@email{ian@@cygnus.com}.
@menu
-* BFD glossary:: BFD glossary
+* BFD overview:: BFD overview
* BFD guidelines:: BFD programming guidelines
* BFD target vector:: BFD target vector
* BFD generated files:: BFD generated files
* BFD multiple compilations:: Files compiled multiple times in BFD
* BFD relocation handling:: BFD relocation handling
* BFD ELF support:: BFD ELF support
+* BFD glossary:: Glossary
* Index:: Index
@end menu
-@node BFD glossary
-@section BFD glossary
-@cindex glossary for bfd
-@cindex bfd glossary
-
-This is a short glossary of some BFD terms.
-
-@table @asis
-@item a.out
-The a.out object file format. The original Unix object file format.
-Still used on SunOS, though not Solaris. Supports only three sections.
-
-@item archive
-A collection of object files produced and manipulated by the @samp{ar}
-program.
-
-@item BFD
-The BFD library itself. Also, each object file, archive, or exectable
-opened by the BFD library has the type @samp{bfd *}, and is sometimes
-referred to as a bfd.
-
-@item COFF
-The Common Object File Format. Used on Unix SVR3. Used by some
-embedded targets, although ELF is normally better.
-
-@item DLL
-A shared library on Windows.
-
-@item dynamic linker
-When a program linked against a shared library is run, the dynamic
-linker will locate the appropriate shared library and arrange to somehow
-include it in the running image.
-
-@item dynamic object
-Another name for an ELF shared library.
-
-@item ECOFF
-The Extended Common Object File Format. Used on Alpha Digital Unix
-(formerly OSF/1), as well as Ultrix and Irix 4. A variant of COFF.
-
-@item ELF
-The Executable and Linking Format. The object file format used on most
-modern Unix systems, including GNU/Linux, Solaris, Irix, and SVR4. Also
-used on many embedded systems.
-
-@item executable
-A program, with instructions and symbols, and perhaps dynamic linking
-information. Normally produced by a linker.
-
-@item NLM
-NetWare Loadable Module. Used to describe the format of an object which
-be loaded into NetWare, which is some kind of PC based network server
-program.
+@node BFD overview
+@section BFD overview
-@item object file
-A binary file including machine instructions, symbols, and relocation
-information. Normally produced by an assembler.
+BFD is a library which provides a single interface to read and write
+object files, executables, archive files, and core files in any format.
-@item object file format
-The format of an object file. Typically object files and executables
-for a particular system are in the same format, although executables
-will not contain any relocation information.
-
-@item PE
-The Portable Executable format. This is the object file format used for
-Windows (specifically, Win32) object files. It is based closely on
-COFF, but has a few significant differences.
-
-@item PEI
-The Portable Executable Image format. This is the object file format
-used for Windows (specifically, Win32) executables. It is very similar
-to PE, but includes some additional header information.
-
-@item relocations
-Information used by the linker to adjust section contents. Also called
-relocs.
+@menu
+* BFD library interfaces:: BFD library interfaces
+* BFD library users:: BFD library users
+* BFD view:: The BFD view of a file
+* BFD blindness:: BFD loses information
+@end menu
-@item section
-Object files and executable are composed of sections. Sections have
-optional data and optional relocation information.
+@node BFD library interfaces
+@subsection BFD library interfaces
+
+One way to look at the BFD library is to divide it into four parts by
+type of interface.
+
+The first interface is the set of generic functions which programs using
+the BFD library will call. These generic function normally translate
+directly or indirectly into calls to routines which are specific to a
+particular object file format. Many of these generic functions are
+actually defined as macros in @file{bfd.h}. These functions comprise
+the official BFD interface.
+
+The second interface is the set of functions which appear in the target
+vectors. This is the bulk of the code in BFD. A target vector is a set
+of function pointers specific to a particular object file format. The
+target vector is used to implement the generic BFD functions. These
+functions are always called through the target vector, and are never
+called directly. The target vector is described in detail in @ref{BFD
+target vector}. The set of functions which appear in a particular
+target vector is often referred to as a BFD backend.
