elf(3E) elf(3E)
NAME
elf - object file access library
SYNOPSIS
cc [flag ...] file ... -lelf [library ...]
#include <libelf.h>
DESCRIPTION
Functions in the ELF access library let a program manipulate ELF (Exe-
cutable 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, elfbegin creates an ELF 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 may then obtain
section descriptors to manipulate the sections of the file [see
elfgetscn(3E)]. Sections hold the bulk of an object file's real
information, such as text, data, the symbol table, and so on. A sec-
tion descriptor belongs to a particular ELF descriptor, just as a sec-
tion 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 may 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 asso-
ciated data bytes. This sharing avoids the space overhead for dupli-
cate buffers and the performance overhead for copying data unneces-
sarily.
FILE CLASSES
ELF provides a framework in which to define a family of object files,
supporting multiple processors and architectures. An important dis-
tinction 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 or file sizes as in the following example:
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elf(3E) elf(3E)
_________________________________________
| Name | Purpose |
|||
| Elf32Addr | Unsigned address |
| Elf32Half | Unsigned medium integer|
| Elf32Off | Unsigned file offset |
| Elf32Sword | Signed large integer |
| Elf32Word | 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 may have different data encodings (size or byte
order) than the target machine. Although native memory objects should
be at least as big as the file objects (to avoid information loss),
they may be bigger if that is more natural for the host machine.
Translation facilities exist to convert between file and memory repre-
sentations. 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.
See elfxlate(3E) and elffsize(3E) for more information.
Programs may translate data explicitly, taking full control over the
object file layout and semantics. If the program prefers not to have
and exercise complete control, the library provides a higher-level
interface that hides many object file details. elfbegin 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 applica-
tion program knows about a particular version by virtue of being com-
piled with certain header files. Second, the access library similarly
is compiled with header files that control what versions it under-
stands. Third, an ELF object file holds a value identifying its ver-
sion, determined by the ELF version known by the file's creator.
Ideally, all three versions would be the same, but they may differ.
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elf(3E) elf(3E)
- If a program's version is newer than the access library, the pro-
gram might use information unknown to the library. Translation rou-
tines might not work properly, leading to undefined behavior. This
condition merits installing a new library.
- The library's version might be newer than the program's and the
file's. The library understands old versions, thus avoiding compa-
tibility problems in this case.
- 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 under-
stands the file's version.
To accommodate these differences, a program must use elfversion to
pass its version to the library, thus establishing the "working ver-
sion" for the process. Using this, the library accepts data from and
presents data to the program in the proper representations. 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, elfbegin and related routines provide a higher-
level interface to ELF files, performing input and output on behalf of
the application program. These routines assume 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
perform subsequent operations on the memory copy. Programs that wish
to read or write large object files with this model must execute 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
elfcntl to retrieve all necessary data from the file, allowing the
program to close the file descriptor and reuse it.
Although the elfbegin interfaces are convenient and efficient for
many programs, they might be inappropriate for some. In those cases,
an application may invoke the elfxlate data translation routines
directly. These routines perform no input or output, leaving that as
the application's responsibility. By assuming a larger share of the
job, an application controls its input and output model.
LIBRARY NAMES
Names associated with the library take several forms.
elfname These class-independent names perform some service,
name, for the program.
elf32name Service names with an embedded class, 32 here,
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indicate they work only for the designated class of
files.
ElfType Data types can be class-independent as well, dis-
tinguished by Type.
Elf32Type Class-dependent data types have an embedded class
name, 32 here.
ELFCCMD Several functions take commands that control their
actions. These values are members of the ElfCmd
enumeration; they range from zero through ELFCNUM-1.
ELFFFLAG Several functions take flags that control library
status and/or actions. Flags are bits that may be com-
bined.
ELF32FSZTYPE These constants give the file sizes in bytes of the
basic ELF types for the 32-bit class of files. See
elffsize for more information.
ELFKKIND The function elfkind identifies the KIND of file
associated with an ELF descriptor. These values are
members of the ElfKind enumeration; they range from
zero through ELFKNUM-1.
ELFTTYPE When a service function, such as elfxlate, deals with
multiple types, names of this form specify the desired
TYPE. Thus, for example, ELFTEHDR is directly
related to Elf32Ehdr. These values are members of the
ElfType enumeration; they range from zero through
ELFTNUM-1.
NOTES
Information in the ELF header files is separated into common parts and
processor-specific parts. A program can make a processor's information
available by including the appropriate header file: <sys/elfNAME.h>
where NAME matches the processor name as used in the ELF file header.
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elf(3E) elf(3E)
_________________________
| Symbol| Processor |
|||
| M32 | AT&T WE 32100 |
| SPARC | SPARC |
| 386 | Intel 80386 |
| 486 | Intel 80486 |
| 860 | Intel 80860 |
| 68K | Motorola 68000|
| 88K | Motorola 88000|
| R2000 | MIPS R2000 |
|_______|________________|
Other processors will be added to the table as necessary. To illus-
trate, a program could use the following code to see the processor-
specific information for the MIPS R2000.
#include <libelf.h>
#include <sys/elfR2000.h>
Without the <sys/elfR2000.h> definition, only the common ELF informa-
tion would be visible.
SEE ALSO
elfbegin(3E), elfcntl(3E), elfend(3E), elffill(3E), elfflag(3E),
elffsize(3E), elfgetarhdr(3E), elfgetarsym(3E), elfgetbase(3E),
elfgetdata(3E), elfgetehdr(3E), elfgetident(3E), elfgetphdr(3E),
elfgetscn(3E), elfgetshdr(3E), elfhash(3E), elfkind(3E),
elfnext(3E), elfrand(3E), elfrawfile(3E), elfstrptr(3E),
elfupdate(3E), elfversion(3E), elfxlate(3E), a.out(4), ar(4).
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