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Info file bfd.info, produced by Makeinfo, -*- Text -*- from input file bfd.texinfo. This file documents the BFD library. Copyright (C) 1991 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, subject to the terms of the GNU General Public License, which includes the provision that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions. File: bfd.info, Node: Top, Next: Overview, Prev: (dir), Up: (dir) This file documents the binary file descriptor library libbfd. * Menu: * Overview:: Overview of BFD * BFD front end:: BFD front end * BFD back end:: BFD back end * Index:: Index File: bfd.info, Node: Overview, Next: BFD front end, Prev: Top, Up: Top Introduction ************ Simply put, BFD is a package which allows applications to use the same routines to operate on object files whatever the object file format. A different object file format can be supported simply by creating a new BFD back end and adding it to the library. BFD is split into two parts; the front end and the many back ends. * memory, and various canonical data structures. The front end also decides which back end to use, and when to call back end routines. * end provides a set of calls which the BFD front end can use to maintain its canonical form. The back ends also may keep around information for their own use, for greater efficiency. * Menu: * History:: History * How It Works:: How It Works * What BFD Version 1 Can Do:: What BFD Version 1 Can Do File: bfd.info, Node: History, Next: How It Works, Prev: Overview, Up: Overview History ======= One spur behind BFD was the desire, on the part of the GNU 960 team at Intel Oregon, for interoperability of applications on their COFF and b.out file formats. Cygnus was providing GNU support for the team, and Cygnus was contracted to provide the required functionality. The name came from a conversation David Wallace was having with Richard Stallman about the library: RMS said that it would be quite hard--David said "BFD". Stallman was right, but the name stuck. At the same time, Ready Systems wanted much the same thing, but for different object file formats: IEEE-695, Oasys, Srecords, a.out and 68k coff. BFD was first implemented by Steve Chamberlain (steve@cygnus.com), John Gilmore (gnu@cygnus.com), K. Richard Pixley (rich@cygnus.com) and David Wallace (gumby@cygnus.com) at Cygnus Support in Palo Alto, California. File: bfd.info, Node: How It Works, Next: What BFD Version 1 Can Do, Prev: History, Up: Overview How It Works ============ To use the library, include `bfd.h' and link with `libbfd.a'. BFD provides a common interface to the parts of an object file for a calling application. When an application sucessfully opens a target file (object, archive or whatever) a pointer to an internal structure is returned. This pointer points to a structure called `bfd', described in `include/bfd.h'. Our convention is to call this pointer a BFD, and instances of it within code `abfd'. All operations on the target object file are applied as methods to the BFD. The mapping is defined within `bfd.h' in a set of macros, all beginning `bfd'_. For example, this sequence would do what you would probably expect: return the number of sections in an object file attached to a BFD `abfd'. #include "bfd.h" unsigned int number_of_sections(abfd) bfd *abfd; { return bfd_count_sections(abfd); } The abstraction used within BFD is that an object file has a header, a number of sections containing raw data, a set of relocations, and some symbol information. Also, BFDs opened for archives have the additional attribute of an index and contain subordinate BFDs. This approach is fine for a.out and coff, but loses efficiency when applied to formats such as S-records and IEEE-695. File: bfd.info, Node: What BFD Version 1 Can Do, Prev: How It Works, Up: Overview What BFD Version 1 Can Do ========================= As different information from the the object files is required, BFD reads from different sections of the file and processes them. For example a very common operation for the linker is processing symbol tables. Each BFD back end provides a routine for converting between the object file's representation of symbols and an internal canonical format. When the linker asks for the symbol table of an object file, it calls through the memory pointer to the relevant BFD back end routine which reads and converts the table into a canonical form. The linker then operates upon the canonical form. When the link is finished and the linker writes the output file's symbol table, another BFD back end routine is called which takes the newly created symbol table and converts it into the chosen output format. * Menu: * BFD information loss:: Information Loss * Mechanism:: Mechanism File: bfd.info, Node: BFD information loss, Next: Mechanism, Prev: What BFD Version 1 Can Do, Up: What BFD Version 1 Can Do Information Loss ---------------- *Some information is lost due to the nature of the file format.