+
+The third interface is a set of oddball functions which are typically
+specific to a particular object file format, are not generic functions,
+and are called from outside of the BFD library. These are used as hooks
+by the linker and the assembler when a particular object file format
+requires some action which the BFD generic interface does not provide.
+These functions are typically declared in @file{bfd.h}, but in many
+cases they are only provided when BFD is configured with support for a
+particular object file format. These functions live in a grey area, and
+are not really part of the official BFD interface.
+
+The fourth interface is the set of BFD support functions which are
+called by the other BFD functions. These manage issues like memory
+allocation, error handling, file access, hash tables, swapping, and the
+like. These functions are never called from outside of the BFD library.
+
+@node BFD library users
+@subsection BFD library users
+
+Another way to look at the BFD library is to divide it into three parts
+by the manner in which it is used.
+
+The first use is to read an object file. The object file readers are
+programs like @samp{gdb}, @samp{nm}, @samp{objdump}, and @samp{objcopy}.
+These programs use BFD to view an object file in a generic form. The
+official BFD interface is normally fully adequate for these programs.
+
+The second use is to write an object file. The object file writers are
+programs like @samp{gas} and @samp{objcopy}. These programs use BFD to
+create an object file. The official BFD interface is normally adequate
+for these programs, but for some object file formats the assembler needs
+some additional hooks in order to set particular flags or other
+information. The official BFD interface includes functions to copy
+private information from one object file to another, and these functions
+are used by @samp{objcopy} to avoid information loss.
+
+The third use is to link object files. There is only one object file
+linker, @samp{ld}. Originally, @samp{ld} was an object file reader and
+an object file writer, and it did the link operation using the generic
+BFD structures. However, this turned out to be too slow and too memory
+intensive.
+
+The official BFD linker functions were written to permit specific BFD
+backends to perform the link without translating through the generic
+structures, in the normal case where all the input files and output file
+have the same object file format. Not all of the backends currently
+implement the new interface, and there are default linking functions
+within BFD which use the generic structures and which work with all
+backends.
+
+For several object file formats the linker needs additional hooks which
+are not provided by the official BFD interface, particularly for dynamic
+linking support. These functions are typically called from the linker
+emulation template.
+
+@node BFD view
+@subsection The BFD view of a file
+
+BFD uses generic structures to manage information. It translates data
+into the generic form when reading files, and out of the generic form
+when writing files.
+
+BFD describes a file as a pointer to the @samp{bfd} type. A @samp{bfd}
+is composed of the following elements. The BFD information can be
+displayed using the @samp{objdump} program with various options.
-@item shared library
-A library of functions which may be used by many executables without
-actually being linked into each executable. There are several different
-implementations of shared libraries, each having slightly different
-features.
+@table @asis
+@item general information
+The object file format, a few general flags, the start address.
+@item architecture
+The architecture, including both a general processor type (m68k, MIPS
+etc.) and a specific machine number (m68000, R4000, etc.).
+@item sections
+A list of sections.
+@item symbols
+A symbol table.
+@end table
-@item symbol
-Each object file and executable may have a list of symbols, often
-referred to as the symbol table. A symbol is basically a name and an
-address. There may also be some additional information like the type of
-symbol, although the type of a symbol is normally something simple like
-function or object, and should be confused with the more complex C
-notion of type. Typically every global function and variable in a C
-program will have an associated symbol.
+BFD represents a section as a pointer to the @samp{asection} type. Each
+section has a name and a size. Most sections also have an associated
+block of data, known as the section contents. Sections also have
+associated flags, a virtual memory address, a load memory address, a
+required alignment, a list of relocations, and other miscellaneous
+information.
+
+BFD represents a relocation as a pointer to the @samp{arelent} type. A
+relocation describes an action which the linker must take to modify the
+section contents. Relocations have a symbol, an address, an addend, and
+a pointer to a howto structure which describes how to perform the
+relocation. For more information, see @ref{BFD relocation handling}.
+
+BFD represents a symbol as a pointer to the @samp{asymbol} type. A
+symbol has a name, a pointer to a section, an offset within that
+section, and some flags.
