NAME

SYNOPSIS

DESCRIPTION

Elf32_Addr	Unsigned program address
Elf32_Half	Unsigned halfword field
Elf32_Off	Unsigned file offset
Elf32_Sword	Signed large integer
Elf32_Word	Field or unsigned large integer

Elf64_Addr	Unsigned program address
Elf64_Shalf	Signed halfword field
Elf64_Half	Unsigned halfword field
Elf64_Off	Unsigned file offset
Elf64_Sword	Signed large integer
Elf64_Word	Field or unsigned large integer

Elf64_Xword	Unsigned object size or alignment
Elf64_Sxword	Signed object size or alignment
Elf64_Quarter	Unsigned quarterword field

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_Quarter   e_type;
	Elf64_Quarter   e_machine;
	Elf64_Half      e_version;
	Elf64_Addr      e_entry;
	Elf64_Off       e_phoff;
	Elf64_Off       e_shoff;
	Elf64_Half      e_flags;
	Elf64_Quarter   e_ehsize;
	Elf64_Quarter   e_phentsize;
	Elf64_Quarter   e_phnum;
	Elf64_Quarter   e_shentsize;
	Elf64_Quarter   e_shnum;
	Elf64_Quarter   e_shstrndx;
} Elf64_Ehdr;

e_ident

This array of bytes specifies to interpret the file, independent of the processor or the file's remaining contents. Within this array everything is named by macros, which start with the prefix EI_ and may contain values which start with the prefix ELF. The following macros are defined:

EI_MAG0

The first byte of the magic number. It must be filled with ELFMAG0.

EI_MAG1

The second byte of the magic number. It must be filled with ELFMAG1.

EI_MAG2

The third byte of the magic number. It must be filled with ELFMAG2.

EI_MAG3

The fourth byte of the magic number. It must be filled with ELFMAG3.

EI_CLASS

The fifth byte identifies the architecture for this binary:

ELFCLASSNONE

This class is invalid.

ELFCLASS32

This defines the 32-bit architecture. It supports machines with files and virtual address spaces up to 4 Gigabytes.

ELFCLASS64

This defines the 64-bit architecture.

EI_DATA

The sixth byte specifies the data encoding of the processor-specific data in the file. Currently these encodings are supported:

ELFDATANONE

Unknown data format.

ELFDATA2LSB

Two's complement, little-endian.

ELFDATA2MSB

Two's complement, big-endian.

EI_VERSION

The version number of the ELF specification:

EV_NONE

Invalid version.

EV_CURRENT

Current version.

EI_PAD

Start of padding. These bytes are reserved and set to zero. Programs which read them should ignore them. The value for EI_PAD will change in the future if currently unused bytes are given meanings.

EI_BRAND

Start of architecture identification.

EI_NIDENT

The size of the e_ident array.

e_type

This member of the structure identifies the object file type:

ET_NONE

An unknown type.

ET_REL

A relocatable file.

ET_EXEC

An executable file.

ET_DYN

A shared object.

ET_CORE

A core file.

e_machine

This member specifies the required architecture for an individual file:

EM_NONE

An unknown machine.

EM_M32

AT&T WE 32100.

EM_SPARC

Sun Microsystems SPARC.

EM_386

Intel 80386.

EM_68K

Motorola 68000.

EM_88K

Motorola 88000.

EM_486

Intel 80486.

EM_860

Intel 80860.

EM_MIPS

MIPS RS3000 (big-endian only).

EM_MIPS_RS4_BE

MIPS RS4000 (big-endian only).

EM_SPARC64

SPARC v9 64-bit (unofficial).

EM_PARISC

HPPA.

EM_SPARC32PLUS

SPARC with enhanced instruction set.

EM_PPC

PowerPC.

EM_ALPHA

Compaq [DEC] Alpha.

EM_SPARCV9

SPARC v9 64-bit.

EM_ALPHA_EXP

Compaq [DEC] Alpha with enhanced instruction set.

EM_VAX

DEC Vax.

e_version

This member identifies the file version:

EV_NONE

Invalid version.

EV_CURRENT

Current version.

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'. Currently no flags have been defined.

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, e_phnum holds the value zero.

e_shentsize

This member holds a sections 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 of 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.

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 is reserved for processor-specific semantics.

SHN_HIPROC

This value down to and including SHN_LOPROC is 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 between SHN_LORESERVE and SHN_HIRESERVE, inclusive; the values do not reference the section header table. That is, the section header table does not contain entries for the reserved indices.

