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NetBSD 6.1.5 - man page for elf (netbsd section 5)

ELF(5)				     BSD File Formats Manual				   ELF(5)

NAME
     ELF -- executable and linking format

SYNOPSIS
     #include <elf.h>

DESCRIPTION
     Because of the flexible nature of ELF, the structures describing it are available both as
     32bit and 64bit versions. This document uses the 32bit versions, refer to <elf.h> for the
     corresponding 64bit versions.

     The four main types of an ELF object file are:

     executable   A file suitable for execution. It contains the information required for creat-
		  ing a new process image.

     relocatable  Contains the necessary information to be run through the link editor ld(1) to
		  create an executable or a shared library.

     shared	  The shared object contains necessary information which can be used by either
		  the link editor ld(1) at link time or by the dynamic loader ld.elf_so(1) at run
		  time.

     core	  A file which describes the virtual address space and register state of a
		  process.  Core files are typically used in conjunction with debuggers such as
		  gdb(1).

     ELF files have a dual nature. The toolchain, including tools such as the as(1) and linker
     ld(1), treats them as a set of sections described by their section headers. The system
     loader treats them as a set of segments described by the program headers.

     The general format of an ELF file is the following: The file starts with an ELF header. This
     is followed by a table of program headers (optional for relocatable and shared files). After
     this come the sections/segments.  The file ends with a table of section headers (optional
     for executable files).

     A segment can be considered to consist of several sections. For example, all executable sec-
     tions are typically packed into one loadable segment which is read-only and executable (see
     p_flags in the program header). This enables the system to map the entire file with just a
     few operations, one for each loadable segment, instead of doing numerous map operations for
     each section separately.

     Each file is described by the ELF header:

	   typedef struct {
		   unsigned char   e_ident[ELF_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 array contains the following information in the indicated locations:

		  EI_MAG0	 The elements ranging from EI_MAG0 to EI_MAG3 contain the ELF
				 magic number: \0177ELF

		  EI_CLASS	 Contains the address size of the binary, either 32 or 64bit.

		  EI_DATA	 byte order

		  EI_VERSION	 Contains the ELF header version. This is currently always set to
				 1.

		  EI_OSABI	 Contains the operating system ABI identification. Note that even
				 though the definition ELFOSABI_NETBSD exists, NetBSD uses
				 ELFOSABI_SYSV here, since the NetBSD ABI does not deviate from
				 the standard.

		  EI_ABIVERSION  ABI version.

     e_type	  Contains the file type identification. It can be either ET_REL, ET_EXEC,
		  ET_DYN, or ET_CORE for relocatable, executable, shared, or core, respectively.

     e_machine	  Contains the machine type, e.g. SPARC, Alpha, MIPS, ...

     e_entry	  The program entry point if the file is executable.

     e_phoff	  The position of the program header table in the file or 0 if it doesn't exist.

     e_shoff	  The position of the section header table in the file or 0 if it doesn't exist.

     e_flags	  Contains processor-specific flags. For example, the SPARC port uses this space
		  to specify what kind of memory store ordering is required.

     e_ehsize	  The size of the ELF header.

     e_phentsize  The size of an entry in the program header table. All entries are the same
		  size.

     e_phnum	  The number of entries in the program header table, or 0 if none exists.

     e_shentsize  The size of an entry in the section header table. All entries are the same
		  size.

     e_shnum	  The number of entries in the section header table, or 0 if none exists.

     e_shstrndx   Contains the index number of the section which contains the section name
		  strings.

     Each ELF section in turn is described by the section header:

	   typedef struct {
		   Elf32_Word	   sh_name;
		   Elf32_Word	   sh_type;
		   Elf32_Word	   sh_flags;
		   Elf32_Addr	   sh_addr;
		   Elf32_Off	   sh_offset;
		   Elf32_Word	   sh_size;
		   Elf32_Word	   sh_link;
		   Elf32_Word	   sh_info;
		   Elf32_Word	   sh_addralign;
		   Elf32_Word	   sh_entsize;
	   } Elf32_Shdr;

     sh_name	   Contains an index to the position in the section header string section where
		   the name of the current section can be found.

     sh_type	   Contains the section type indicator. The more important possible values are:

		   SHT_NULL	 Section is inactive. The other fields contain undefined values.

		   SHT_PROGBITS  Section contains program information. It can be for example
				 code, data, or debugger information.

		   SHT_SYMTAB	 Section contains a symbol table. This section usually contains
				 all the symbols and is intended for the regular link editor
				 ld(1).

		   SHT_STRTAB	 Section contains a string table.

		   SHT_RELA	 Section contains relocation information with an explicit addend.

		   SHT_HASH	 Section contains a symbol hash table.

		   SHT_DYNAMIC	 Section contains dynamic linking information.

		   SHT_NOTE	 Section contains some special information. The format can be
				 e.g.  vendor-specific.

		   SHT_NOBITS	 Sections contains information similar to SHT_PROGBITS, but takes
				 up no space in the file. This can be used for e.g. bss.

		   SHT_REL	 Section contains relocation information without an explicit
				 addend.

		   SHT_SHLIB	 This section type is reserved but has unspecified semantics.

