bit 7 set. The serial number
|
||
is the computed value minus 128.
|
||
|
||
(MMO3_MIDDLE)
|
||
0x20 - Traverse middle trie. (Read a new command byte
|
||
and recurse.) Decrement character position.
|
||
|
||
(MMO3_RIGHT)
|
||
0x10 - Traverse right trie. (Read a new command byte and
|
||
recurse.)
|
||
|
||
Let's look again at the `lop_stab' for the trivial file (*note File
|
||
layout::).
|
||
|
||
0x980b0000 - lop_stab for ":Main" = 0, serial 1.
|
||
0x203a4040
|
||
0x10404020
|
||
0x4d206120
|
||
0x69016e00
|
||
0x81000000
|
||
|
||
This forms the trivial trie (note that the path between ":" and "M"
|
||
is redundant):
|
||
|
||
203a ":"
|
||
40 /
|
||
40 /
|
||
10 \
|
||
40 /
|
||
40 /
|
||
204d "M"
|
||
2061 "a"
|
||
2069 "i"
|
||
016e "n" is the last character in a full symbol, and
|
||
with a value represented in one byte.
|
||
00 The value is 0.
|
||
81 The serial number is 1.
|
||
|
||
|
||
File: bfd.info, Node: mmo section mapping, Prev: Symbol-table, Up: mmo
|
||
|
||
mmo section mapping
|
||
-------------------
|
||
|
||
The implementation in BFD uses special data type 80 (decimal) to
|
||
encapsulate and describe named sections, containing e.g. debug
|
||
information. If needed, any datum in the encapsulation will be quoted
|
||
using lop_quote. First comes a 32-bit word holding the number of
|
||
32-bit words containing the zero-terminated zero-padded segment name.
|
||
After the name there's a 32-bit word holding flags describing the
|
||
section type. Then comes a 64-bit big-endian word with the section
|
||
length (in bytes), then another with the section start address.
|
||
Depending on the type of section, the contents might follow,
|
||
zero-padded to 32-bit boundary. For a loadable section (such as data
|
||
or code), the contents might follow at some later point, not
|
||
necessarily immediately, as a lop_loc with the same start address as in
|
||
the section description, followed by the contents. This in effect
|
||
forms a descriptor that must be emitted before the actual contents.
|
||
Sections described this way must not overlap.
|
||
|
||
For areas that don't have such descriptors, synthetic sections are
|
||
formed by BFD. Consecutive contents in the two memory areas
|
||
`0x0000...00' to `0x01ff...ff' and `0x2000...00' to `0x20ff...ff' are
|
||
entered in sections named `.text' and `.data' respectively. If an area
|
||
is not otherwise described, but would together with a neighboring lower
|
||
area be less than `0x40000000' bytes long, it is joined with the lower
|
||
area and the gap is zero-filled. For other cases, a new section is
|
||
formed, named `.MMIX.sec.N'. Here, N is a number, a running count
|
||
through the mmo file, starting at 0.
|
||
|
||
A loadable section specified as:
|
||
|
||
.section secname,"ax"
|
||
TETRA 1,2,3,4,-1,-2009
|
||
BYTE 80
|
||
|
||
and linked to address `0x4', is represented by the sequence:
|
||
|
||
0x98080050 - lop_spec 80
|
||
0x00000002 - two 32-bit words for the section name
|
||
0x7365636e - "secn"
|
||
0x616d6500 - "ame\0"
|
||
0x00000033 - flags CODE, READONLY, LOAD, ALLOC
|
||
0x00000000 - high 32 bits of section length
|
||
0x0000001c - section length is 28 bytes; 6 * 4 + 1 + alignment to 32 bits
|
||
0x00000000 - high 32 bits of section address
|
||
0x00000004 - section address is 4
|
||
0x98010002 - 64 bits with address of following data
|
||
0x00000000 - high 32 bits of address
|
||
0x00000004 - low 32 bits: data starts at address 4
|
||
0x00000001 - 1
|
||
0x00000002 - 2
|
||
0x00000003 - 3
|
||
0x00000004 - 4
|
||
0xffffffff - -1
|
||
0xfffff827 - -2009
|
||
0x50000000 - 80 as a byte, padded with zeros.
|
||
|
||
Note that the lop_spec wrapping does not include the section
|
||
contents. Compare this to a non-loaded section specified as:
|
||
|
||
.section thirdsec
|
||
TETRA 200001,100002
|
||
BYTE 38,40
|
||
|
||
This, when linked to address `0x200000000000001c', is represented by:
|
||
|
||
0x98080050 - lop_spec 80
|
||
0x00000002 - two 32-bit words for the section name
|
||
0x7365636e - "thir"
|
||
0x616d6500 - "dsec"
|
||
0x00000010 - flag READONLY
|
||
0x00000000 - high 32 bits of section length
|
||
0x0000000c - section length is 12 bytes; 2 * 4 + 2 + alignment to 32 bits
|
||
0x20000000 - high 32 bits of address
|
||
0x0000001c - low 32 bits of address 0x200000000000001c
|
||
0x00030d41 - 200001
|
||
0x000186a2 - 100002
|
||
0x26280000 - 38, 40 as bytes, padded with zeros
|
||
|
||
For the latter example, the section contents must not be loaded in
|
||
memory, and is therefore specified as part of the special data. The
|
||
address is usually unimportant but might provide information for e.g.
|
||
the DWARF 2 debugging format.
|
||
|