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_Half      p_type;
        Elf64_Half      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;

p_type

This member of the Phdr struct tells what kind of segment this array element describes or how to interpret the array element's information.

PT_NULL

The array element is unused and the other members' values are undefined. This lets the program header have ignored entries.

PT_LOAD

The array element specifies a loadable segment, described by p_filesz and p_memsz. The bytes from the file are mapped to the beginning of the memory segment. If the segment's memory size (p_memsz) is larger than the file size (p_filesz), the “extra” bytes are defined to hold the value 0 and to follow the segment's initialized area. The file size may not be larger than the memory size. Loadable segment entries in the program header table appear in ascending order, sorted on the p_vaddr member.

PT_DYNAMIC

The array element specifies dynamic linking information.

PT_INTERP

The array element specifies the location and size of a null-terminated path name to invoke as an interpreter. This segment type is meaningful only for executable files (though it may occur for shared objects). However it may not occur more than once in a file. If it is present, it must precede any loadable segment entry.

PT_NOTE

The array element specifies the location and size for auxiliary information.

PT_SHLIB

This segment type is reserved but has unspecified semantics. Programs that contain an array element of this type do not conform to the ABI.

PT_PHDR

The array element, if present, specifies the location and size of the program header table itself, both in the file and in the memory image of the program. This segment type may not occur more than once in a file. Moreover, it may only occur if the program header table is part of the memory image of the program. If it is present, it must precede any loadable segment entry.

PT_LOOS

This value up to and including PT_HIOS is reserverd for operating system-specific semantics.

PT_HIOS

This value down to and including PT_LOOS is reserved for operating system-specific semantics.

PT_LOPROC

This value up to and including PT_HIPROC is reserved for processor-specific semantics.

PT_HIPROC

This value down to and including PT_LOPROC is reserved for processor-specific semantics.

p_offset

This member holds the offset from the beginning of the file at which the first byte of the segment resides.

p_vaddr

This member holds the virtual address at which the first byte of the segment resides in memory.

p_paddr

On systems for which physical addressing is relevant, this member is reserved for the segment's physical address. Under BSD this member is not used and must be zero.

p_filesz

This member holds the number of bytes in the file image of the segment. It may be zero.

p_memsz

This member holds the number of bytes in the memory image of the segment. It may be zero.

p_flags

This member holds flags relevant to the segment:

PF_X

An executable segment.

PF_W

A writable segment.

PF_R

A readable segment.

A text segment commonly has the flags PF_X and PF_R. A data segment commonly has PF_X, PF_W and PF_R.

p_align

This member holds the value to which the segments are aligned in memory and in the file. Loadable process segments must have congruent values for p_vaddr and p_offset, modulo the page size. Values of zero and one mean no alignment is required. Otherwise, p_align should be a positive, integral power of two, and p_vaddr should equal p_offset, modulo p_align.

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 is reserved for processor-specific semantics.

SHN_HIPROC

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

SHN_ABS

This value specifies the absolute value for the corresponding reference. For example, a symbol defined relative to section number SHN_ABS has an absolute value and is 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.

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_Half      sh_name;
	Elf64_Half      sh_type;
	Elf64_Xword     sh_flags;
	Elf64_Addr      sh_addr;
	Elf64_Off       sh_offset;
	Elf64_Xword     sh_size;
	Elf64_Half      sh_link;
	Elf64_Half      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

This 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. An 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 is reserved for processor-specific semantics.

SHT_HIPROC

This value down to and including SHT_LOPROC is 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

This 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

This section contains executable machine instructions.

SHF_MASKPROC

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

sh_addr

If this section appears 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.

.bss

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 attribute 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.

.ctors

This section holds initialized pointers to the C++ constructor functions. This section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE.

.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.

.dtors

This section holds initialized pointers to the C++ destructor functions. This section is of type SHT_PROGBITS. The attribute types are SHF_ALLOC and SHF_WRITE.

.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. OpenBSD native executables usually contain a .note.openbsd.ident section to identify themselves, for the kernel to bypass any compatibility ELF binary emulation tests when loading the file.

.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 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.

.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.

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_Half      st_name;
	Elf_Byte        st_info;
	Elf_Byte        st_other;
	Elf64_Quarter   st_shndx;
	Elf64_Xword     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 is reserved for processor-specific semantics.

STT_HIPROC

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

STB_HIPROC

This value down to and including STB_LOPROC is 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.

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

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

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

typedef struct {
	Elf64_Xword     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.

SEE ALSO

HISTORY

AUTHORS