Functions in the
ELF access library let a program manipulate
ELF (Executable and Linking Format) object files,
archive files, and archive members.
The header file provides type and function declarations
for all library services.
Programs communicate with many of the higher-level routines using an
``ELF descriptor''.
That is, when the program starts working with a file,
elf_begin(S)
creates a descriptor through which the program
manipulates the structures and information in the file.
These
ELF descriptors can be used both to read and to write files.
After the program establishes an
ELF descriptor for a file, it can obtain
section descriptors
to manipulate the sections of the file (see
elf_getscn(S)).
Sections hold the bulk of an object file's real information,
such as text, data, the symbol table, and so on.
A section descriptor ``belongs'' to a particular
ELF descriptor, just as a section belongs to a file. Finally, ``data descriptors''
are available through section descriptors, allowing
the program to manipulate the information associated with a section.
A data descriptor ``belongs'' to a section descriptor.
Descriptors provide private handles to a file and its pieces.
In other words, a data descriptor is associated with one section
descriptor, which is associated with one
ELF descriptor, which is associated with one file.
Although descriptors are private, they give access to data that might be shared.
Consider programs that combine input files,
using incoming data to create or update another file.
Such a program might get data descriptors for an input and an output section.
It then could update the output descriptor to reuse the input descriptor's data.
That is, the descriptors are distinct, but they could
share the associated data bytes.
This sharing avoids space overhead for duplicate buffers
and performance overhead for copying data unnecessarily.
File classes
ELF provides a framework in which to define a family of
object files, supporting multiple processors and architectures.
An important distinction among object files is the
class, or capacity, of the file.
The 32-bit class supports architectures in which
a 32-bit object can represent addresses, file sizes, and so forth,
as in the following:
Name
Purpose
Elf32_Addr
Unsigned address
Elf32_Half
Unsigned medium integer
Elf32_Off
Unsigned file offset
Elf32_Sword
Signed large integer
Elf32_Word
Unsigned large integer
unsigned char
Unsigned small integer
Other classes will be defined as necessary, to support
larger (or smaller) machines.
Some library services deal only with data objects for
a specific class, while others are class-independent.
To make this distinction clear, library function names
reflect their status, as described below.
Data representations
Conceptually, two parallel sets of objects
support cross compilation environments.
One set corresponds to file contents, while
the other set corresponds to the native memory image of
the program manipulating the file.
Type definitions supplied by the header files
work on the native machine, which might have different
data encodings (size, byte order, and so forth) than the target machine.
Native memory objects should be at least as big as the file objects
(to avoid information loss),
but they can be bigger if that is more natural for the host machine.
Translation facilities exist to convert between
file and memory representations.
Some library routines convert data automatically,
while others leave conversion as the program's responsibility.
Either way, programs that create object files must write
file-typed objects to those files; programs that read
object files must take a similar view.
For more information, see
elf_xlate(S)
and
elf_fsize(S).
Programs can translate data explicitly,
taking full control over the object file layout and semantics.
If the programmer prefers not to exercise complete control,
the library provides a higher-level interface
that hides many object file details.
elf_begin( )
and related functions let a program deal with the native memory types,
converting between memory objects and their file equivalents
automatically when reading or writing an object file.
ELF versions
Object file versions allow
ELF to adapt to new requirements.
Three independent versions can be important to a program:
First, an application program knows about a particular version
by virtue of being compiled with certain header files.
Second, the access library similarly is compiled with header
files that control what versions it understands.
Third, an ELF object file holds a value identifying its version,
determined by the ELF version known by the file's creator.
Ideally, all three versions would be the same, but they can differ,
and that can present problems:
The library's version might be newer than the program's and the file's.
The library understands old versions,
thus avoiding compatibility problems in this case.
If a program's version is newer than the access library,
the program might use information unknown to the library.
Translation routines might not work properly, leading to undefined behavior.
This condition merits installing a new library.
Finally, a file's version might be newer than either the program
or the library understands.
The program might or might not be able to process the
file properly, depending on whether the file has
extra information and whether that information can be safely ignored.
Again, the safe alternative is to install a new
library that understands the file's version.
To accommodate these differences, a program must use
elf_version(S)
to pass its version to the library, thus establishing the
"working version"
for the process.
Using the working version, the library represents data properly
when transferring it to and from the program.
When the library reads object files,
it uses each file's version to interpret the data.
When writing files or converting memory types to the file equivalents,
the library uses the program's working version for the file data.
System services
As mentioned above,
elf_begin( )
and related routines provide a higher-level interface to
ELF files,
handling input and output on behalf of the application program.
These routines assume that a program can hold entire files in memory,
without explicitly using temporary files.
When reading a file,
the library routines bring the data into memory
and do subsequent operations on the memory copy.
Programs that read or write large object files with this model
must run on a machine with a large process virtual address space.
If the underlying operating system limits the number of open files,
a program can use
elf_cntl(S)
to retrieve all necessary data from the file,
allowing the program to close the file descriptor and reuse it.
Although the
elf_begin( )
interfaces are convenient and efficient for many programs.
If not, an application can invoke the
elf_xlate( )
data translation routines directly.
These routines do no input or output,
leaving that as the application's responsibility.
By assuming a larger share of the job,
an application controls its own input and output model.
Library names
Names associated with the library take several forms.
elf_name
These class-independent names do some service,
name,
for the program.
elf32_name
Service names with an embedded class,
32
here, indicate that they work only for the designated class of files.
Elf_Type
Data types can be class-independent as well, distinguished by
Type.
Elf32_Type
Class-dependent data types have an embedded class name,
32
here.
ELF_C_CMD
Several functions take commands that control their actions.
These values are members of the
Elf_Cmd
enumeration; they range from zero through
ELF_C_NUM-1.
ELF_F_FLAG
Several functions take flags that control library status and/or actions.
Flags are bits that may be combined.
ELF32_FSZ_TYPE
These constants give the file sizes in bytes of the basic
ELF
types for the 32-bit class of files. See
elf_fsize( )
for more information.
ELF_K_KIND
The function
elf_kind(S)
identifies the
KIND
of file associated with an
ELF descriptor.
These values are members of the
Elf_Kind
enumeration; they range from zero through
ELF_K_NUM-1.
ELF_T_TYPE
When a service function, such as
elf_xlate( ),
deals with multiple types, names of this form specify the desired
TYPE.
Thus, for example,
ELF_T_EHDR
is directly related to
Elf32_Ehdr(S).
These values are members of the
Elf_Type
enumeration; they range from zero through
ELF_T_NUM-1.
Diagnostics
Error conditions are identified through the routine
elf_error(S).