* The output targets supported by BFD do not provide identical facilities, and information which may be described in one form has nowhere to go in another format. One example of this is alignment information in `b.out'. There is nowhere in an `a.out' format file to store alignment information on the contained data, so when a file is linked from `b.out' and an `a.out' image is produced, alignment information will not propagate to the output file. (The linker will still use the alignment information internally, so the link is performed correctly). Another example is COFF section names. COFF files may contain an unlimited number of sections, each one with a textual section name. If the target of the link is a format which does not have many sections (eg `a.out') or has sections without names (eg the Oasys format) the link cannot be done simply. You can circumvent this problem by describing the desired input-to-output section mapping with the linker command language. *Information can be lost during canonicalization.* The BFD internal canonical form of the external formats is not exhaustive; there are structures in input formats for which there is no direct representation internally. This means that the BFD back ends cannot maintain all possible data richness through the transformation between external to internal and back to external formats. This limitation is only a problem when an application reads one format and writes another. Each BFD back end is responsible for maintaining as much data as possible, and the internal BFD canonical form has structures which are opaque to the BFD core, and exported only to the back ends. When a file is read in one format, the canonical form is generated for BFD and the application. At the same time, the back end saves away any information which may otherwise be lost. If the data is then written back in the same format, the back end routine will be able to use the canonical form provided by the BFD core as well as the information it prepared earlier. Since there is a great deal of commonality between back ends, this mechanism is very useful. There is no information lost for this reason when linking or copying big endian COFF to little endian COFF, or `a.out' to `b.out'. When a mixture of formats is linked, the information is only lost from the files whose format differs from the destination. File: bfd.info, Node: Mechanism, Prev: BFD information loss, Up: What BFD Version 1 Can Do Mechanism --------- The greatest potential for loss of information is when there is least overlap between the information provided by the source format, that stored by the canonical format, and the information needed by the destination format. A brief description of the canonical form may help you appreciate what kinds of data you can count on preserving across conversions. *files* Information on target machine architecture, particular implementation and format type are stored on a per-file basis. Other information includes a demand pageable bit and a write protected bit. Note that information like Unix magic numbers is not stored here--only the magic numbers' meaning, so a `ZMAGIC' file would have both the demand pageable bit and the write protected text bit set. The byte order of the target is stored on a per-file basis, so that big- and little-endian object files may be linked with one another. *sections* Each section in the input file contains the name of the section, the original address in the object file, various flags, size and alignment information and pointers into other BFD data structures. *symbols* Each symbol contains a pointer to the object file which originally defined it, its name, its value, and various flag bits. When a BFD back end reads in a symbol table, the back end relocates all symbols to make them relative to the base of the section where they were defined. This ensures that each symbol points to its containing section. Each symbol also has a varying amount of hidden data to contain private data for the BFD back end. Since the symbol points to the original file, the private data format for that symbol is accessible. `gld' can operate on a collection of symbols of wildly different formats without problems. Normal global and simple local symbols are maintained on output, so an output file (no matter its format) will retain symbols pointing to functions and to global, static, and common variables. Some symbol information is not worth retaining; in `a.out' type information is stored in the symbol table as long symbol names. This information would be useless to most COFF debuggers; the linker has command line switches to allow users to throw it away. There is one word of type information within the symbol, so if the format supports symbol type information within symbols (for