1 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.

• The front end of BFD provides the interface to the user. It manages memory, and various canonical data structures. The front end also decides which back end to use, and when to call back end routines.
• The back ends provide BFD its view of the real world. Each back 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.

1.1 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 members of Cygnus Support; Steve Chamberlain (sac@cygnus.com), John Gilmore (gnu@cygnus.com), K. Richard Pixley (rich@cygnus.com) and David Henkel-Wallace (gumby@cygnus.com).

1.2 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.

1.3 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.

1.3.1 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.

1.3.2 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).

2 BFD front end

2.1 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.

3 BFD back end

All of BFD lives in one directory.

Index

Table of Contents

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