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DESCRIPTION

An executable file using the ELF file format consists of an ELF header, followed by a program header table or a section header table, or both. The ELF header is always at offset zero of the file. The program header table and the section header table's offset in the file are defined in the ELF header. The two tables describe the rest of the particularities of the file.

Applications which wish to process ELF binary files for their native architecture only should include <elf.h> in their source code. These applications should need to refer to all the types and structures by their generic names “Elf_xxx” and to the macros by “ELF_xxx”. Applications written this way can be compiled on any architecture, regardless whether the host is 32-bit or 64-bit.

Should an application need to process ELF files of an unknown architecture then the application needs to include both <sys/elf32.h> and <sys/elf64.h> instead of <elf.h>. Furthermore, all types and structures need to be identified by either “Elf32_xxx” or “Elf64_xxx”. The macros need to be identified by “ELF32_xxx” or “ELF64_xxx”.

Whatever the system's architecture is, it will always include <sys/elf_common.h> as well as <sys/elf_generic.h>.

These header files describe the above mentioned headers as C structures and also include structures for dynamic sections, relocation sections and symbol tables.

The following types are being used for 32-bit architectures:

Elf32_Addr	Unsigned 32-bit program address
Elf32_Half	Unsigned 16-bit field
Elf32_Lword	Unsigned 64-bit field
Elf32_Off	Unsigned 32-bit file offset
Elf32_Sword	Signed 32-bit field or integer
Elf32_Word	Unsigned 32-bit field or integer

For 64-bit architectures we have the following types:

Elf64_Addr	Unsigned 64-bit program address
Elf64_Half	Unsigned 16-bit field
Elf64_Lword	Unsigned 64-bit field
Elf64_Off	Unsigned 64-bit file offset
Elf64_Sword	Signed 32-bit field
Elf64_Sxword	Signed 64-bit field or integer
Elf64_Word	Unsigned 32-bit field
Elf64_Xword	Unsigned 64-bit field or integer

All data structures that the file format defines follow the “natural” size and alignment guidelines for the relevant class. If necessary, data structures contain explicit padding to ensure 4-byte alignment for 4-byte objects, to force structure sizes to a multiple of 4, etc.

The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:

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;

The fields have the following meanings:

An executable or shared object file's program header table is an array of structures, each describing a segment or other information the system needs to prepare the program for execution. An object file segment contains one or more sections. Program headers are meaningful only for executable and shared object files. A file specifies its own program header size with the ELF header's e_phentsize and e_phnum members. As with the Elf executable header, the program header also has different versions depending on the architecture:

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;

typedef struct {
        Elf64_Word      p_type;
        Elf64_Word      p_flags;
        Elf64_Off       p_offset;
        Elf64_Addr      p_vaddr;
        Elf64_Addr      p_paddr;
        Elf64_Xword     p_filesz;
        Elf64_Xword     p_memsz;
        Elf64_Xword     p_align;
} Elf64_Phdr;

The main difference between the 32-bit and the 64-bit program header lies only in the location of a p_flags member in the total struct.

An file's section header table lets one locate all the file's sections. The section header table is an array of Elf32_Shdr or Elf64_Shdr structures. The ELF header's e_shoff member gives the byte offset from the beginning of the file to the section header table. e_shnum holds the number of entries the section header table contains. e_shentsize holds the size in bytes of each entry.

A section header table index is a subscript into this array. Some section header table indices are reserved. An object file does not have sections for these special indices:

SHN_UNDEF

This value marks an undefined, missing, irrelevant, or otherwise meaningless section reference. For example, a symbol “defined” relative to section number SHN_UNDEF is an undefined symbol.

SHN_LORESERVE

This value specifies the lower bound of the range of reserved indices.

SHN_LOPROC

This value up to and including SHN_HIPROC are reserved for processor-specific semantics.

SHN_HIPROC

This value down to and including SHN_LOPROC are reserved for processor-specific semantics.

SHN_ABS

This value specifies absolute values for the corresponding reference. For example, symbols defined relative to section number SHN_ABS have absolute values and are not affected by relocation.

SHN_COMMON

Symbols defined relative to this section are common symbols, such as FORTRAN COMMON or unallocated C external variables.

SHN_HIRESERVE

This value specifies the upper bound of the range of reserved indices. The system reserves indices between SHN_LORESERVE and SHN_HIRESERVE, inclusive. The section header table does not contain entries for the reserved indices.