+
+Archive files do not have any sections or symbols. Instead, BFD
+represents an archive file as a file which contains a list of
+@samp{bfd}s. BFD also provides access to the archive symbol map, as a
+list of symbol names. BFD provides a function to return the @samp{bfd}
+within the archive which corresponds to a particular entry in the
+archive symbol map.
+
+@node BFD blindness
+@subsection BFD loses information
+
+Most object file formats have information which BFD can not represent in
+its generic form, at least as currently defined.
+
+There is often explicit information which BFD can not represent. For
+example, the COFF version stamp, or the ELF program segments. BFD
+provides special hooks to handle this information when copying,
+printing, or linking an object file. The BFD support for a particular
+object file format will normally store this information in private data
+and handle it using the special hooks.
+
+In some cases there is also implicit information which BFD can not
+represent. For example, the MIPS processor distinguishes small and
+large symbols, and requires that all small symbls be within 32K of the
+GP register. This means that the MIPS assembler must be able to mark
+variables as either small or large, and the MIPS linker must know to put
+small symbols within range of the GP register. Since BFD can not
+represent this information, this means that the assembler and linker
+must have information that is specific to a particular object file
+format which is outside of the BFD library.
+
+This loss of information indicates areas where the BFD paradigm breaks
+down. It is not actually possible to represent the myriad differences
+among object file formats using a single generic interface, at least not
+in the manner which BFD does it today.
+
+Nevertheless, the BFD library does greatly simplify the task of dealing
+with object files, and particular problems caused by information loss
+can normally be solved using some sort of relatively constrained hook
+into the library.
-@item Win32
-The current Windows API, implemented by Windows 95 and later and Windows
-NT 3.51 and later, but not by Windows 3.1.
-@item XCOFF
-The eXtended Common Object File Format. Used on AIX. A variant of
-COFF, with a completely different symbol table implementation.
-@end table
@node BFD guidelines
@section BFD programming guidelines
work, and it is required by the GNU coding standards.
@item
-Always remember that people can compile using --enable-targets to build
-several, or all, targets at once. It must be possible to link together
-the files for all targets.
+Always remember that people can compile using @samp{--enable-targets} to
+build several, or all, targets at once. It must be possible to link
+together the files for all targets.
@item
BFD code should compile with few or no warnings using @samp{gcc -Wall}.
@item flavour
A general description of the type of target. The following flavours are
currently defined:
+
@table @samp
@item bfd_target_unknown_flavour
Undefined or unknown.
Every target vector has three arrays of function pointers which are
indexed by the BFD format type. The BFD format types are as follows:
+
@table @samp
@item bfd_unknown
Unknown format. Not used for anything useful.
@end table
The three arrays of function pointers are as follows:
+
@table @samp
@item bfd_check_format
Check whether the BFD is of a particular format (object file, archive
functions initialize the appropriate fields in the BFD target vector.
This is done because it turns out that many different target vectors can
-shared certain classes of functions. For example, archives are similar
+share certain classes of functions. For example, archives are similar
on most platforms, so most target vectors can use the same archive
functions. Those target vectors all use @samp{BFD_JUMP_TABLE_ARCHIVE}
with the same argument, calling a set of functions which is defined in
@item _get_section_contents_in_window
Set a @samp{bfd_window} to hold the contents of a section. This is
called from @samp{bfd_get_section_contents_in_window}. The
-@samp{bfd_window} idea never really caught in, and I don't think this is
+@samp{bfd_window} idea never really caught on, and I don't think this is
ever called. Pretty much all targets implement this as
@samp{bfd_generic_get_section_contents_in_window}, which uses
@samp{bfd_get_section_contents} to do the right thing. The
Print information about the symbol. This is called via
@samp{bfd_print_symbol}. One of the arguments indicates what sort of
information should be printed:
+
@table @samp
@item bfd_print_symbol_name
Just print the symbol name.
@section Files compiled multiple times in BFD
Several files in BFD are compiled multiple times. By this I mean that
there are header files which contain function definitions. These header
-filesare included by other files, and thus the functions are compiled
+files are included by other files, and thus the functions are compiled
once per file which includes them.
Preprocessor macros are used to control the compilation, so that each
section is the difference between the value of a symbol and the final
address of the section contents.
