ELF header

Some object file control structures can grow, because the ELF header contains their actual sizes. If the object file format changes, a program may 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 will be specified when and if extensions are defined.

Example 8-1 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;

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 in ``ELF identification''.

e_type

This member identifies the object file type.

 -------------------------------------------------------
 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 in the System V Application Binary Interface, 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 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.

 ------------------------------------------------------
 Name            Value   Meaning
 ------------------------------------------------------
 EM_NONE         0       No machine
 EM_M32          1       AT&T WE 32100
 EM_SPARC        2       SPARC
 EM_386          3       Intel386(TM) CPU
 EM_68K          4       Motorola 68000
 EM_88K          5       Motorola 88000
 EM_860          7       Intel860(TM) CPU
 EM_MIPS         8       MIPS R2000

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 in the next table use the prefix EF_.

e_version

This member identifies the object file version.

 ------------------------------------------------------
 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.

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NOTE: See ``ELF header flags'' for flag definitions.

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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.

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.

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NOTE: See ``Section header'' and ``String table'' for more information.

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