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Technical Reference: Base Operating System and Extensions, Volume 1

Introduction to ELF Subroutines

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 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 the elf_getscn subroutine]. 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.

Special Notice

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/elf_NAME.h where NAME matches the processor name as used in the ELF file header.

Symbol Processor
386 Intel 80386
IA_64 Itanium-based platform

To illustrate, a program could use the following code to see the processor-specific information for the Itanium-based platform:

   #include <libelf.h>
   #include <sys/elf_IA_64.h>

Without the sys/elf_IA_64.h definition, only the common ELF information would be visible.

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. The 64-bit class similarly supports architectures in which a 64-bit object is required to represent addresses, file sizes, etc.

32-bit Data Types 64-bit Data Types Purpose
Elf32_Addr Elf64_Addr Unsigned address
Elf32_Half Elf64_Half Unsigned medium integer
Elf32_Off Elf64_Off Unsigned file offset
Elf32_Sword Elf64_Sword Signed integer
Elf32_Word ELF_64_Word Unsigned integer
unsigned char unsigned char Unsigned small integer

Elf64_Sxword Signed long integer

Elf64_Xword Unsigned long 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, byte order, and so forth) 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 and elf32_fsize 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,these version are:

  1. an application program knows about a particular version by virtue of being compiled with certain header files.
  2. the access library similarly is compiled with header files that control what versions it understands.
  3. 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.

To accommodate these differences, a program must use elf_version 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 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 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_xlatetof or elf64_xlatetof 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 or elf64_name Service names with an embedded class, 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 or Elf64_Type Class-dependent data types have an embedded class name. Here the class is denoted by 32 or 64.
ELF_C_CMD Several functions take commands that control their actions. These values are members of the Elfelf_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 or ELF64_FSZ_TYPE These constants give the file sizes in bytes of the basic ELF types for the specified class of files. See elf_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 deals with multiple types, names of this form specify the desired TYPE. Thus, for example, ELF_T_EHDR is directly related to Elf32_Ehdr or Elf64_Ehdr. These values are members of the Elf_Type enumeration; they range from zero through ELF_T_NUM-1.

Related Information

The elf_version (elf_version Subroutine) subroutine.

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