		   SHT_DYNSYM	 Section contains a symbol table. This symbol table is intended
				 for the dynamic linker, and is kept as small as possible to con-
				 serve space, since it must be loaded to memory at run time.

     sh_flags	   Contains the section flags, which can have the following values or any combi-
		   nation of them:

		   SHF_WRITE	  Section is writable after it has been loaded.

		   SHF_ALLOC	  Section will occupy memory at run time.

		   SHF_EXECINSTR  Section contains executable machine instructions.

     sh_addr	   Address to where the section will be loaded, or 0 if this section does not
		   reside in memory at run time.

     sh_offset	   The byte offset from the beginning of the file to the beginning of this sec-
		   tion. If the section is of type SHT_NOBITS, this field specifies the concep-
		   tual placement in the file.

     sh_size	   The size of the section in the file for all types except SHT_NOBITS.  For that
		   type the value may differ from zero, but the section will still always take up
		   no space from the file.

     sh_link	   Contains an index to the section header table. The interpretation depends on
		   the section type as follows:

		   SHT_REL
		   SHT_RELA	Section index of the associated symbol table.

		   SHT_SYMTAB
		   SHT_DYNSYM	Section index of the associated string table.

		   SHT_HASH	Section index of the symbol table to which the hash table
				applies.

		   SHT_DYNAMIC	Section index of of the string table by which entries in this
				section are used.

     sh_info	   Contains extra information. The interpretation depends on the type as follows:

		   SHT_REL
		   SHT_RELA    Section index of the section to which the relocation information
			       applies.

		   SHT_SYMTAB
		   SHT_DYNSYM  Contains a value one greater that the last local symbol table
			       index.

     sh_addralign  Marks the section alignment requirement. If, for example, the section contains
		   a doubleword, the entire section must be doubleword aligned to ensure proper
		   alignment. Only 0 and integral powers of two are allowed. Values 0 and 1
		   denote that the section has no alignment.

     sh_entsize    Contains the entry size of an element for sections which are constructed of a
		   table of fixed-size entries. If the section does not hold a table of fixed-
		   size entries, this value is 0.

     Every executable object must contain a program header. The program header contains informa-
     tion necessary in constructing a process image.

	   typedef struct {
		   Elf32_Word	   p_type;
		   Elf32_Off	   p_offset;
		   Elf32_Addr	   p_vaddr;
		   Elf32_Addr	   p_paddr;
		   Elf32_Word	   p_filesz;
		   Elf32_Word	   p_memsz;
		   Elf32_Word	   p_flags;
		   Elf32_Word	   p_align;
	   } Elf32_Phdr;

     p_type    Contains the segment type indicator. The possible values are:

	       PT_NULL	   Segment is inactive. The other fields contain undefined values.

	       PT_LOAD	   Segment is loadable. It is loaded to the address described by p_vaddr.
			   If p_memsz is greater than p_filesz, the memory range from (p_vaddr +
			   p_filesz) to (p_vaddr + p_memsz) is zero-filled when the segment is
			   loaded.  p_filesz can not be greater than p_memsz.  Segments of this
			   type are sorted in the header table by p_vaddr in ascending order.

	       PT_DYNAMIC  Segment contains dynamic linking information.

	       PT_INTERP   Segment contains a null-terminated path name to the interpreter. This
			   segment may be present only once in a file, and it must appear before
			   any loadable segments. This field will most likely contain the ELF
			   dynamic loader: /libexec/ld.elf_so

	       PT_NOTE	   Segment contains some special information. Format can be e.g. vendor-
			   specific.

	       PT_SHLIB    This segment type is reserved but has unspecified semantics. Programs
			   which contain a segment of this type do not conform to the ABI, and
			   must indicate this by setting the appropriate ABI in the ELF header
			   EI_OSABI field.

	       PT_PHDR	   The values in a program header of this type specify the characteris-
			   tics of the program header table itself. For example, the p_vaddr
			   field specifies the program header table location in memory once the
			   program is loaded. This field may not occur more than once, may occur
			   only if the program header table is part of the file memory image, and
			   must come before any loadable segments.

     p_offset  Contains the byte offset from the beginning of the file to the beginning of this
	       segment.

     p_vaddr   Contains the virtual memory address to which this segment is loaded.

     p_paddr   Contains the physical address to which this segment is loaded. This value is usu-
	       ally ignored, but may be used while bootstrapping or in embedded systems.

     p_filesz  Contains the number of bytes this segment occupies in the file image.

     p_memsz   Contains the number of bytes this segment occupies in the memory image.

     p_flags   Contains the segment flags, which specify the permissions for the segment after it
	       has been loaded. The following values or any combination of them is acceptable:

	       PF_R  Segment can be read.

	       PF_W  Segment can be written.

	       PF_X  Segment is executable.

     p_align   Contains the segment alignment. Acceptable values are 0 and 1 for no alignment,
	       and integral powers of two.  p_vaddr should equal p_offset modulo p_align.

SEE ALSO
     as(1), gdb(1), ld(1), ld.elf_so(1), execve(2), nlist(3), a.out(5), core(5), link(5), stab(5)

HISTORY
     The ELF object file format first appeared in AT&T System V UNIX.

BSD					November 18, 2006				      BSD


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