path: root/usr/src/man/man3elf/elf.3elf

'\" te
.\"  Copyright 1989 AT&T  All Rights Reserved  Copyright (c) 2001 Sun Microsystems, Inc.  All Rights Reserved.
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.TH ELF 3ELF "Jul 23, 2001"
.SH NAME
elf \- object file access library
.SH SYNOPSIS
.LP
.nf
cc [ \fIflag\fR ... ] \fIfile\fR ... \fB-lelf\fR [ \fIlibrary\fR ... ]
\fB#include <libelf.h>\fR
.fi

.SH DESCRIPTION
.sp
.LP
Functions in the \fBELF\fR access library let a program manipulate \fBELF\fR
(Executable and Linking Format) object files, archive files, and archive
members. The header provides type and function declarations for all library
services.
.sp
.LP
Programs communicate with many of the higher-level routines using an \fIELF
descriptor\fR. That is, when the program starts working with a file,
\fBelf_begin\fR(3ELF) creates an \fBELF\fR descriptor through which the program
manipulates the structures and information in the file. These \fBELF\fR
descriptors can be used both to read and to write files. After the program
establishes an \fBELF\fR descriptor for a file, it may then obtain
\fIsection\fR \fIdescriptors\fR to manipulate the sections of the file (see
\fBelf_getscn\fR(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 \fBELF\fR descriptor, just as a section
belongs to a file. Finally, \fIdata\fR \fIdescriptors\fR are available through
section descriptors, allowing the program to manipulate the information
associated with a section. A data descriptor ``belongs'' to a section
descriptor.
.sp
.LP
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 \fBELF\fR 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.
.SS "File Classes"
.sp
.LP
\fBELF\fR 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 \fIclass\fR, 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:
.sp

.sp
.TS
box;
c | c
l | l .
Name	Purpose
_
\fBElf32_Addr\fR	Unsigned address
_
\fBElf32_Half\fR	Unsigned medium integer
_
\fBElf32_Off\fR	Unsigned file offset
_
\fBElf32_Sword\fR	Signed large integer
_
\fBElf32_Word\fR	Unsigned large integer
_
\fBunsigned char\fR	Unsigned small integer
.TE

.sp
.LP
The 64\(mibit class works the same as the 32\(mibit 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.
.SS "Data Representation"
.sp
.LP
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.
.sp
.LP
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 \fBelf32_xlatetof\fR(3ELF) and
\fBelf32_fsize\fR(3ELF) for more information.
.sp
.LP
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. \fBelf_begin()\fR 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.
.SS "ELF Versions"
.sp
.LP
Object file versions allow \fBELF\fR to adapt to new requirements. \fIThree\fR
\fIindependent\fR \fIversions\fR 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
\fBELF\fR object file holds a value identifying its version, determined by the
\fBELF\fR version known by the file's creator. Ideally, all three versions
would be the same, but they may differ.
.sp
.LP
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.
.sp
.LP
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.
.sp
.LP
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.
.sp
.LP
To accommodate these differences, a program must use \fBelf_version\fR(3ELF) to
pass its version to the library, thus establishing the \fIworking version\fR
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.
.SS "System Services"
.sp
.LP
As mentioned above, \fBelf_begin()\fR and related routines provide a
higher-level interface to \fBELF\fR 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 \fBelf_cntl\fR(3ELF) to retrieve all necessary data
from the file, allowing the program to close the file descriptor and reuse it.
.sp
.LP
Although the \fBelf_begin()\fR interfaces are convenient and efficient for many
programs, they might be inappropriate for some. In those cases, an application
may invoke the \fBelf32_xlatetom\fR(3ELF) or \fBelf32_xlatetof\fR(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.
.SS "Library Names"
.sp
.LP
Names associated with the library take several forms.
.sp
.ne 2
.na
\fB\fBelf_\fR\fIname\fR\fR
.ad
.RS 18n
These class-independent names perform some service, \fIname\fR, for the
program.
.RE

.sp
.ne 2
.na
\fB\fBelf32_\fR\fIname\fR\fR
.ad
.RS 18n
Service names with an embedded class, \fB32\fR here, indicate they work only
for the designated class of files.
.RE

.sp
.ne 2
.na
\fB\fBElf_\fR\fIType\fR\fR
.ad
.RS 18n
Data types can be class-independent as well, distinguished by \fIType\fR.
.RE

.sp
.ne 2
.na
\fB\fBElf32_\fR\fIType\fR\fR
.ad
.RS 18n
Class-dependent data types have an embedded class name, \fB32\fR here.
.RE

.sp
.ne 2
.na
\fB\fBELF_C_\fR\fICMD\fR\fR
.ad
.RS 18n
Several functions take commands that control their actions. These values are
members of the \fBElf_Cmd\fR enumeration; they range from zero through
\fBELF_C_NUM\fR\(mi1\fB\&.\fR
.RE

.sp
.ne 2
.na
\fB\fBELF_F_\fR\fIFLAG\fR\fR
.ad
.RS 18n
Several functions take flags that control library status and/or actions. Flags
are bits that may be combined.
.RE

.sp
.ne 2
.na
\fB\fBELF32_FSZ_\fR\fITYPE\fR\fR
.ad
.RS 18n
These constants give the file sizes in bytes of the basic \fBELF\fR types for
the 32-bit class of files. See \fBelf32_fsize()\fR for more information.
.RE

.sp
.ne 2
.na
\fB\fBELF_K_\fR\fIKIND\fR\fR
.ad
.RS 18n
The function \fBelf_kind()\fR identifies the \fIKIND\fR of file associated with
an \fBELF\fR descriptor. These values are members of the \fBElf_Kind\fR
enumeration; they range from zero through \fBELF_K_NUM\fR\(mi1\fB\&.\fR
.RE

.sp
.ne 2
.na
\fB\fBELF_T_\fR\fITYPE\fR\fR
.ad
.RS 18n
When a service function, such as \fBelf32_xlatetom()\fR or
\fBelf32_xlatetof()\fR, deals with multiple types, names of this form specify
the desired \fITYPE\fR. Thus, for example, \fBELF_T_EHDR\fR is directly related
to \fBElf32_Ehdr\fR. These values are members of the \fBElf_Type\fR
enumeration; they range from zero through \fBELF_T_NUM\fR\(mi1\fB\&.\fR
.RE

.SH EXAMPLES
.LP
\fBExample 1 \fRAn interpretation of elf file.
.sp
.LP
The basic interpretation of an ELF file consists of:

.RS +4
.TP
.ie t \(bu
.el o
opening an ELF object file
.RE
.RS +4
.TP
.ie t \(bu
.el o
obtaining an ELF descriptor
.RE
.RS +4
.TP
.ie t \(bu
.el o
analyzing the file using the descriptor.
.RE
.sp
.LP
The following example opens the file, obtains the ELF descriptor,  and prints
out the names of each section in the file.

.sp
.in +2
.nf
#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\en", cnt,
        (char *)data->d_buf + shdr->sh_name);
    }
}        /* end main */

static void
failure()
{
    (void) fprintf(stderr, "%s\en", elf_errmsg(elf_errno()));
    exit(1);
}
.fi
.in -2

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Stable
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.sp
.LP
\fBar.h\fR(3HEAD), \fBelf32_checksum\fR(3ELF), \fBelf32_fsize\fR(3ELF),
\fBelf32_getshdr\fR(3ELF), \fBelf32_xlatetof\fR(3ELF), \fBelf_begin\fR(3ELF),
\fBelf_cntl\fR(3ELF), \fBelf_errmsg\fR(3ELF), \fBelf_fill\fR(3ELF),
\fBelf_getarhdr\fR(3ELF), \fBelf_getarsym\fR(3ELF), \fBelf_getbase\fR(3ELF),
\fBelf_getdata\fR(3ELF), \fBelf_getident\fR(3ELF), \fBelf_getscn\fR(3ELF),
\fBelf_hash\fR(3ELF), \fBelf_kind\fR(3ELF), \fBelf_memory\fR(3ELF),
\fBelf_rawfile\fR(3ELF), \fBelf_strptr\fR(3ELF), \fBelf_update\fR(3ELF),
\fBelf_version\fR(3ELF), \fBgelf\fR(3ELF), \fBlibelf\fR(3LIB),
\fBattributes\fR(5), \fBlfcompile\fR(5)
.sp
.LP
\fIANSI C Programmer's Guide\fR
.SS "SPARC only"
.sp
.LP
\fBa.out\fR(4)
.SH NOTES
.sp
.LP
Information in the \fBELF\fR headers is separated into common parts and
processor-specific parts. A program can make a processor's information
available by including the appropriate header:
\fB<sys/elf_\fR\fINAME\fR\fB\&.h>\fR where \fINAME\fR matches the processor
name as used in the \fBELF\fR file header.
.sp

.sp
.TS
box;
c | c
l | l .
Name	Processor
_
\fBM32\fR	AT&T WE 32100
_
\fBSPARC\fR	SPARC
_
\fB386\fR	Intel 80386, 80486, Pentium
.TE

.sp
.LP
Other processors will be added to the table as necessary.
.sp
.LP
To illustrate, a program could use the following code to ``see'' the
processor-specific information for the SPARC based system.
.sp
.in +2
.nf
\fB#include <libelf.h>
#include <sys/elf_SPARC.h>\fR
.fi
.in -2

.sp
.LP
Without the \fB<sys/elf_SPARC.h>\fR definition, only the common \fBELF\fR
information would be visible.
.sp
.LP
A program could use the following code to ``see'' the processor-specific
information for the Intel 80386:
.sp
.in +2
.nf
\fB#include <libelf.h>
#include <sys/elf_386.h>\fR
.fi
.in -2

.sp
.LP
Without the \fB<sys/elf_386.h>\fR definition, only the common \fBELF\fR
information would be visible.
.sp
.LP
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.