ELF(5)

NAME
elf - format of Executable and Linking Format (ELF) files

SYNOPSIS
#include <elf.h>

DESCRIPTION
The header file <elf.h> defines the format of ELF executable binary files. Amongst these files are normal executable files, relocatable object files, core files, and shared objects.

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.

This header file describes the above mentioned headers as C structures and also includes structures for dynamic sections, relocation sections and symbol tables.

  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:

    EI_MAG0
        The first byte of the magic number. It must be filled with ELFMAG0. (0: 0x7f)

    EI_MAG1
        The second byte of the magic number. It must be filled with ELFMAG1. (1: 'E')

    EI_MAG2
        The third byte of the magic number. It must be filled with ELFMAG2. (2: 'L')

    EI_MAG3
        The fourth byte of the magic number. It must be filled with ELFMAG3. (3: 'F')

    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 seventh byte is the version number of the ELF specification:

        EV_NONE
            Invalid version.
        EV_CURRENT
            Current version.

    EI_OSABI
        The eighth byte identifies the operating system and ABI to which the object is targeted. Some fields in other ELF structures have flags and values that have platform-specific meanings; the interpretation of those fields is determined by the value of this byte. For example:

        ELFOSABI_NONE
            Same as ELFOSABI_SYSV
        ELFOSABI_SYSV
            UNIX System V ABI
        ELFOSABI_HPUX
            HP-UX ABI
        ELFOSABI_NETBSD
            NetBSD ABI
        ELFOSABI_LINUX
            Linux ABI
        ELFOSABI_SOLARIS
            Solaris ABI
        ELFOSABI_IRIX
            IRIX ABI
        ELFOSABI_FREEBSD
            FreeBSD ABI
        ELFOSABI_TRU64
            TRU64 UNIX ABI
        ELFOSABI_ARM
            ARM architecture ABI
        ELFOSABI_STANDALONE
            Stand-alone (embedded) ABI

    EI_ABIVERSION
        The ninth byte identifies the version of the ABI to which the object is targeted. This field is used to distinguish among incompatible versions of an ABI. The interpretation of this version number is dependent on the ABI identified by the EI_OSABI field. Applications conforming to this specification use the value 0.

    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_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. For example:

    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_860
        Intel 80860
    EM_MIPS
        MIPS RS3000 (big-endian only)
    EM_PARISC
        HP/PA
    EM_SPARC32PLUS
        SPARC with enhanced instruction set
    EM_PPC
        PowerPC
    EM_PPC64
        PowerPC 64-bit
    EM_S390
        IBM S/390
    EM_ARM
        Advanced RISC Machines
    EM_SH
        Renesas SuperH
    EM_SPARCV9
        SPARC v9 64-bit
    EM_IA_64
        Intel Itanium
    EM_X86_64
        AMD x86-64
    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.

    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.

    PN_XNUM
        This is defined as 0xffff, the largest number e_phnum can have, specifying where the actual number of program headers is assigned.

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.

    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 pathname 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 of notes (ElfN_Nhdr).

    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 occur only 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_LOPROC
    PT_HIPROC
        Values in the inclusive range [PT_LOPROC, PT_HIPROC] are reserved for processor-specific semantics.

    PT_GNU_STACK
        GNU extension which is used by the Linux kernel to control the state of the stack via the flags set in the p_flags member.

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:

    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_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.

    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 SHT_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 notes (ElfN_Nhdr).

    SHT_NOBITS
        A section of this type occupies no space in the file but otherwise resembles SHT_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 SHT_SYMTAB section.

    SHT_LOPROC
    SHT_HIPROC
        Values in the inclusive range [SHT_LOPROC, SHT_HIPROC] 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
        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 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:

    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
    STT_HIPROC
        Values in the inclusive range [STT_LOPROC, STT_HIPROC] 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 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
    STB_HIPROC
        Values in the inclusive range [STB_LOPROC, STB_HIPROC] are reserved for processor-specific semantics.

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

    ELF32_ST_BIND(info)
    ELF64_ST_BIND(info)
        Extract a binding from an st_info value.

    ELF32_ST_TYPE(info)
    ELF64_ST_TYPE(info)
        Extract a type from an st_info value.

    ELF32_ST_INFO(bind, type)
    ELF64_ST_INFO(bind, type)
        Convert a binding and a type into an st_info value.

st_other
    This member defines the symbol visibility.

    STV_DEFAULT
        Default symbol visibility rules. Global and weak symbols are available to other modules; references in the local module can be interposed by definitions in other modules.
    STV_INTERNAL
        Processor-specific hidden class.
    STV_HIDDEN
        Symbol is unavailable to other modules; references in the local module always resolve to the local symbol (i.e., the symbol can't be interposed by definitions in other modules).
    STV_PROTECTED
        Symbol is available to other modules, but references in the local module always resolve to the local symbol.

