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General Programming Concepts: Writing and Debugging Programs

ELF Header

Some object file control structures can grow, because the ELF header contains their actual sizes. If the object file format changes, a program may encounter control structures that are larger or smaller than expected. Programs might therefore ignore extra information. The treatment of missing information depends on context and will be specified when and if extensions are defined.

ELF Header

#define EI_NIDENT 16
typedef struct {
        unsigned char   e_ident[EI_NIDENT];
        Elf32_Half      e_type;
        Elf32_Half      e_machine;
        Elf32_Word      e_version;
        Elf32_Addr      e_entry;
        Elf32_Off       e_phoff;
        Elf32_Off       e_shoff;
        Elf32_Word      e_flags;
        Elf32_Half      e_ehsize;
        Elf32_Half      e_phentsize;
        Elf32_Half      e_phnum;
        Elf32_Half      e_shentsize;
        Elf32_Half      e_shnum;
        Elf32_Half      e_shtrndx;
} Elf32_Ehdr;
typedef struct {
        unsigned char   e_ident[EI_NIDENT];
        Elf64_Half      e_type;
        Elf64_Half      e_machine;
        Elf64_Word      e_version;
        Elf64_Addr      e_entry;
        Elf64_Off       e_phoff;
        Elf64_Off       e_shoff;
        Elf64_Word      e_flags;
        Elf64_Half      e_ehsize;
        Elf64_Half      e_phentsize;
        Elf64_Half      e_phnum;
        Elf64_Half      e_shentsize;
        Elf64_Half      e_shnum;
        Elf64_Half      e_shtrndx;
} Elf64_Ehdr;

The initial bytes mark the file as an object file and provide machine-independent data with which to decode and interpret the file's contents. Complete descriptions appear below in ELF Identification.

This member identifies the object file type.


Name Value Meaning
ET_NONE 0 No file type
ET_REL 1 Relocatable file
ET_EXEC 2 Executable file
ET_DYN 3 Shared object file
ET_CORE 4 Core file
ET_LOOS 0xfe00 Operating system-specific
ET_HIOS 0xfeff Operating system-specific
ET_LOPROC 0xff00 Processor-specific
ET_HIPROC 0xffff Processor-specific

Although the core file contents are unspecified, type ET_CORE is reserved to mark the file. Values from ET_LOOS through ET_HIOS (inclusive) are reserved for operating system-specific semantics. Values from ET_LOPROC through ET_HIPROC (inclusive) are reserved for processor-specific semantics. If meanings are specified, the processor supplement explains them. Other values are reserved and will be assigned to new object file types as necessary.

This member's value specifies the required architecture for an individual file.


