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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™ CPU
      EM_68K          4       Motorola 68000
      EM_88K          5       Motorola 88000
      EM_860          7       Intel860™ 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.

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

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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