ELF Header

Some control structures within object files can grow because the ELF header contains their actual sizes. If the object file format does change, a program can 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 is specified if and when extensions are defined.

The ELF header has the following structure. See sys/elf.h.

#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. These bytes provide machine-independent data with which to decode and interpret the file's contents. Complete descriptions appear in ELF Identification.

e_type

Identifies the object file type, as listed in the following table.

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_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_LOPROC through ET_HIPROC (inclusive) are reserved for processor-specific semantics. Other values are reserved for future use.

e_machine

Specifies the required architecture for an individual file. Relevant architectures are listed in the following table.

Name	Value	Meaning
EM_NONE	0	No machine
EM_SPARC	2	SPARC
EM_386	3	Intel 80386
EM_SPARC32PLUS	18	Sun SPARC 32+
EM_SPARCV9	43	SPARC V9
EM_AMD64	62	AMD 64

Other values are reserved for future use. Processor-specific ELF names are distinguished by using the machine name. For example, the flags defined for e_flags use the prefix EF_. A flag that is named WIDGET for the EM_XYZ machine would be called EF_XYZ_WIDGET.

e_version

Identifies the object file version, as listed in the following table.

Name	Value	Meaning
EV_NONE	0	Invalid version
EV_CURRENT	>=1	Current version

The value 1 signifies the original file format. The value of EV_CURRENT changes as necessary to reflect the current version number.

e_entry

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

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

e_shoff

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

e_flags

Processor-specific flags associated with the file. Flag names take the form EF_machine_flag. This member is presently zero for x86. The SPARC flags are listed in the following table.

Name	Value	Meaning
EF_SPARC_EXT_MASK	0xffff00	Vendor Extension mask
EF_SPARC_32PLUS	0x000100	Generic V8+ features
EF_SPARC_SUN_US1	0x000200	Sun UltraSPARC 1 Extensions
EF_SPARC_HAL_R1	0x000400	HAL R1 Extensions
EF_SPARC_SUN_US3	0x000800	Sun UltraSPARC 3 Extensions
EF_SPARCV9_MM	0x3	Mask for Memory Model
EF_SPARCV9_TSO	0x0	Total Store Ordering
EF_SPARCV9_PSO	0x1	Partial Store Ordering
EF_SPARCV9_RMO	0x2	Relaxed Memory Ordering

e_ehsize

The ELF header's size in bytes.

e_phentsize

The size in bytes of one entry in the file's program header table. All entries are the same size.

e_phnum

The number of entries in the program header table. 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.

If the number of program headers is greater than or equal to PN_XNUM (0xffff), this member has the value PN_XNUM (0xffff). The actual number of program header table entries is contained in the sh_info field of the section header at index 0. Otherwise, the sh_info member of the initial section header entry contains the value zero. See Table 7–6 and Table 7–7.

e_shentsize

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

The number of entries in the section header table. 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), e_shnum has the value zero. 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 section header entry contains the value zero. See Table 7–6 and Table 7–7.

e_shstrndx

The section header table index of the entry that is associated with the section name string table. If the file has no section name string table, this member holds the value SHN_UNDEF.

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 section header entry contains the value zero. See Table 7–6 and Table 7–7.