Name Value Meaning
EM_NONE 0 No machine
EM_M32 1 AT&T WE 32100
EM_386 3 Intel 80386
EM_68K 4 Motorola 68000
EM_88K 5 Motorola 88000
RESERVED 6 Reserved for future use
EM_860 7 Intel 80860
EM_MIPS 8 MIPS I Architecture
EM_S370 9 IBM System/370 Processor
EM_MIPS_RS3_LE 10 MIPS RS3000 Little-endian
RESERVED 11-14 Reserved for future use
EM_PARISC 15 Hewlett-Packard PA-RISC
RESERVED 16 Reserved for future use
EM_VPP500 17 Fujitsu VPP500
EM_SPARC32PLUS 18 Enhanced instruction set SPARC
EM_960 19 Intel 80960
EM_PPC 20 PowerPC
EM_PPC64 21 64-bit PowerPC
RESERVED 22-35 Reserved for future use
EM_V800 36 NEC V800
EM_FR20 37 Fujitsu FR20
EM_RH32 38 TRW RH-32
EM_RCE 39 Motorola RCE
EM_ARM 40 Advanced RISC Machines ARM
EM_ALPHA 41 Digital Alpha
EM_SH 42 Hitachi SH
EM_SPARCV9 43 SPARC Version 9
EM_TRICORE 44 Siemens Tricore embedded processor
EM_ARC 45 Argonaut RISC Core, Argonaut Technologies Inc.
EM_H8_300 46 Hitachi H8/300
EM_H8_300H 47 Hitachi H8/300H
EM_H8S 48 Hitachi H8S
EM_H8_500 49 Hitachi H8/500
EM_IA_64 50 Itanium-based platform
EM_MIPS_X 51 Stanford MIPS-X
EM_COLDFIRE 52 Motorola ColdFire
EM_68HC12 53 Motorola M68HC12
EM_MMA 54 Fujitsu MMA Multimedia Accelerator
EM_PCP 55 Siemens PCP
EM_NCPU 56 Sony nCPU embedded RISC processor
EM_NDR1 57 Denso NDR1 microprocessor
EM_STARCORE 58 Motorola Star*Core processor
EM_ME16 59 Toyota ME16 processor
EM_ST100 60 STMicroelectronics ST100 processor
EM_TINYJ 61 Advanced Logic Corp. TinyJ embedded processor family
Reserved 62-65 Reserved for future use
EM_FX66 66 Siemens FX66 microcontroller
EM_ST9PLUS 67 STMicroelectronics ST9+ 8/16 bit microcontroller
EM_ST7 68 STMicroelectronics ST7 8-bit microcontroller
EM_68HC16 69 Motorola MC68HC16 Microcontroller
EM_68HC11 70 Motorola MC68HC11 Microcontroller
EM_68HC08 71 Motorola MC68HC08 Microcontroller
EM_68HC05 72 Motorola MC68HC05 Microcontroller
EM_SVX 73 Silicon Graphics SVx
EM_ST19 74 STMicroelectronics ST19 8-bit microcontroller
EM_VAX 75 Digital VAX
EM_CRIS 76 Axis Communications 32-bit embedded processor
EM_JAVELIN 77 Infineon Technologies 32-bit embedded processor
EM_FIREPATH 78 Element 14 64-bit DSP Processor
EM_ZSP 79 LSI Logic 16-bit DSP Processor
EM_MMIX 80 Donald Knuth's educational 64-bit processor
EM_HUANY 81 Harvard University machine-independent object files
EM_PRISM 82 SiTera Prism

Other values are reserved and will be assigned to new machines as necessary. Processor-specific ELF names use the machine name to distinguish them. For example, the flags mentioned below use the prefix EF_; a flag named WIDGET for the EM_XYZ machine would be called EF_XYZ_WIDGET.

This member identifies the object file version.


Name Value Meaning
EV_NONE 0 Invalid version
EV_CURRENT 1 Current version

The value 1 signifies the original file format; extensions will create new versions with higher numbers. Although the value of EV_CURRENT is shown as 1 in the previous table, it will change as necessary to reflect the current version number.

This member gives the virtual address to which the system first transfers control, thus starting the process. If the file has no associated entry point, this member holds zero.

This member holds the program header table's file offset in bytes. If the file has no program header table, this member holds zero.

This member holds the section header table's file offset in bytes. If the file has no section header table, this member holds zero.

This member holds processor-specific flags associated with the file. Flag names take the form EF_machine_flag.

This member holds the ELF header's size in bytes.

This member holds the size in bytes of one entry in the file's program header table; all entries are the same size.

This member holds the number of entries in the program header table. Thus the product of e_phentsize and e_phnum gives the table's size in bytes. If a file has no program header table, e_phnum holds the value zero.

This member holds a section header's size in bytes. A section header is one entry in the section header table; all entries are the same size.

This member holds the number of entries in the section header table. Thus the product of e_shentsize and e_shnum gives the section header table's size in bytes. If a file has no section header table, e_shnum holds the value zero.

This member holds the section header table index of the entry associated with the section name string table. If the file has no section name string table, this member holds the value SHN_UNDEF. See Sections and String Table for more information.

ELF Identification

As mentioned above, ELF provides an object file framework to support multiple processors, multiple data encodings, and multiple classes of machines. To support this object file family, the initial bytes of the file specify how to interpret the file, independent of the processor on which the inquiry is made and independent of the file's remaining contents.