The section header has the following structure:

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;

typedef struct {
	Elf64_Word      sh_name;
	Elf64_Word      sh_type;
	Elf64_Xword     sh_flags;
	Elf64_Addr      sh_addr;
	Elf64_Off       sh_offset;
	Elf64_Xword     sh_size;
	Elf64_Word      sh_link;
	Elf64_Word      sh_info;
	Elf64_Xword     sh_addralign;
	Elf64_Xword     sh_entsize;
} Elf64_Shdr;

sh_name

This member specifies the name of the section. Its value is an index into the section header string table section, giving the location of a null-terminated string.

sh_type

This member categorizes the section's contents and semantics.

SHT_NULL

This value marks the section header as inactive. It does not have an associated section. Other members of the section header have undefined values.

SHT_PROGBITS

The section holds information defined by the program, whose format and meaning are determined solely by the program.

SHT_SYMTAB

This section holds a symbol table. Typically, SHT_SYMTAB provides symbols for link editing, though it may also be used for dynamic linking. As a complete symbol table, it may contain many symbols unnecessary for dynamic linking. An object file can also contain a SHN_DYNSYM section.

SHT_STRTAB

This section holds a string table. An object file may have multiple string table sections.

SHT_RELA

This section holds relocation entries with explicit addends, such as type Elf32_Rela for the 32-bit class of object files. An object may have multiple relocation sections.

SHT_HASH

This section holds a symbol hash table. All object participating in dynamic linking must contain a symbol hash table. An object file may have only one hash table.

SHT_DYNAMIC

This section holds information for dynamic linking. An object file may have only one dynamic section.

SHT_NOTE

This section holds information that marks the file in some way.

SHT_NOBITS

A section of this type occupies no space in the file but otherwise resembles SHN_PROGBITS. Although this section contains no bytes, the sh_offset member contains the conceptual file offset.

SHT_REL

This section holds relocation offsets without explicit addends, such as type Elf32_Rel for the 32-bit class of object files. An object file may have multiple relocation sections.

SHT_SHLIB

This section is reserved but has unspecified semantics.

SHT_DYNSYM

This section holds a minimal set of dynamic linking symbols. An object file can also contain a SHN_SYMTAB section.

SHT_LOPROC

This value up to and including SHT_HIPROC are reserved for processor-specific semantics.

SHT_HIPROC

This value down to and including SHT_LOPROC are reserved for processor-specific semantics.

SHT_LOUSER

This value specifies the lower bound of the range of indices reserved for application programs.

SHT_HIUSER

This value specifies the upper bound of the range of indices reserved for application programs. Section types between SHT_LOUSER and SHT_HIUSER may be used by the application, without conflicting with current or future system-defined section types.

sh_flags

Sections support one-bit flags that describe miscellaneous attributes. If a flag bit is set in sh_flags, the attribute is “on” for the section. Otherwise, the attribute is “off” or does not apply. Undefined attributes are set to zero.

SHF_WRITE

This section contains data that should be writable during process execution.

SHF_ALLOC

The section occupies memory during process execution. Some control sections do not reside in the memory image of an object file. This attribute is off for those sections.

SHF_EXECINSTR

The section contains executable machine instructions.

SHF_MASKPROC

All bits included in this mask are reserved for processor-specific semantics.

sh_addr

If the section will appear in the memory image of a process, this member holds the address at which the section's first byte should reside. Otherwise, the member contains zero.

sh_offset

This member's value holds the byte offset from the beginning of the file to the first byte in the section. One section type, SHT_NOBITS, occupies no space in the file, and its sh_offset member locates the conceptual placement in the file.

sh_size

This member holds the section's size in bytes. Unless the section type is SHT_NOBITS, the section occupies sh_size bytes in the file. A section of type SHT_NOBITS may have a non-zero size, but it occupies no space in the file.

sh_link

This member holds a section header table index link, whose interpretation depends on the section type.

sh_info

This member holds extra information, whose interpretation depends on the section type.

sh_addralign

Some sections have address alignment constraints. If a section holds a doubleword, the system must ensure doubleword alignment for the entire section. That is, the value of sh_addr must be congruent to zero, modulo the value of sh_addralign. Only zero and positive integral powers of two are allowed. Values of zero or one mean the section has no alignment constraints.

sh_entsize

Some sections hold a table of fixed-sized entries, such as a symbol table. For such a section, this member gives the size in bytes for each entry. This member contains zero if the section does not hold a table of fixed-size entries.