-In general, relocations can be arbitrarily complex. For
-example,relocations used in dynamic linking systems often require the
-linker to allocate space in a different section and use the offset
-within that section as the value to store. In the IEEE object file
-format, relocations may involve arbitrary expressions.
+In general, relocations can be arbitrarily complex. For example,
+relocations used in dynamic linking systems often require the linker to
+allocate space in a different section and use the offset within that
+section as the value to store. In the IEEE object file format,
+relocations may involve arbitrary expressions.
When doing a relocateable link, the linker may or may not have to do
anything with a relocation, depending upon the definition of the
So, if you want to add a new target, or add a new relocation to an
existing target, you need to do the following:
+
@itemize @bullet
@item
Make sure you clearly understand what the contents of the section should
constants used by the generic support.
@menu
+* BFD ELF sections and segments:: ELF sections and segments
* BFD ELF generic support:: BFD ELF generic support
* BFD ELF processor specific support:: BFD ELF processor specific support
+* BFD ELF core files:: BFD ELF core files
* BFD ELF future:: BFD ELF future
@end menu
+@node BFD ELF sections and segments
+@subsection ELF sections and segments
+
+The ELF ABI permits a file to have either sections or segments or both.
+Relocateable object files conventionally have only sections.
+Executables conventionally have both. Core files conventionally have
+only program segments.
+
+ELF sections are similar to sections in other object file formats: they
+have a name, a VMA, file contents, flags, and other miscellaneous
+information. ELF relocations are stored in sections of a particular
+type; BFD automatically converts these sections into internal relocation
+information.
+
+ELF program segments are intended for fast interpretation by a system
+loader. They have a type, a VMA, an LMA, file contents, and a couple of
+other fields. When an ELF executable is run on a Unix system, the
+system loader will examine the program segments to decide how to load
+it. The loader will ignore the section information. Loadable program
+segments (type @samp{PT_LOAD}) are directly loaded into memory. Other
+program segments are interpreted by the loader, and generally provide
+dynamic linking information.
+
+When an ELF file has both program segments and sections, an ELF program
+segment may encompass one or more ELF sections, in the sense that the
+portion of the file which corresponds to the program segment may include
+the portions of the file corresponding to one or more sections. When
+there is more than one section in a loadable program segment, the
+relative positions of the section contents in the file must correspond
+to the relative positions they should hold when the program segment is
+loaded. This requirement should be obvious if you consider that the
+system loader will load an entire program segment at a time.
+
+On a system which supports dynamic paging, such as any native Unix
+system, the contents of a loadable program segment must be at the same
+offset in the file as in memory, modulo the memory page size used on the
+system. This is because the system loader will map the file into memory
+starting at the start of a page. The system loader can easily remap
+entire pages to the correct load address. However, if the contents of
+the file were not correctly aligned within the page, the system loader
+would have to shift the contents around within the page, which is too
+expensive. For example, if the LMA of a loadable program segment is
+@samp{0x40080} and the page size is @samp{0x1000}, then the position of
+the segment contents within the file must equal @samp{0x80} modulo
+@samp{0x1000}.
+
+BFD has only a single set of sections. It does not provide any generic
+way to examine both sections and segments. When BFD is used to open an
+object file or executable, the BFD sections will represent ELF sections.
+When BFD is used to open a core file, the BFD sections will represent
+ELF program segments.
+
+When BFD is used to examine an object file or executable, any program
+segments will be read to set the LMA of the sections. This is because
+ELF sections only have a VMA, while ELF program segments have both a VMA
+and an LMA. Any program segments will be copied by the
+@samp{copy_private} entry points. They will be printed by the
+@samp{print_private} entry point. Otherwise, the program segments are
+ignored. In particular, programs which use BFD currently have no direct
+access to the program segments.
+
+When BFD is used to create an executable, the program segments will be
+created automatically based on the section information. This is done in
+the function @samp{assign_file_positions_for_segments} in @file{elf.c}.
+This function has been tweaked many times, and probably still has
+problems that arise in particular cases.
