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, inde‐
              pendent 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 pro‐
                     cessor-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 spec‐
                     ification:

                     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 plat‐
                     form-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 cur‐
                     rently 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 indi‐
              vidual 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 sec‐
              tion 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 sec‐
              tion 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 ini‐
              tial 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 spec‐
       ifies 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 seg‐
                        ment entries in the program header table appear in
                        ascending order, sorted on the p_vaddr member.

                 PT_DYNAMIC
                        The array element specifies dynamic linking informa‐
                        tion.

                 PT_INTERP
                        The array element specifies the location and size of
                        a null-terminated pathname to invoke as an inter‐
                        preter.  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 loca‐
                        tion and size of the program header table itself,
                        both in the file and in the memory image of the pro‐
                        gram.  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 sec‐
       tions.  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 ta‐
       ble.  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 other‐
              wise 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 head‐
       ers.

       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 reloca‐
                     tion sections.

              SHT_HASH
                     This section holds a symbol hash table.  An object par‐
                     ticipating in dynamic linking must contain a symbol
                     hash table.  An object file may have only one hash ta‐
                     ble.

              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 con‐
                     tains 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 reloca‐
                     tion 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 dur‐
                     ing 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 proces‐
                     sor-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 sec‐
              tion 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 sec‐
              tion 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_PROG‐
              BITS.  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.  Other‐
              wise, that bit will be off.  This section is of type SHT_PROG‐
              BITS.

       .line  This section holds line number information for symbolic debug‐
              ging, 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-spe‐
              cific.

       .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.  Oth‐
              erwise, the bit will be off.  By convention, "NAME" is sup‐
              plied 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.  Oth‐
              erwise, the bit will be off.  By convention, "NAME" is sup‐
              plied 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, com‐
       monly called strings.  The object file uses these strings to repre‐
       sent 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 dif‐
       ferent 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 exe‐
                     cutable 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 defini‐
                     tions 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 sym‐
                     bols 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 sec‐
              tion.  This member holds the relevant section header table
              index.

   Relocation entries (Rel & Rela)
       Relocation is the process of connecting symbolic references with sym‐
       bolic definitions.  Relocatable files must have information that
       describes how to modify their section contents, thus allowing exe‐
       cutable 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 reloca‐
              tion 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 mem‐
              ber.

       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 rele‐
       vant 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 (depre‐
                          cated)

              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 interpreta‐
              tions.

       d_ptr  This member represents program virtual addresses.  When inter‐
              preting 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 names‐
       pace 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 ter‐
              minated.  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 names‐
                   pace 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 com‐
                                        parator 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 fol‐
                          lowed 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 names‐
                   pace 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), 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 5.08 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                            2020-04-11                           ELF(5)

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