Various sections hold program and control information:

.bss

(Block Started by Symbol) This section holds uninitialized data that contributes to the program's memory image. By definition, the system initializes the data with zeros when the program begins to run. This section is of type SHT_NOBITS. The attributes types are SHF_ALLOC and SHF_WRITE.

.comment

This section holds version control information. This section is of type SHT_PROGBITS. No attribute types are used.

.data

This section holds initialized data that contribute to the program's memory image. This section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE.

.data1

This section holds initialized data that contribute to the program's memory image. This section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE.

.debug

This section holds information for symbolic debugging. The contents are unspecified. This section is of type SHT_PROGBITS. No attribute types are used.

.dynamic

This section holds dynamic linking information. The section's attributes will include the SHF_ALLOC bit. Whether the SHF_WRITE bit is set is processor-specific. This section is of type SHT_DYNAMIC. See the attributes above.

.dynstr

This section holds strings needed for dynamic linking, most commonly the strings that represent the names associated with symbol table entries. This section is of type SHT_STRTAB. The attribute type used is SHF_ALLOC.

.dynsym

This section holds the dynamic linking symbol table. This section is of type SHT_DYNSYM. The attribute used is SHF_ALLOC.

.fini

This section holds executable instructions that contribute to the process termination code. When a program exits normally the system arranges to execute the code in this section. This section is of type SHT_PROGBITS. The attributes used are SHF_ALLOC and SHF_EXECINSTR.

.got

This section holds the global offset table. This section is of type SHT_PROGBITS. The attributes are processor-specific.

.hash

This section holds a symbol hash table. This section is of type SHT_HASH. The attribute used is SHF_ALLOC.

.init

This section holds executable instructions that contribute to the process initialization code. When a program starts to run the system arranges to execute the code in this section before calling the main program entry point. This section is of type SHT_PROGBITS. The attributes used are SHF_ALLOC and SHF_EXECINSTR.

.interp

This section holds the pathname of a program interpreter. If the file has a loadable segment that includes the section, the section's attributes will include the SHF_ALLOC bit. Otherwise, that bit will be off. This section is of type SHT_PROGBITS.

.line

This section holds line number information for symbolic debugging, which describes the correspondence between the program source and the machine code. The contents are unspecified. This section is of type SHT_PROGBITS. No attribute types are used.

.note

This section holds information in the “Note Section” format described below. This section is of type SHT_NOTE. No attribute types are used.

.plt

This section holds the procedure linkage table. This section is of type SHT_PROGBITS. The attributes are processor-specific.

.relNAME

This section holds relocation information as described below. If the file has a loadable segment that includes relocation, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. By convention, “NAME” is supplied by the section to which the relocations apply. Thus a relocation section for .text normally would have the name .rel.text. This section is of type SHT_REL.

.relaNAME

This section holds relocation information as described below. If the file has a loadable segment that includes relocation, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. By convention, “NAME” is supplied by the section to which the relocations apply. Thus a relocation section for .text normally would have the name .rela.text. This section is of type SHT_RELA.

.rodata

This section holds read-only data that typically contributes to a non-writable segment in the process image. This section is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.

.rodata1

This section hold read-only data that typically contributes to a non-writable segment in the process image. This section is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.

.shstrtab

This section holds section names. This section is of type SHT_STRTAB. No attribute types are used.

.strtab

This section holds strings, most commonly the strings that represent the names associated with symbol table entries. If the file has a loadable segment that includes the symbol string table, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. This section is of type SHT_STRTAB.

.symtab

This section holds a symbol table. If the file has a loadable segment that includes the symbol table, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. This section is of type SHT_SYMTAB.

.text

This section holds the “text”, or executable instructions, of a program. This section is of type SHT_PROGBITS. The attributes used are SHF_ALLOC and SHF_EXECINSTR.

.jcr

This section holds information about Java classes that must be registered.

.eh_frame

This section holds information used for C++ exception-handling.

String table sections hold null-terminated character sequences, commonly called strings. The object file uses these strings to represent symbol and section names. One references a string as an index into the string table section. The first byte, which is index zero, is defined to hold a null character. Similarly, a string table's last byte is defined to hold a null character, ensuring null termination for all strings.