The initial bytes of an ELF header (and an object file) correspond to the e_ident member.

e_ident[] Identification Indexes

Name Value Purpose
EI_MAG0 0 File identification
EI_MAG1 1 File identification
EI_MAG2 2 File identification
EI_MAG3 3 File identification
EI_CLASS 4 File class
EI_DATA 5 Data encoding
EI_VERSION 6 File version
EI_OSABI 7 Operating system/ABI identification
EI_PAD 9 Start of padding bytes
EI_NIDENT 16 Size of e_ident[]

These indexes access bytes that hold the following values.

A file's first 4 bytes hold a magic number, identifying the file as an ELF object file.


Name Value Position
ELFMAG0 0x7f e_ident[EI_MAG0]
ELFMAG1 'E' e_ident[EI_MAG1]
ELFMAG2 'L' e_ident[EI_MAG2]
ELFMAG3 'F' e_ident[EI_MAG3]

The next byte, e_ident[EI_CLASS], identifies the file's class, or capacity.
Name Value Meaning
ELFCLASSNONE 0 Invalid class
ELFCLASS32 1 32-bit objects
ELFCLASS64 2 64-bit objects

The file format is designed to be portable among machines of various sizes, without imposing the sizes of the largest machine on the smallest. The class of the file defines the basic types used by the data structures of the object file container itself. The data contained in object file sections may follow a different programming model. If so, the processor supplement describes the model used.

Class ELFCLASS32 supports machines with 32-bit architectures. It uses the basic types defined in the table labeled 32-Bit Data Types.

Class ELFCLASS64 supports machines with 64-bit architectures. It uses the basic types defined in the table labeled 64-Bit Data Types.

Other classes will be defined as necessary, with different basic types and sizes for object file data.

Byte e_ident[EI_DATA] specifies the encoding of both the data structures used by object file container and data contained in object file sections.

The following encoding are currently defined.
Name Value Meaning
ELFDATANONE 0 Invalid data encoding
ELFDATA2LSB 1 See below
ELFDATA2MSB 2 See below

Other values are reserved and will be assigned to new encodings as necessary.

NOTE: Primarily for the convenience of code that looks at the ELF file at runtime, the ELF data structures are intended to have the same byte order as that of the running program.

Byte e_ident[EI_VERSION] specifies the ELF header version number. Currently, this value must be EV_CURRENT, as explained above for e_version.

Byte e_ident[EI_OSABI] identifies the operating system and ABI to which the object is targeted. Some fields in other ELF structures have flags and values that have operating system and/or ABI specific meanings; the interpretation of those fields is determined by the value of this byte. The value of this byte must be interpreted differently for each machine. That is, each value for the e_machine field determines a set of values for the EI_OSABI byte. Values are assigned by the ABI processor supplement for each machine. If the processor supplement does not specify a set of values, the value 0 shall be used and indicates unspecified.

Byte e_ident[EI_ABIVERSION] identifies the version of the ABI to which the object is targeted. This field is used to distinguish among incompatible versions of an ABI. The interpretation of this version number is dependent on the ABI identified by the EI_OSABI field. If no values are specified for the EI_OSABI field by the processor supplement or no version values are specified for the ABI determined by a particular value of the EI_OSABI byte, the value 0 shall be used for the EI_ABIVERSION byte; it indicates unspecified.

This value marks the beginning of the unused bytes in e_ident. These bytes are reserved and set to zero; programs that read object files should ignore them. The value of EI_PAD will change in the future if currently unused bytes are given meanings.

A file's data encoding specifies how to interpret the basic objects in a file. Class ELFCLASS32 files use objects that occupy 1, 2, and 4 bytes. Class ELFCLASS64 files use objects that occupy 1, 2, 4, and 8 bytes. Under the defined encodings, objects are represented as shown below.

Encoding ELFDATA2LSB specifies 2's complement values, with the least significant byte occupying the lowest address.



02 01


04 03 02 01


08 07 06 05 04 03 02 01


Data Encoding ELFDATA2LSB, byte address zero on the left

Encoding ELFDATA2MSB specifies 2's complement values, with the most significant byte occupying the lowest address.



01 02


01 02 03 04


01 02 03 04 05 06 07 08


Data Encoding ELFDATA2MSB, byte address zero on the left

Machine Information (Processor-Specific)

NOTE: This section requires processor-specific information. The ABI supplement for the desired processor describes the details.

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