+
+There is a hook which may be used to explicitly define the program
+segments when creating an executable: the @samp{bfd_record_phdr}
+function in @file{bfd.c}. If this function is called, BFD will not
+create program segments itself, but will only create the program
+segments specified by the caller. The linker uses this function to
+implement the @samp{PHDRS} linker script command.
+
@node BFD ELF generic support
@subsection BFD ELF generic support
When writing a @file{elf@var{nn}-@var{cpu}.c} file, you must do the
following:
+
@itemize @bullet
@item
Define either @samp{TARGET_BIG_SYM} or @samp{TARGET_LITTLE_SYM}, or
@item
If the format should use @samp{Rel} rather than @samp{Rela} relocations,
define @samp{USE_REL}. This is normally defined in chapter 4 of the
-processor specific supplement. In the absence of a supplement, it's
-usually easier to work with @samp{Rela} relocations, although they will
-require more space in object files (but not in executables, except when
-using dynamic linking). It is possible, though somewhat awkward, to
-support both @samp{Rel} and @samp{Rela} relocations for a single target;
-@file{elf64-mips.c} does it by overriding the relocation reading and
-writing routines.
+processor specific supplement.
+
+In the absence of a supplement, it's easier to work with @samp{Rela}
+relocations. @samp{Rela} relocations will require more space in object
+files (but not in executables, except when using dynamic linking).
+However, this is outweighed by the simplicity of addend handling when
+using @samp{Rela} relocations. With @samp{Rel} relocations, the addend
+must be stored in the object file, which makes relocateable links more
+complex. In particular, split relocations, in which an address is built
+up using two or more instructions, become very awkward; such relocations
+are used on RISC chips which can not load an address in a single
+instruction.
+
+It is possible, though somewhat awkward, to support both @samp{Rel} and
+@samp{Rela} relocations for a single target; @file{elf64-mips.c} does it
+by overriding the relocation reading and writing routines.
@item
Define howto structures for all the relocation types.
@item
Dynamic linking support, which involves processor specific relocations
requiring special handling, is also implemented via hook functions.
+@node BFD ELF core files
+@subsection BFD ELF core files
+@cindex elf core files
+
+On native ELF Unix systems, core files are generated without any
+sections. Instead, they only have program segments.
+
+When BFD is used to read an ELF core file, the BFD sections will
+actually represent program segments. Since ELF program segments do not
+have names, BFD will invent names like @samp{segment@var{n}} where
+@var{n} is a number.
+
+A single ELF program segment may include both an initialized part and an
+uninitialized part. The size of the initialized part is given by the
+@samp{p_filesz} field. The total size of the segment is given by the
+@samp{p_memsz} field. If @samp{p_memsz} is larger than @samp{p_filesz},
+then the extra space is uninitialized, or, more precisely, initialized
+to zero.
+
+BFD will represent such a program segment as two different sections.
+The first, named @samp{segment@var{n}a}, will represent the initialized
+part of the program segment. The second, named @samp{segment@var{n}b},
+will represent the uninitialized part.
+
+ELF core files store special information such as register values in
+program segments with the type @samp{PT_NOTE}. BFD will attempt to
+interpret the information in these segments, and will create additional
+sections holding the information. Some of this interpretation requires
+information found in the host header file @file{sys/procfs.h}, and so
+will only work when BFD is built on a native system.
+
+BFD does not currently provide any way to create an ELF core file. In
+general, BFD does not provide a way to create core files. The way to
+implement this would be to write @samp{bfd_set_format} and
+@samp{bfd_write_contents} routines for the @samp{bfd_core} type; see
+@ref{BFD target vector format}.
+
@node BFD ELF future
@subsection BFD ELF future
The processor function hooks and constants are ad hoc and need better
documentation.
+When a linker script uses @samp{SIZEOF_HEADERS}, the ELF backend must
+guess at the number of program segments which will be required, in
+@samp{get_program_header_size}. This is because the linker calls
+@samp{bfd_sizeof_headers} before it knows all the section addresses and
+sizes. The ELF backend may later discover, when creating program
+segments, that more program segments are required. This is currently
+reported as an error in @samp{assign_file_positions_for_segments}.
