Basic types

The following types are used for N-bit architectures (N=32,64, ElfN stands for Elf32 or Elf64, uintN_t stands for uint32_t or uint64_t):

ElfN_Addr       Unsigned program address, uintN_t
ElfN_Off        Unsigned file offset, uintN_t
ElfN_Section    Unsigned section index, uint16_t
ElfN_Versym     Unsigned version symbol information, uint16_t
Elf_Byte        unsigned char
ElfN_Half       uint16_t
ElfN_Sword      int32_t
ElfN_Word       uint32_t
ElfN_Sxword     int64_t
ElfN_Xword      uint64_t

(Note: the *BSD terminology is a bit different. There, Elf64_Half is twice as large as Elf32_Half, and Elf64Quarter is used for uint16_t. In order to avoid confusion these types are replaced by explicit ones in the below.)

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, and so on.

ELF header (Ehdr)

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

#define EI_NIDENT 16
typedef struct {
    unsigned char e_ident[EI_NIDENT];
    uint16_t      e_type;
    uint16_t      e_machine;
    uint32_t      e_version;
    ElfN_Addr     e_entry;
    ElfN_Off      e_phoff;
    ElfN_Off      e_shoff;
    uint32_t      e_flags;
    uint16_t      e_ehsize;
    uint16_t      e_phentsize;
    uint16_t      e_phnum;
    uint16_t      e_shentsize;
    uint16_t      e_shnum;
    uint16_t      e_shstrndx;
} ElfN_Ehdr;

The fields have the following meanings:

e_ident

This array of bytes specifies how 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:

e_type

This member of the structure identifies the object file type:

e_machine

This member specifies the required architecture for an individual file. For example:

e_version

This member identifies the file 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.

If the number of entries in the program header table is larger than or equal to PN_XNUM (0xffff), this member holds PN_XNUM (0xffff) and the real number of entries in the program header table is held in the sh_info member of the initial entry in section header table. Otherwise, the sh_info member of the initial entry contains 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.

If the number of entries in the section header table is larger than or equal to SHN_LORESERVE (0xff00), e_shnum holds the value zero and the real number of entries in the section header table is held in the sh_size member of the initial entry in section header table. Otherwise, the sh_size member of the initial entry in the section header table 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.

If the index of section name string table section is larger than or equal to SHN_LORESERVE (0xff00), this member holds SHN_XINDEX (0xffff) and the real index of the section name string table section is held in the sh_link member of the initial entry in section header table. Otherwise, the sh_link member of the initial entry in section header table contains the value zero.

Program header (Phdr)

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. The ELF program header is described by the type Elf32_Phdr or Elf64_Phdr depending on the architecture:

typedef struct {
    uint32_t   p_type;
    Elf32_Off  p_offset;
    Elf32_Addr p_vaddr;
    Elf32_Addr p_paddr;
    uint32_t   p_filesz;
    uint32_t   p_memsz;
    uint32_t   p_flags;
    uint32_t   p_align;
} Elf32_Phdr;

typedef struct {
    uint32_t   p_type;
    uint32_t   p_flags;
    Elf64_Off  p_offset;
    Elf64_Addr p_vaddr;
    Elf64_Addr p_paddr;
    uint64_t   p_filesz;
    uint64_t   p_memsz;
    uint64_t   p_align;
} Elf64_Phdr;

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

p_type

This member of the structure indicates what kind of segment this array element describes or how to interpret the array element's information.

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 a bit mask of flags relevant to the segment:

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

Section header (Shdr)

A 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: the initial entry and the indices between SHN_LORESERVE and SHN_HIRESERVE. The initial entry is used in ELF extensions for e_phnum, e_shnum, and e_shstrndx; in other cases, each field in the initial entry is set to zero. 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.

SHN_LORESERVE

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

SHN_LOPROC

SHN_HIPROC

Values greater in the inclusive range [SHN_LOPROC, SHN_HIPROC] are 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.

The section header has the following structure:

typedef struct {
    uint32_t   sh_name;
    uint32_t   sh_type;
    uint32_t   sh_flags;
    Elf32_Addr sh_addr;
    Elf32_Off  sh_offset;
    uint32_t   sh_size;
    uint32_t   sh_link;
    uint32_t   sh_info;
    uint32_t   sh_addralign;
    uint32_t   sh_entsize;
} Elf32_Shdr;

typedef struct {
    uint32_t   sh_name;
    uint32_t   sh_type;
    uint64_t   sh_flags;
    Elf64_Addr sh_addr;
    Elf64_Off  sh_offset;
    uint64_t   sh_size;
    uint32_t   sh_link;
    uint32_t   sh_info;
    uint64_t   sh_addralign;
    uint64_t   sh_entsize;
} Elf64_Shdr;

No real differences exist between the 32-bit and 64-bit section headers.

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.

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.

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 nonzero 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. The value 0 or 1 means that 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

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.

.gnu.version

This section holds the version symbol table, an array of ElfN_Half elements. This section is of type SHT_GNU_versym. The attribute type used is SHF_ALLOC.

.gnu.version_d

This section holds the version symbol definitions, a table of ElfN_Verdef structures. This section is of type SHT_GNU_verdef. The attribute type used is SHF_ALLOC.

.gnu.version_r

This section holds the version symbol needed elements, a table of ElfN_Verneed structures. This section is of type SHT_GNU_versym. The attribute type used is SHF_ALLOC.

.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 various notes. This section is of type SHT_NOTE. No attribute types are used.

