illumos: 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)