    There are macros for extracting the visibility type:

    ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

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:

    DT_NULL
        Marks end of dynamic section

    DT_NEEDED
        String table offset to name of a needed library

    DT_PLTRELSZ
        Size in bytes of PLT relocation entries

    DT_PLTGOT
        Address of PLT and/or GOT

    DT_HASH
        Address of symbol hash table

    DT_STRTAB
        Address of string table

    DT_SYMTAB
        Address of symbol table

    DT_RELA
        Address of Rela relocation table

    DT_RELASZ
        Size in bytes of the Rela relocation table

    DT_RELAENT
        Size in bytes of a Rela relocation table entry

    DT_STRSZ
        Size in bytes of string table

    DT_SYMENT
        Size in bytes of a symbol table entry

    DT_INIT
        Address of the initialization function

    DT_FINI
        Address of the termination function

    DT_SONAME
        String table offset to name of shared object

    DT_RPATH
        String table offset to library search path (deprecated)

    DT_SYMBOLIC
        Alert linker to search this shared object before the executable for symbols

    DT_REL
        Address of Rel relocation table

    DT_RELSZ
        Size in bytes of Rel relocation table

    DT_RELENT
        Size in bytes of a Rel table entry

    DT_PLTREL
        Type of relocation entry to which the PLT refers (Rela or Rel)

    DT_DEBUG
        Undefined use for debugging

    DT_TEXTREL
        Absence of this entry indicates that no relocation entries should apply to a nonwritable segment

    DT_JMPREL
        Address of relocation entries associated solely with the PLT

    DT_BIND_NOW
        Instruct dynamic linker to process all relocations before transferring control to the executable

    DT_RUNPATH
        String table offset to library search path

    DT_LOPROC
    DT_HIPROC
        Values in the inclusive range [DT_LOPROC, DT_HIPROC] are reserved for processor-specific semantics

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:

    Core files (e_type = ET_CORE)
        Notes used by all core files. These are highly operating system or architecture specific and often require close coordination with kernels, C libraries, and debuggers. These are used when the namespace is the default (i.e., n_namesz will be set to 0), or a fallback when the namespace is unknown.

        NT_PRSTATUS
            prstatus struct
        NT_FPREGSET
            fpregset struct
        NT_PRPSINFO
            prpsinfo struct
        NT_PRXREG
            prxregset struct
        NT_TASKSTRUCT
            task structure
        NT_PLATFORM
            String from sysinfo(SI_PLATFORM)
        NT_AUXV
            auxv array
        NT_GWINDOWS
            gwindows struct
        NT_ASRS
            asrset struct
        NT_PSTATUS
            pstatus struct
        NT_PSINFO
            psinfo struct
        NT_PRCRED
            prcred struct
        NT_UTSNAME
            utsname struct
        NT_LWPSTATUS
            lwpstatus struct
        NT_LWPSINFO
            lwpinfo struct
        NT_PRFPXREG
            fprxregset struct
        NT_SIGINFO
            siginfo_t (size might increase over time)
        NT_FILE
            Contains information about mapped files
        NT_PRXFPREG
            user_fxsr_struct
        NT_PPC_VMX
            PowerPC Altivec/VMX registers
        NT_PPC_SPE
            PowerPC SPE/EVR registers
        NT_PPC_VSX
            PowerPC VSX registers
        NT_386_TLS
            i386 TLS slots (struct user_desc)
        NT_386_IOPERM
            x86 io permission bitmap (1=deny)
        NT_X86_XSTATE
            x86 extended state using xsave
        NT_S390_HIGH_GPRS
            s390 upper register halves
        NT_S390_TIMER
            s390 timer register
        NT_S390_TODCMP
            s390 time-of-day (TOD) clock comparator register
        NT_S390_TODPREG
            s390 time-of-day (TOD) programmable register
        NT_S390_CTRS
            s390 control registers
        NT_S390_PREFIX
            s390 prefix register
        NT_S390_LAST_BREAK
            s390 breaking event address
        NT_S390_SYSTEM_CALL
            s390 system call restart data
        NT_S390_TDB
            s390 transaction diagnostic block
        NT_ARM_VFP
            ARM VFP/NEON registers
        NT_ARM_TLS
            ARM TLS register
        NT_ARM_HW_BREAK
            ARM hardware breakpoint registers
        NT_ARM_HW_WATCH
            ARM hardware watchpoint registers
        NT_ARM_SYSTEM_CALL
            ARM system call number

    n_name = GNU
        Extensions used by the GNU tool chain.

        NT_GNU_ABI_TAG
            Operating system (OS) ABI information. The desc field will be 4 words:

            [0] OS descriptor (ELF_NOTE_OS_LINUX, ELF_NOTE_OS_GNU, and so on)`
            [1] major version of the ABI
            [2] minor version of the ABI
            [3] subminor version of the ABI

        NT_GNU_HWCAP
            Synthetic hwcap information. The desc field begins with two words:

            [0] number of entries
            [1] bit mask of enabled entries

            Then follow variable-length entries, one byte followed by a null-terminated hwcap name string. The byte gives the bit number to test if enabled, (1U << bit) & bit mask.

        NT_GNU_BUILD_ID
            Unique build ID as generated by the GNU ld(1) --build-id option. The desc consists of any nonzero number of bytes.

        NT_GNU_GOLD_VERSION
            The desc contains the GNU Gold linker version used.

    Default/unknown namespace (e_type != ET_CORE)
        These are used when the namespace is the default (i.e., n_namesz will be set to 0), or a fallback when the namespace is unknown.

        NT_VERSION
            A version string of some sort.
        NT_ARCH
            Architecture information.

NOTES
ELF first appeared in System V. The ELF format is an adopted standard.

The extensions for e_phnum, e_shnum, and e_shstrndx respectively are Linux extensions. Sun, BSD, and AMD64 also support them; for further information, look under SEE ALSO.

SEE ALSO
as(1), elfedit(1), gdb(1), ld(1), nm(1), objcopy(1), objdump(1), patchelf(1), readelf(1), size(1), strings(1), strip(1), execve(2), dl_iterate_phdr(3), core(5), ld.so(8)

Hewlett-Packard, Elf-64 Object File Format.

Santa Cruz Operation, System V Application Binary Interface.

UNIX System Laboratories, "Object Files", Executable and Linking Format (ELF).

Sun Microsystems, Linker and Libraries Guide.

AMD64 ABI Draft, System V Application Binary Interface AMD64 Architecture Processor Supplement.