.note.ABI-tag

This section is used to declare the expected run-time ABI of the ELF image. It may include the operating system name and its run-time versions. This section is of type SHT_NOTE. The only attribute used is SHF_ALLOC.

.note.gnu.build-id

This section is used to hold an ID that uniquely identifies the contents of the ELF image. Different files with the same build ID should contain the same executable content. See the --build-id option to the GNU linker (ld (1)) for more details. This section is of type SHT_NOTE. The only attribute used is SHF_ALLOC.

.note.GNU-stack

This section is used in Linux object files for declaring stack attributes. This section is of type SHT_PROGBITS. The only attribute used is SHF_EXECINSTR. This indicates to the GNU linker that the object file requires an executable stack.

.note.openbsd.ident

OpenBSD native executables usually contain this section to identify themselves so the kernel can 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 nonwritable 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 nonwritable 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.

String and symbol tables

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 byte ('\0'). Similarly, a string table's last byte is defined to hold a null byte, 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 {
    uint32_t      st_name;
    Elf32_Addr    st_value;
    uint32_t      st_size;
    unsigned char st_info;
    unsigned char st_other;
    uint16_t      st_shndx;
} Elf32_Sym;

typedef struct {
    uint32_t      st_name;
    unsigned char st_info;
    unsigned char st_other;
    uint16_t      st_shndx;
    Elf64_Addr    st_value;
    uint64_t      st_size;
} Elf64_Sym;

The 32-bit and 64-bit versions have the same members, just in a different order.

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 nonzero, it represents a string table index that gives the symbol name. Otherwise, the symbol 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:

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

st_other

This member defines the symbol visibility.

st_shndx

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

Relocation entries (Rel & Rela)

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's program image. Relocation entries are these data.

Relocation structures that do not need an addend:

typedef struct {
    Elf32_Addr r_offset;
    uint32_t   r_info;
} Elf32_Rel;

typedef struct {
    Elf64_Addr r_offset;
    uint64_t   r_info;
} Elf64_Rel;

Relocation structures that need an addend:

typedef struct {
    Elf32_Addr r_offset;
    uint32_t   r_info;
    int32_t    r_addend;
} Elf32_Rela;

typedef struct {
    Elf64_Addr r_offset;
    uint64_t   r_info;
    int64_t    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.

Dynamic tags (Dyn)

The .dynamic section contains a series of structures that hold relevant dynamic linking information. The d_tag member controls the interpretation of d_un.

typedef struct {
    Elf32_Sword    d_tag;
    union {
        Elf32_Word d_val;
        Elf32_Addr d_ptr;
    } d_un;
} Elf32_Dyn;
extern Elf32_Dyn _DYNAMIC[];

typedef struct {
    Elf64_Sxword    d_tag;
    union {
        Elf64_Xword d_val;
        Elf64_Addr  d_ptr;
    } d_un;
} Elf64_Dyn;
extern Elf64_Dyn _DYNAMIC[];

d_tag

This member may have any of the following values:

d_val

This member represents integer values with various interpretations.

d_ptr

This member represents program virtual addresses. When interpreting these addresses, the actual address should be computed based on the original file value and memory base address. Files do not contain relocation entries to fixup these addresses.

_DYNAMIC

Array containing all the dynamic structures in the .dynamic section. This is automatically populated by the linker.

Notes (Nhdr)

ELF notes allow for appending arbitrary information for the system to use. They are largely used by core files (e_type of ET_CORE), but many projects define their own set of extensions. For example, the GNU tool chain uses ELF notes to pass information from the linker to the C library.

Note sections contain a series of notes (see the struct definitions below). Each note is followed by the name field (whose length is defined in n_namesz) and then by the descriptor field (whose length is defined in n_descsz) and whose starting address has a 4 byte alignment. Neither field is defined in the note struct due to their arbitrary lengths.

An example for parsing out two consecutive notes should clarify their layout in memory:

void *memory, *name, *desc;
Elf64_Nhdr *note, *next_note;
/* The buffer is pointing to the start of the section/segment. */
note = memory;
/* If the name is defined, it follows the note. */
name = note->n_namesz == 0 ? NULL : memory + sizeof(*note);
/* If the descriptor is defined, it follows the name
   (with alignment). */
desc = note->n_descsz == 0 ? NULL :
       memory + sizeof(*note) + ALIGN_UP(note->n_namesz, 4);
/* The next note follows both (with alignment). */
next_note = memory + sizeof(*note) +
                     ALIGN_UP(note->n_namesz, 4) +
                     ALIGN_UP(note->n_descsz, 4);

Keep in mind that the interpretation of n_type depends on the namespace defined by the n_namesz field. If the n_namesz field is not set (e.g., is 0), then there are two sets of notes: one for core files and one for all other ELF types. If the namespace is unknown, then tools will usually fallback to these sets of notes as well.

typedef struct {
    Elf32_Word n_namesz;
    Elf32_Word n_descsz;
    Elf32_Word n_type;
} Elf32_Nhdr;

typedef struct {
    Elf64_Word n_namesz;
    Elf64_Word n_descsz;
    Elf64_Word n_type;
} Elf64_Nhdr;

n_namesz

The length of the name field in bytes. The contents will immediately follow this note in memory. The name is null terminated. For example, if the name is "GNU", then n_namesz will be set to 4.

n_descsz

The length of the descriptor field in bytes. The contents will immediately follow the name field in memory.

n_type

Depending on the value of the name field, this member may have any of the following values: