ELF Object File Format

2. ELF Header¶

The ELF header resides at the beginning of an ELF file. It identifies the file as an ELF file and contains the information necessary for interpreting the contents of the file and locating the other components of the file.

2.1. Contents of the ELF Header¶

Listing 2.1 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_shstrndx;
} 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_shstrndx;
} Elf64_Ehdr;

e_ident

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 Section 2.2, ELF Identification.

e_type

This member identifies the object file type.

Table 2.1 Object File Types¶
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 psABI supplement explains them. Other values are reserved and will be assigned to new object file types as necessary.

e_machine

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

See Appendix A for currently-assigned values for this field. 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.

e_version

This member identifies the object file version.

Table 2.2 Object File Version Numbers¶
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.

e_entry

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.

e_phoff

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

e_shoff

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

e_flags

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

e_ehsize

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

e_phentsize

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

e_phnum

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.

e_shentsize

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.

e_shnum

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.

If the number of sections is greater than or equal to SHN_LORESERVE (0xff00), this member has the value zero and the actual number of section header table entries is contained in the sh_size field of the section header at index 0. (Otherwise, the sh_size member of the initial entry contains 0.)

e_shstrndx

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 Chapter 3, Sections, and Chapter 4, String Table, for more information.

If the section name string table section index is greater than or equal to SHN_LORESERVE (0xff00), this member has the value SHN_XINDEX (0xffff) and the actual index of the section name string table section is contained in the sh_link field of the section header at index 0. (Otherwise, the sh_link member of the initial entry contains 0.)

2.2. 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.

Table 2.3 `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_ABIVERSION`	`8`	ABI version
`EI_PAD`	`9`	Start of padding bytes
`EI_NIDENT`	`16`	Size of `e_ident[]`

These indexes access bytes that hold the following values.

EI_MAG0 to EI_MAG3

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

Table 2.4 ELF Magic Numbers¶
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]`

EI_CLASS

The next byte, e_ident[EI_CLASS], identifies the file’s class, or capacity.

Table 2.5 ELF Class¶
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 psABI supplement describes the model used.

Class ELFCLASS32 supports machines with 32-bit architectures. It uses the basic types defined in Table 1.1 in Section 1.2, Data Representation.

Class ELFCLASS64 supports machines with 64-bit architectures. It uses the basic types defined in Table 1.2 in Section 1.2, Data Representation.

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

EI_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 encodings are currently defined.

Table 2.6 ELF Data Encoding¶
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.

EI_VERSION

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

EI_OSABI

Byte e_ident[EI_OSABI] identifies the OS- or ABI-specific ELF extensions used by this file. 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. If the object file does not use any extensions, it is recommended that this byte be set to 0. If the value for this byte is 64 through 255, its meaning depends on the value of the e_machine header member. The psABI supplement for an architecture can define its own associated set of values for this byte in this range. If the psABI supplement does not specify a set of values, one of the values defined in Appendix B shall be used, where 0 (ELFOSABI_NONE) can also be taken to mean unspecified.

EI_ABIVERSION

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 psABI 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.

EI_PAD

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.

2.3. Data Encoding¶

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. Encoding ELFDATA2MSB specifies 2’s complement values, with the most significant byte occupying the lowest address. See Figure 2.1.