ELF object files

ELF identification

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_PAD 7 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. Class ELFCLASS32 supports machines with files and virtual address spaces up to 4 gigabytes and uses the basic types defined in the previous table.

Class ELFCLASS64 is reserved for 64-bit architectures. Its appearance here shows how the object file may change, but the 64-bit format is otherwise unspecified. Other classes will be defined as necessary, with different basic types and sizes for object file 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.

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.

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

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. Under the defined encodings, objects are represented as detailed in ``Data encoding ELFDATA2LSB''. Byte numbers appear in the upper left corners.

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

Data encoding ELFDATA2LSB

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

Data encoding ELFDATA2MSB

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