Tribblix: manual page: elf.3elf

ELF(3ELF) ELF Library Functions ELF(3ELF)

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
(Executable and Linking Format) object files, archive files, and
archive members. The header 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(3ELF) 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
elf_getscn(3ELF)). 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 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 associated data bytes. This sharing avoids the
space overhead for duplicate buffers and the 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 on, 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 |
+--------------+-------------------------+

The 64-bit class works the same as the 32-bit class, substituting 64
for 32 as necessary. 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 Representation

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 headers work
on the native machine, which may have different data encodings (size,
byte order, and so on) 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
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. See elf32_xlatetof(3ELF) and elf32_fsize(3ELF) 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. 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 headers. 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 may differ.

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.

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.

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(3ELF) to pass its version to the library, thus
establishing the working version 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, elf_begin() 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 elf_cntl(3ELF) 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, they might be inappropriate for some. In those cases,
an application may invoke the elf32_xlatetom(3ELF) or
elf32_xlatetof(3ELF) 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.

elf_name
These class-independent names perform some service,
name, for the program.

elf32_name
Service names with an embedded class, 32 here,
indicate 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
elf32_fsize() for more information.

ELF_K_KIND
The function elf_kind() 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 elf32_xlatetom()
or elf32_xlatetof(), deals with multiple types,
names of this form specify the desired TYPE. Thus,
for example, ELF_T_EHDR is directly related to
Elf32_Ehdr. These values are members of the
Elf_Type enumeration; they range from zero through
ELF_T_NUM-1.

EXAMPLES

Example 1: An interpretation of elf file.

The basic interpretation of an ELF file consists of:

o opening an ELF object file

o obtaining an ELF descriptor

o analyzing the file using the descriptor.

The following example opens the file, obtains the ELF descriptor,
and prints out the names of each section in the file.

#include <fcntl.h>
#include <stdio.h>
#include <libelf.h>
#include <stdlib.h>
#include <string.h>
static void failure(void);
void
main(int argc, char ** argv)
{
Elf32_Shdr * shdr;
Elf32_Ehdr * ehdr;
Elf * elf;
Elf_Scn * scn;
Elf_Data * data;
int fd;
unsigned int cnt;

/* Open the input file */
if ((fd = open(argv[1], O_RDONLY)) == -1)
exit(1);

/* Obtain the ELF descriptor */
(void) elf_version(EV_CURRENT);
if ((elf = elf_begin(fd, ELF_C_READ, NULL)) == NULL)
failure();

/* Obtain the .shstrtab data buffer */
if (((ehdr = elf32_getehdr(elf)) == NULL) ||
((scn = elf_getscn(elf, ehdr->e_shstrndx)) == NULL) ||
((data = elf_getdata(scn, NULL)) == NULL))
failure();

/* Traverse input filename, printing each section */
for (cnt = 1, scn = NULL; scn = elf_nextscn(elf, scn); cnt++) {
if ((shdr = elf32_getshdr(scn)) == NULL)
failure();
(void) printf("[%d] %s\n", cnt,
(char *)data->d_buf + shdr->sh_name);
}
} /* end main */

static void
failure()
{
(void) fprintf(stderr, "%s\n", elf_errmsg(elf_errno()));
exit(1);
}

ATTRIBUTES

NOTES

Information in the ELF headers is separated into common parts and
processor-specific parts. A program can make a processor's
information available by including the appropriate header:
<sys/elf_NAME.h> where NAME matches the processor name as used in the
ELF file header.

+------+-----------------------------+
|Name | Processor |
+------+-----------------------------+
|M32 | AT&T WE 32100 |
+------+-----------------------------+
|SPARC | SPARC |
+------+-----------------------------+
|386 | Intel 80386, 80486, Pentium |
+------+-----------------------------+

Other processors will be added to the table as necessary.

To illustrate, a program could use the following code to ``see'' the
processor-specific information for the SPARC based system.

#include <libelf.h>
#include <sys/elf_SPARC.h>

Without the <sys/elf_SPARC.h> definition, only the common ELF
information would be visible.

A program could use the following code to ``see'' the processor-
specific information for the Intel 80386:

#include <libelf.h>
#include <sys/elf_386.h>

Without the <sys/elf_386.h> definition, only the common ELF
information would be visible.

Although reading the objects is rather straightforward,
writing/updating them can corrupt the shared offsets among sections.
Upon creation, relationships are established among the sections that
must be maintained even if the object's size is changed.

July 23, 2001 ELF(3ELF)