elf(5) — Linux manual page

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ELF(5)                  Linux Programmer's Manual                 ELF(5)

NAME         top

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

SYNOPSIS         top

       #include <elf.h>

DESCRIPTION         top

       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:

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

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

                          • word 0: number of entries
                          • word 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         top

       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         top

       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.

COLOPHON         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                         ELF(5)

Pages that refer to this page: dl_iterate_phdr(3)end(3)core(5)ld.so(8)