example COFF, IEEE, Oasys) and the type is simple enough to fit within one word (nearly everything but aggregates) the information will be preserved. *relocation level* Each canonical BFD relocation record contains a pointer to the symbol to relocate to, the offset of the data to relocate, the section the data is in and a pointer to a relocation type descriptor. Relocation is performed effectively by message passing through the relocation type descriptor and symbol pointer. It allows relocations to be performed on output data using a relocation method only available in one of the input formats. For instance, Oasys provides a byte relocation format. A relocation record requesting this relocation type would point indirectly to a routine to perform this, so the relocation may be performed on a byte being written to a COFF file, even though 68k COFF has no such relocation type. *line numbers* Object formats can contain, for debugging purposes, some form of mapping between symbols, source line numbers, and addresses in the output file. These addresses have to be relocated along with the symbol information. Each symbol with an associated list of line number records points to the first record of the list. The head of a line number list consists of a pointer to the symbol, which allows divination of the address of the function whose line number is being described. The rest of the list is made up of pairs: offsets into the section and line numbers. Any format which can simply derive this information can pass it successfully between formats (COFF, IEEE and Oasys). File: bfd.info, Node: BFD front end, Next: BFD back end, Prev: Overview, Up: Top BFD front end ************* `typedef bfd' ============= A BFD is has type `bfd'; objects of this type are the cornerstone of any application using `libbfd'. References though the BFD and to data in the BFD give the entire BFD functionality. Here is the struct used to define the type `bfd'. This contains the major data about the file, and contains pointers to the rest of the data. struct _bfd { The filename the application opened the BFD with. CONST char *filename; A pointer to the target jump table. struct bfd_target *xvec; To avoid dragging too many header files into every file that includes `bfd.h', IOSTREAM has been declared as a "char *", and MTIME as a "long". Their correct types, to which they are cast when used, are "FILE *" and "time_t". The iostream is the result of an fopen on the filename. char *iostream; Is the file being cached *Note File Caching::. boolean cacheable; Marks whether there was a default target specified when the BFD was opened. This is used to select what matching algorithm to use to chose the back end. boolean target_defaulted; The caching routines use these to maintain a least-recently-used list of BFDs (*note File Caching::.). struct _bfd *lru_prev, *lru_next; When a file is closed by the caching routines, BFD retains state information on the file here: file_ptr where; and here: boolean opened_once; boolean mtime_set; File modified time long mtime; Reserved for an unimplemented file locking extension. int ifd; The format which belongs to the BFD. bfd_format format; The direction the BFD was opened with enum bfd_direction {no_direction = 0, read_direction = 1, write_direction = 2, both_direction = 3} direction; Format_specific flags flagword flags; Currently my_archive is tested before adding origin to anything. I believe that this can become always an add of origin, with origin set to 0 for non archive files. file_ptr origin; Remember when output has begun, to stop strange things happening. boolean output_has_begun; Pointer to linked list of sections struct sec *sections; The number of sections unsigned int section_count; Stuff only useful for object files: The start address. bfd_vma start_address; Used for input and output unsigned int symcount; Symbol table for output BFD struct symbol_cache_entry **outsymbols; Architecture of object machine, eg m68k enum bfd_architecture obj_arch; Particular machine within arch, e.g. 68010 unsigned long obj_machine; Stuff only useful for archives: PTR arelt_data; struct _bfd *my_archive; struct _bfd *next; struct _bfd *archive_head; boolean has_armap; Used by the back end to hold private data. PTR tdata; Used by the application to hold private data PTR usrdata; Where all the allocated stuff under this BFD goes (*note Memory Usage::.). struct obstack memory; }; `bfd_set_start_address' ....................... Marks the entry point of an output BFD. Returns `true' on success, `false' otherwise. boolean bfd_set_start_address(bfd *, bfd_vma); `bfd_get_mtime' ............... Return cached file modification time (e.g. as read from archive header for archive members, or from file system if we have been called before); else determine modify time, cache it, and return it. long bfd_get_mtime(bfd *); `stuff' ....... #define