An object file's symbol table holds information needed to locate and relocate a program's symbolic definitions and references. A symbol table index is a subscript into this array.

typedef struct {
	Elf32_Word      st_name;
	Elf32_Addr      st_value;
	Elf32_Word      st_size;
	unsigned char   st_info;
	unsigned char   st_other;
	Elf32_Half      st_shndx;
} Elf32_Sym;

typedef struct {
	Elf64_Word      st_name;
	unsigned char   st_info;
	unsigned char   st_other;
	Elf64_Half      st_shndx;
	Elf64_Addr      st_value;
	Elf64_Xword     st_size;
} Elf64_Sym;

st_name

This member holds an index into the object file's symbol string table, which holds character representations of the symbol names. If the value is non-zero, it represents a string table index that gives the symbol name. Otherwise, the symbol table has no name.

st_value

This member gives the value of the associated symbol.

st_size

Many symbols have associated sizes. This member holds zero if the symbol has no size or an unknown size.

st_info

This member specifies the symbol's type and binding attributes:

STT_NOTYPE

The symbol's type is not defined.

STT_OBJECT

The symbol is associated with a data object.

STT_FUNC

The symbol is associated with a function or other executable code.

STT_SECTION

The symbol is associated with a section. Symbol table entries of this type exist primarily for relocation and normally have STB_LOCAL bindings.

STT_FILE

By convention the symbol's name gives the name of the source file associated with the object file. A file symbol has STB_LOCAL bindings, its section index is SHN_ABS, and it precedes the other STB_LOCAL symbols of the file, if it is present.

STT_LOPROC

This value up to and including STT_HIPROC are reserved for processor-specific semantics.

STT_HIPROC

This value down to and including STT_LOPROC are reserved for processor-specific semantics.

STB_LOCAL

Local symbols are not visible outside the object file containing their definition. Local symbols of the same name may exist in multiple file without interfering with each other.

STB_GLOBAL

Global symbols are visible to all object files being combined. One file's definition of a global symbol will satisfy another file's undefined reference to the same symbol.

STB_WEAK

Weak symbols resemble global symbols, but their definitions have lower precedence.

STB_LOPROC

This value up to and including STB_HIPROC are reserved for processor-specific semantics.

STB_HIPROC

This value down to and including STB_LOPROC are reserved for processor-specific semantics.

There are macros for packing and unpacking the binding and type fields:

ELF32_ST_BIND(info)

or ELF64_ST_BIND(info) extract a binding from an st_info value.

ELF64_ST_TYPE(info)

or ELF32_ST_TYPE(info) extract a type from an st_info value.

ELF32_ST_INFO(bind, type)

or ELF64_ST_INFO(bind, type) convert a binding and a type into an st_info value.

st_other

This member currently holds zero and has no defined meaning.

st_shndx

Every symbol table entry is “defined” in relation to some section. This member holds the relevant section header table index.

Relocation is the process of connecting symbolic references with symbolic definitions. Relocatable files must have information that describes how to modify their section contents, thus allowing executable and shared object files to hold the right information for a process' program image. Relocation entries are these data.

Relocation structures that do not need an addend:

typedef struct {
	Elf32_Addr      r_offset;
	Elf32_Word      r_info;
} Elf32_Rel;

typedef struct {
	Elf64_Addr      r_offset;
	Elf64_Xword     r_info;
} Elf64_Rel;

Relocation structures that need an addend:

typedef struct {
	Elf32_Addr      r_offset;
	Elf32_Word      r_info;
	Elf32_Sword     r_addend;
} Elf32_Rela;

typedef struct {
	Elf64_Addr      r_offset;
	Elf64_Xword     r_info;
	Elf64_Sxword    r_addend;
} Elf64_Rela;

r_offset

This member gives the location at which to apply the relocation action. For a relocatable file, the value is the byte offset from the beginning of the section to the storage unit affected by the relocation. For an executable file or shared object, the value is the virtual address of the storage unit affected by the relocation.

r_info

This member gives both the symbol table index with respect to which the relocation must be made and the type of relocation to apply. Relocation types are processor-specific. When the text refers to a relocation entry's relocation type or symbol table index, it means the result of applying ELF_[32|64]_R_TYPE or ELF[32|64]_R_SYM, respectively to the entry's r_info member.

r_addend

This member specifies a constant addend used to compute the value to be stored into the relocatable field.