ELF Identification

As mentioned above, ELF provides an object file framework to support multiple processors, multiple data encoding, 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 7-7 e_ident[ ] Identification Index

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 - EI_MAG3

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

Table 7-8 Magic Number

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 7-9 File 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.

Class ELFCLASS32 supports machines with files and virtual address spaces up to 4 gigabytes. It uses the basic types defined in the table Table 7-1.

Class ELFCLASS64 is reserved for 64-bit architectures such as SPARC. It uses the basic types defined in the table Table 7-2.

EI_DATA

Byte e_ident[EI_DATA] specifies the data encoding of the processor-specific data in the object file. The following encodings are currently defined.

Table 7-10 Data Encoding

Name	Value	Meaning
ELFDATANONE	0	Invalid data encoding
ELFDATA2LSB	1	See Figure 7-2.
ELFDATA2MSB	2	See Figure 7-3.

More information on these encodings appears below. Other values are reserved and will be assigned to new encodings as necessary.

EI_VERSION

Byte e_ident[EI_VERSION] specifies the ELF header version number. Currently, this value must be EV_CURRENT, as explained in Table 7-5 for e_version.

EI_OSABI

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.

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 for the processor or no version values are specified for the ABI determined by a particular value of the EI_OSABI byte, the value 0 is used to indicate 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.

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. Byte numbers appear in the upper left corners.

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

Figure 7-2 Data Encoding ELFDATA2LSB

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

Figure 7-3 Data Encoding ELFDATA2MSB