As elf(3ELF) explains, ELF provides a framework for various classes of files, where basic objects may have 32 bits, 64 bits, etc. To accommodate these differences, without forcing the larger sizes on smaller machines, the initial bytes in an ELF file hold identification information common to all file classes. Every ELF header's e_ident has EI_NIDENT bytes with the following interpretation:

Other kinds of files (see elf_kind(3ELF)) also may have identification data, though they would not conform to e_ident.

elf_getident() returns a pointer to the file's ``initial bytes.'' If the library recognizes the file, a conversion from the file image to the memory image may occur. In any case, the identification bytes are guaranteed not to have been modified, though the size of the unmodified area depends on the file type. If ptr is non-null, the library stores the number of identification bytes in the location to which ptr points. If no data are present, elf is null, or an error occurs, the return value is a null pointer, with 0 stored through ptr, if ptr is non-null.

See attributes(5) for descriptions of the following attributes:

elf(3ELF), elf32_getehdr(3ELF), elf_begin(3ELF), elf_kind(3ELF), elf_rawfile(3ELF), libelf(3LIB), attributes(5)