bfd_sizeof_headers(abfd, reloc) \ BFD_SEND (abfd, _bfd_sizeof_headers, (abfd, reloc)) #define bfd_find_nearest_line(abfd, section, symbols, offset, filename_ptr, func, line_ptr) \ BFD_SEND (abfd, _bfd_find_nearest_line, (abfd, section, symbols, offset, filename_ptr, func, line_ptr)) #define bfd_debug_info_start(abfd) \ BFD_SEND (abfd, _bfd_debug_info_start, (abfd)) #define bfd_debug_info_end(abfd) \ BFD_SEND (abfd, _bfd_debug_info_end, (abfd)) #define bfd_debug_info_accumulate(abfd, section) \ BFD_SEND (abfd, _bfd_debug_info_accumulate, (abfd, section)) #define bfd_stat_arch_elt(abfd, stat) \ BFD_SEND (abfd, _bfd_stat_arch_elt,(abfd, stat)) #define bfd_coff_swap_aux_in(a,e,t,c,i) \ BFD_SEND (a, _bfd_coff_swap_aux_in, (a,e,t,c,i)) #define bfd_coff_swap_sym_in(a,e,i) \ BFD_SEND (a, _bfd_coff_swap_sym_in, (a,e,i)) #define bfd_coff_swap_lineno_in(a,e,i) \ BFD_SEND ( a, _bfd_coff_swap_lineno_in, (a,e,i)) * Menu: * Memory Usage:: Memory Usage File: bfd.info, Node: Memory Usage, Prev: BFD front end, Up: BFD front end Memory Usage ============ BFD keeps all its internal structures in obstacks. There is one obstack per open BFD file, into which the current state is stored. When a BFD is closed, the obstack is deleted, and so everything which has been allocated by libbfd for the closing file will be thrown away. BFD will not free anything created by an application, but pointers into `bfd' structures will be invalidated on a `bfd_close'; for example, after a `bfd_close' the vector passed to `bfd_canonicalize_symtab' will still be around, since it has been allocated by the application, but the data that it pointed to will be lost. The general rule is not to close a BFD until all operations dependent upon data from the BFD have been completed, or all the data from within the file has been copied. To help with the management of memory, there is a function (`bfd_alloc_size') which returns the number of bytes in obstacks associated with the supplied BFD. This could be used to select the greediest open BFD, close it to reclaim the memory, perform some operation and reopen the BFD again, to get a fresh copy of the data structures. File: bfd.info, Node: Sections, Next: BFD back end, Prev: BFD front end, Up: Top Sections ======== Sections are supported in BFD in `section.c'. The raw data contained within a BFD is maintained through the section abstraction. A single BFD may have any number of sections, and keeps hold of them by pointing to the first, each one points to the next in the list. * Menu: * Section Input:: * Section Output:: * typedef asection:: * section prototypes:: File: bfd.info, Node: Section Input, Next: Section Output, Up: Sections Section Input ------------- When a BFD is opened for reading, the section structures are created and attached to the BFD. Each section has a name which describes the section in the outside world - for example, `a.out' would contain at least three sections, called `.text', `.data' and `.bss'. Sometimes a BFD will contain more than the 'natural' number of sections. A back end may attach other sections containing constructor data, or an application may add a section (using bfd_make_section) to the sections attached to an already open BFD. For example, the linker creates a supernumary section `COMMON' for each input file's BFD to hold information about common storage. The raw data is not necessarily read in at the same time as the section descriptor is created. Some targets may leave the data in place until a `bfd_get_section_contents' call is made. Other back ends may read in all the data at once - For example; an S-record file has to be read once to determine the size of the data. An IEEE-695 file doesn't contain raw data in sections, but data and relocation expressions intermixed, so the data area has to be parsed to get out the data and relocations. File: bfd.info, Node: Section Output, Next: typedef asection, Prev: Section Input, Up: Sections Section Output -------------- To write a new object style BFD, the various sections to be written have to be created. They are attached to the BFD in the same way as input sections, data is written to the sections using `bfd_set_section_contents'. The linker uses the fields `output_section' and `output_offset' to create an output file. The data to be written comes from input sections attached to the output sections. The output section structure can be considered a filter for the input section, the output section determines the vma of the output data and the name, but the input section determines the offset into the output section of the data to be written. Eg to create a section "O", starting at 0x100, 0x123 long, containing two subsections, "A" at offset 0x0 (ie at vma 0x100) and "B" at offset 0x20 (ie at vma 0x120) the structures would look like: section name "A" output_offset 0x00 size 0x20 output_section -----------> section name "O" | vma 0x100 section name "B" | size 0x123 output_offset 0x20 | size 0x103 | output_section --------| File: bfd.info, Node: typedef asection, Next: section prototypes, Prev: Section Output, Up: Sections typedef asection ---------------- The shape of a section struct: typedef struct sec { The name of the section, the name isn't a copy, the pointer is the same as that passed to bfd_make_section. CONST char *name; The next section in the list belonging to the BFD, or NULL. struct sec *next; The field flags contains attributes of the section. Some of these flags are read in from the object file, and some are synthesized from other information. flagword flags; #define SEC_NO_FLAGS 0x000 Tells the OS to allocate space for this section when loaded. This would clear for a section containing debug information only. #define SEC_ALLOC 0x001 Tells the OS to load the section from the file when loading. This would be clear for a .bss section #define SEC_LOAD 0x002 The section contains data still to be relocated, so there will be some relocation information too. #define SEC_RELOC 0x004 Obsolete #define SEC_BALIGN 0x008 A signal to the OS that the section contains read only data. #define SEC_READONLY 0x010 The section contains code only. #define SEC_CODE 0x020 The section contains data only. #define SEC_DATA 0x040 The section will reside in ROM. #define SEC_ROM 0x080 The section contains constructor information. This section type is used by the linker to create lists of constructors and destructors used by `g++'. When a back end sees a symbol which should be used in a constructor list, it creates a new section for the type of name (eg `__CTOR_LIST__'), attaches the symbol to it and builds a relocation. To build the lists of constructors, all the linker has to to is catenate all the sections called `__CTOR_LIST__' and relocte the data contained within - exactly the operations it would peform on standard data. #define SEC_CONSTRUCTOR 0x100 The section is a constuctor, and should be placed at the end of the .. #define SEC_CONSTRUCTOR_TEXT 0x1100 #define SEC_CONSTRUCTOR_DATA 0x2100 #define SEC_CONSTRUCTOR_BSS 0x3100 The section has contents - a bss section could be `SEC_ALLOC' | `SEC_HAS_CONTENTS', a debug section could be `SEC_HAS_CONTENTS' #define SEC_HAS_CONTENTS 0x200 An instruction to the linker not to output sections containing this flag even if they have information which would normally be written. #define SEC_NEVER_LOAD 0x400 The base address of the section in the address space of the target. bfd_vma vma; The size of the section in bytes of the loaded section. This contains a value even if the section has no contents (eg, the size of `.bss'). bfd_size_type size; If this section is going to be output, then this value is the offset into the output section of the first byte in the input section. Eg, if this was going to start at the 100th byte in the output section, this value would be 100. bfd_vma output_offset; The output section through which to map on output. struct sec *output_section; The alignment requirement of the section, as an exponent - eg 3 aligns to 2^3 (or 8) unsigned int alignment_power; If an input section, a pointer to a vector of relocation records for the data in this section. struct reloc_cache_entry *relocation; If an output section, a pointer to a vector of pointers to relocation records for the data in this section. struct reloc_cache_entry **orelocation; The number of relocation records in one of the above unsigned reloc_count; Which section is it 0..nth int index; Information below is back end specific - and not always used or updated File position of section data file_ptr filepos; File position of relocation info file_ptr rel_filepos; File position of line data file_ptr line_filepos; Pointer to data for applications PTR userdata; struct lang_output_section *otheruserdata; Attached line number information alent *lineno; Number of line number records unsigned int lineno_count; When a section is being output, this value changes as more linenumbers are written out file_ptr moving_line_filepos; what the section number is in the target world unsigned int target_index; PTR used_by_bfd; If this is a constructor section then here is a list of the relocations created to relocate items within it. struct relent_chain *constructor_chain; The BFD which owns the section. bfd *owner; } asection ; File: bfd.info, Node: section prototypes, Next: Section, Prev: typedef section, Up: Sections section prototypes ------------------ `bfd_get_section_by_name' ......................... Runs through the provided ABFD and returns the `asection' who's name matches that provided, otherwise NULL. *Note Sections::, for more information. asection * bfd_get_section_by_name(bfd *abfd, CONST char *name); `bfd_make_section' .................. This function creates a new empty section called NAME and attaches it to the end of the