+
+In practice this makes it difficult to use @samp{SIZEOF_HEADERS} except
+with a carefully defined linker script. Unfortunately,
+@samp{SIZEOF_HEADERS} is required for fast program loading on a native
+system, since it permits the initial code section to appear on the same
+page as the program segments, saving a page read when the program starts
+running. Fortunately, native systems permit careful definition of the
+linker script. Still, ideally it would be possible to use relaxation to
+compute the number of program segments.
+
+@node BFD glossary
+@section BFD glossary
+@cindex glossary for bfd
+@cindex bfd glossary
+
+This is a short glossary of some BFD terms.
+
+@table @asis
+@item a.out
+The a.out object file format. The original Unix object file format.
+Still used on SunOS, though not Solaris. Supports only three sections.
+
+@item archive
+A collection of object files produced and manipulated by the @samp{ar}
+program.
+
+@item backend
+The implementation within BFD of a particular object file format. The
+set of functions which appear in a particular target vector.
+
+@item BFD
+The BFD library itself. Also, each object file, archive, or exectable
+opened by the BFD library has the type @samp{bfd *}, and is sometimes
+referred to as a bfd.
+
+@item COFF
+The Common Object File Format. Used on Unix SVR3. Used by some
+embedded targets, although ELF is normally better.
+
+@item DLL
+A shared library on Windows.
+
+@item dynamic linker
+When a program linked against a shared library is run, the dynamic
+linker will locate the appropriate shared library and arrange to somehow
+include it in the running image.
+
+@item dynamic object
+Another name for an ELF shared library.
+
+@item ECOFF
+The Extended Common Object File Format. Used on Alpha Digital Unix
+(formerly OSF/1), as well as Ultrix and Irix 4. A variant of COFF.
+
+@item ELF
+The Executable and Linking Format. The object file format used on most
+modern Unix systems, including GNU/Linux, Solaris, Irix, and SVR4. Also
+used on many embedded systems.
+
+@item executable
+A program, with instructions and symbols, and perhaps dynamic linking
+information. Normally produced by a linker.
+
+@item LMA
+Load Memory Address. This is the address at which a section will be
+loaded. Compare with VMA, below.
+
+@item NLM
+NetWare Loadable Module. Used to describe the format of an object which
+be loaded into NetWare, which is some kind of PC based network server
+program.
+
+@item object file
+A binary file including machine instructions, symbols, and relocation
+information. Normally produced by an assembler.
+
+@item object file format
+The format of an object file. Typically object files and executables
+for a particular system are in the same format, although executables
+will not contain any relocation information.
+
+@item PE
+The Portable Executable format. This is the object file format used for
+Windows (specifically, Win32) object files. It is based closely on
+COFF, but has a few significant differences.
+
+@item PEI
+The Portable Executable Image format. This is the object file format
+used for Windows (specifically, Win32) executables. It is very similar
+to PE, but includes some additional header information.
+
+@item relocations
+Information used by the linker to adjust section contents. Also called
+relocs.
+
+@item section
+Object files and executable are composed of sections. Sections have
+optional data and optional relocation information.
+
+@item shared library
+A library of functions which may be used by many executables without
+actually being linked into each executable. There are several different
+implementations of shared libraries, each having slightly different
+features.
+
+@item symbol
+Each object file and executable may have a list of symbols, often
+referred to as the symbol table. A symbol is basically a name and an
+address. There may also be some additional information like the type of
+symbol, although the type of a symbol is normally something simple like
+function or object, and should be confused with the more complex C
+notion of type. Typically every global function and variable in a C
+program will have an associated symbol.
+
+@item target vector
+A set of functions which implement support for a particular object file
+format. The @samp{bfd_target} structure.
+
+@item Win32
+The current Windows API, implemented by Windows 95 and later and Windows
+NT 3.51 and later, but not by Windows 3.1.
+
+@item XCOFF
+The eXtended Common Object File Format. Used on AIX. A variant of
+COFF, with a completely different symbol table implementation.
+
+@item VMA
+Virtual Memory Address. This is the address a section will have when
+an executable is run. Compare with LMA, above.
+@end table
+
@node Index
@unnumberedsec Index
@printindex cp
projects / binutils-gdb.git / commitdiff