chain of sections for the BFD supplied. An attempt to create a section with a name which is already in use, returns the old section by that name instead. Possible errors are: `invalid_operation' If output has already started for this BFD. `no_memory' If obstack alloc fails. asection * bfd_make_section(bfd *, CONST char *name); `bfd_set_section_flags' ....................... Attempts to set the attributes of the section named in the BFD supplied to the value. Returns true on success, false on error. Possible error returns are: `invalid operation' The section cannot have one or more of the attributes requested. For example, a .bss section in `a.out' may not have the `SEC_HAS_CONTENTS' field set. boolean bfd_set_section_flags(bfd *, asection *, flagword); `bfd_map_over_sections' ....................... Calls the provided function FUNC for each section attached to the BFD ABFD, passing OBJ as an argument. The function will be called as if by func(abfd, the_section, obj); void bfd_map_over_sections(bfd *abfd, void (*func)(), PTR obj); This is the prefered method for iterating over sections, an alternative would be to use a loop: section *p; for (p = abfd->sections; p != NULL; p = p->next) func(abfd, p, ...) `bfd_set_section_size' ...................... Sets SECTION to the size VAL. If the operation is ok, then `true' is returned, else `false'. Possible error returns: `invalid_operation' Writing has started to the BFD, so setting the size is invalid boolean bfd_set_section_size(bfd *, asection *, bfd_size_type val); `bfd_set_section_contents' .......................... Sets the contents of the section SECTION in BFD ABFD to the data starting in memory at DATA. The data is written to the output section starting at offset OFFSET for COUNT bytes. Normally `true' is returned, else `false'. Possible error returns are: `no_contents' The output section does not have the `SEC_HAS_CONTENTS' attribute, so nothing can be written to it. `and some more too' This routine is front end to the back end function `_bfd_set_section_contents'. boolean bfd_set_section_contents(bfd *abfd, asection *section, PTR data, file_ptr offset, bfd_size_type count); `bfd_get_section_contents' .......................... This function reads data from SECTION in BFD ABFD into memory starting at LOCATION. The data is read at an offset of OFFSET from the start of the input section, and is read for COUNT bytes. If the contents of a constuctor with the `SEC_CONSTUCTOR' flag set are requested, then the LOCATION is filled with zeroes. If no errors occur, `true' is returned, else `false'. Possible errors are: `unknown yet' boolean bfd_get_section_contents(bfd *abfd, asection *section, PTR location, file_ptr offset, bfd_size_type count); File: bfd.info, Node: Symbols, Next: BFD back end, Prev: BFD front end, Up: Top Symbols ======= BFD trys to maintain as much symbol information as it can when it moves information from file to file. BFD passes information to applications though the `asymbol' structure. When the application requests the symbol table, BFD reads the table in the native form and translates parts of it into the internal format. To maintain more than the infomation passed to applications some targets keep some information 'behind the sceans', in a structure only the particular back end knows about. For example, the coff back end keeps the original symbol table structure as well as the canonical structure when a BFD is read in. On output, the coff back end can reconstruct the output symbol table so that no information is lost, even information unique to coff which BFD doesn't know or understand. If a coff symbol table was read, but was written through an a.out back end, all the coff specific information would be lost. (.. until BFD 2 :). The symbol table of a BFD is not necessarily read in until a canonicalize request is made. Then the BFD back end fills in a table provided by the application with pointers to the canonical information. To output symbols, the application provides BFD with a table of pointers to pointers to `asymbol's. This allows applications like the linker to output a symbol as read, since the 'behind the sceens' information will be still available. * Menu: * Reading Symbols:: * Writing Symbols:: * typedef asymbol:: * symbol handling functions:: File: bfd.info, Node: Reading Symbols, Next: Writing Symbols, Prev: Symbols, Up: Symbols Reading Symbols --------------- There are two stages to reading a symbol table from a BFD; allocating storage, and the actual reading process. This is an excerpt from an appliction which reads the symbol table: unsigned int storage_needed; asymbol **symbol_table; unsigned int number_of_symbols; unsigned int i; storage_needed = get_symtab_upper_bound (abfd); if (storage_needed == 0) { return ; } symbol_table = (asymbol **) malloc (storage_needed); ... number_of_symbols = bfd_canonicalize_symtab (abfd, symbol_table); for (i = 0; i < number_of_symbols; i++) { process_symbol (symbol_table[i]); } All storage for the symbols themselves is in an obstack connected to the BFD, and is freed when the BFD is closed. File: bfd.info, Node: Writing Symbols, Next: typedef asymbol, Prev: Reading Symbols, Up: Symbols Writing Symbols --------------- Writing of a symbol table is automatic when a BFD open for writing is closed. The application attaches a vector of pointers to pointers to symbols to the BFD being written, and fills in the symbol count. The close and cleanup code reads through the table provided and performs all the necessary operations. The outputing code must always be provided with an 'owned' symbol; one which has come from another BFD, or one which has been created using `bfd_make_empty_symbol'. An example showing the creation of a symbol table with only one element: #include "bfd.h" main() { bfd *abfd; asymbol *ptrs[2]; asymbol *new; abfd = bfd_openw("foo","a.out-sunos-big"); bfd_set_format(abfd, bfd_object); new = bfd_make_empty_symbol(abfd); new->name = "dummy_symbol"; new->section = (asection *)0; new->flags = BSF_ABSOLUTE | BSF_GLOBAL; new->value = 0x12345; ptrs[0] = new; ptrs[1] = (asymbol *)0; bfd_set_symtab(abfd, ptrs, 1); bfd_close(abfd); } ./makesym nm foo 00012345 A dummy_symbol Many formats cannot represent arbitary symbol information; for instance the `a.out' object format does not allow an arbitary number of sections. A symbol pointing to a section which is not one of `.text', `.data' or `.bss' cannot be described. File: bfd.info, Node: typedef asymbol, Next: symbol handling functions, Prev: Writing Symbols, Up: Symbols typedef asymbol --------------- An `asymbol' has the form: typedef struct symbol_cache_entry { A pointer to the BFD which owns the symbol. This information is necessary so that a back end can work out what additional (invisible to the application writer) information is carried with the symbol. struct _bfd *the_bfd; The text of the symbol. The name is left alone, and not copied - the application may not alter it. CONST char *name; The value of the symbol. symvalue value; Attributes of a symbol: #define BSF_NO_FLAGS 0x00 The symbol has local scope; `static' in `C'. The value is the offset into the section of the data. #define BSF_LOCAL 0x01 The symbol has global scope; initialized data in `C'. The value is the offset into the section of the data. #define BSF_GLOBAL 0x02 Obsolete #define BSF_IMPORT 0x04 The symbol has global scope, and is exported. The value is the offset into the section of the data. #define BSF_EXPORT 0x08 The symbol is undefined. `extern' in `C'. The value has no meaning. #define BSF_UNDEFINED 0x10 The symbol is common, initialized to zero; default in `C'. The value is the size of the object in bytes. #define BSF_FORT_COMM 0x20 A normal `C' symbol would be one of: `BSF_LOCAL', `BSF_FORT_COMM', `BSF_UNDEFINED' or `BSF_EXPORT|BSD_GLOBAL' The symbol is a debugging record. The value has an arbitary meaning. #define BSF_DEBUGGING 0x40 The symbol has no section attached, any value is the actual value and is not a relative offset to a section. #define BSF_ABSOLUTE 0x80 Used by the linker #define BSF_KEEP 0x10000 #define BSF_KEEP_G 0x80000 Unused #define BSF_WEAK 0x100000 #define BSF_CTOR 0x200000 #define BSF_FAKE 0x400000 The symbol used to be a common symbol, but now it is allocated. #define BSF_OLD_COMMON 0x800000 The default value for common data. #define BFD_FORT_COMM_DEFAULT_VALUE 0 In some files the type of a symbol sometimes alters its location in an output file - ie in coff a `ISFCN' symbol which is also `C_EXT' symbol appears where it was declared and not at the end of a section. This bit is set by the target BFD part to convey this information. #define BSF_NOT_AT_END 0x40000 Signal that the symbol is the label of constructor section. #define BSF_CONSTRUCTOR 0x1000000 Signal that the symbol is a warning symbol. If the symbol is a warning symbol, then the value field (I know this is tacky) will point to the asymbol which when referenced will cause the warning. #define BSF_WARNING 0x2000000 Signal that the symbol is indirect. The value of the symbol is a pointer to an undefined asymbol which contains the name to use instead. #define BSF_INDIRECT 0x4000000 flagword flags; A pointer to the section to which this symbol is relative, or 0 if the symbol is absolute or undefined. Note that it is not sufficient to set this location to 0 to mark a symbol as absolute - the flag `BSF_ABSOLUTE' must be set also. struct sec *section; Back end special data. This is being phased out in favour of making this a union. PTR udata; } asymbol; File: bfd.info, Node: symbol handling functions, Next: Symbols, Prev: typedef asymbol, Up: Symbols Symbol Handling Functions ------------------------- `get_symtab_upper_bound' ........................ Returns the number of bytes required in a vector of pointers to `asymbols' for all the symbols in the supplied BFD, including a terminal NULL pointer. If there are no symbols in the BFD, then 0 is returned. #define get_symtab_upper_bound(abfd) \ BFD_SEND (abfd, _get_symtab_upper_bound, (abfd)) `bfd_canonicalize_symtab' ......................... Supplied a BFD and a pointer to an uninitialized vector of pointers. This reads in the symbols from the BFD, and fills in the table with pointers to the symbols, and a trailing NULL. The routine returns the actual number of symbol pointers not including the NULL. #define bfd_canonicalize_symtab(abfd, location) \ BFD_SEND (abfd, _bfd_canonicalize_symtab,\ (abfd, location)) `bfd_set_symtab' ................ Provided a table of pointers to to symbols and a count, writes to the output BFD the symbols when closed. boolean bfd_set_symtab(bfd *, asymbol **, unsigned int ); `bfd_print_symbol_vandf' ........................ Prints the value and flags of the symbol supplied to the stream file. void bfd_print_symbol_vandf(PTR file, asymbol *symbol); `bfd_make_empty_symbol' ....................... This function creates a new `asymbol' structure for the BFD, and returns a pointer to it. This routine is necessary, since each back end has private information surrounding the `asymbol'. Building your own `asymbol' and pointing to it will not create the private information, and will cause problems later on. #define bfd_make_empty_symbol(abfd) \ BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd)) File: bfd.info, Node: Archives, Next: BFD back end, Prev: BFD front end, Up: Top Archives ======== Gumby, you promised to write this bit... Archives are supported in BFD in `archive.c'. An archive is represented internally just like another BFD, with a pointer to a chain of contained BFDs. Archives can be created by opening BFDs, linking them together and attaching them as children to another BFD and then closing the parent BFD. `bfd_get_next_mapent' ..................... What this does symindex bfd_get_next_mapent(bfd *, symindex, carsym **); `bfd_set_archive_head' ...................... Used whilst processing archives. Sets the head of the chain of BFDs contained in an archive to NEW_HEAD. (see chapter on archives) boolean bfd_set_archive_head(bfd *output, bfd *new_head); `bfd_get_elt_at_index' ...................... Return the sub bfd contained within the archive at archive index n. bfd * bfd_get_elt_at_index(bfd *, int); `bfd_openr_next_archived_file' .............................. Initially provided a BFD containing an archive and NULL, opens a BFD on the first contained element and returns that. Subsequent calls to bfd_openr_next_archived_file should pass the archive and the previous return value to return a created BFD to the next contained element. NULL is returned when there are no more. bfd* bfd_openr_next_archived_file(bfd *archive, bfd *previous); File: bfd.info, Node: Formats, Next: BFD back end, Prev: BFD front end, Up: Top File Formats ============ A format is a BFD concept of high level file contents. The formats supported by BFD are: `bfd_object' The BFD may contain data, symbols, relocations and debug info. `bfd_archive' The BFD contains other BFDs and an optional index. `bfd_core' The BFD contains the result of an executable core dump. `bfd_check_format' .................. This routine is supplied a BFD and a format. It attempts to verify if the file attached to the BFD is indeed compatible with the format specified (ie, one of `bfd_object', `bfd_archive' or `bfd_core'). If the BFD has been set to a specific TARGET before the call, only the named target and format combination will be checked. If the target has not been set, or has been set to `default' then all the known target backends will be interrogated to determine a match. The function returns `true' on success, otherwise `false' with one of the following error codes: `' invalid_operation if `format' is not one of `bfd_object', `bfd_archive' or `bfd_core'. `system_call_error' if an error occured during a read - even some file mismatches can cause system_call_errros `file_not_recognised' none of the backends recognised the file format `file_ambiguously_recognized' more than one backend recognised the file format. boolean bfd_check_format(bfd *abfd, bfd_format format); `bfd_set_format' ................ This function sets the file format of the supplied BFD to the format requested. If the target set in the BFD does not support the format requested, the format is illegal or the BFD is not open for writing than an error occurs. boolean bfd_set_format(bfd *, bfd_format); `bfd_format_string' ................... This function takes one argument, and enumerated type (bfd_format) and returns a pointer to a const string "invalid", "object", "archive", "core" or "unknown" depending upon the value of the enumeration. CONST char * bfd_format_string(bfd_format);