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

     SYNOPSIS
          #include <elf.h>

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

          An executable file using the ELF file format consists of an
          ELF header, followed by a program header table or a section
          header table, or both.  The ELF header is always at offset
          zero of the file.  The program header table and the section
          header table's offset in the file are defined in the ELF
          header.  The two tables describe the rest of the particular-
          ities of the file.

          This header file describes the above mentioned headers as C
          structures and also includes structures for dynamic sec-
          tions, 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 pad-
          ding to ensure 4-byte alignment for 4-byte objects, to force
          structure sizes to a multiple of 4, and so on.

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        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:

          This array of bytes specifies how to interpret the file,
          independent of the processor or the file's remaining con-
          tents.  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: aqEaq)

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

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

          EI_CLASS
               The fifth byte identifies the architecture for this
               binary:

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               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 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
               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

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               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.

          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.

          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

          This member identifies the file version:

          EV_NONE         Invalid version
          EV_CURRENT      Current version

          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.

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          This member holds the program header table's file offset in
          bytes.  If the file has no program header table, this member
          holds zero.

          This member holds the section header table's file offset in
          bytes.  If the file has no section header table, this member
          holds zero.

          This member holds processor-specific flags associated with
          the file.  Flag names take the form EF_`machine_flag'.  Cur-
          rently, no flags have been defined.

          This member holds the ELF header's size in bytes.

          This member holds the size in bytes of one entry in the
          file's program header table; all entries are the same size.

          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 pro-
          gram header table is held in the sh_info member of the ini-
          tial 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.

          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.

          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.

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          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.

          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;

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                  uint64_t   p_align;
              } Elf64_Phdr;

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

               This member of the structure indicates what kind of
               segment this array element describes or how to inter-
               pret 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 seg-
                         ment, described by p_filesz and p_memsz. The
                         bytes from the file are mapped to the begin-
                         ning 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 unspec-
                         ified semantics.  Programs that contain an

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                         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.  More-
                         over, it may occur only if the program header
                         table is part of the memory image of the pro-
                         gram.  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 ker-
                         nel to control the state of the stack via the
                         flags set in the p_flags member.

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

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

               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.

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

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

               This member holds a bit mask of flags relevant to the
               segment:

               PF_X An executable segment.
               PF_W A writable segment.

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               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.

               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 seman-
               tics.

          SHN_ABS
               This value specifies the absolute value for the corre-
               sponding reference.  For example, a symbol defined rel-
               ative 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

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               symbols, such as FORTRAN COMMON or unallocated C exter-
               nal 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 sec-
               tion 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 sec-
          tion headers.

          This member specifies the name of the section.  Its value is
          an index into the section header string table section, giv-
          ing the location of a null-terminated string.

          This member categorizes the section's contents and seman-
          tics.

          SHT_NULL
               This value marks the section header as inactive.  It

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               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
               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 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

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               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.

          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
               processor-specific semantics.

          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.

          This member's value holds the byte offset from the beginning

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          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.

          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.

          This member holds a section header table index link, whose
          interpretation depends on the section type.

          This member holds extra information, whose interpretation
          depends on the section type.

          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.

          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:

          This section holds uninitialized data that contributes to
          the program's memory image.  By definition, the system ini-
          tializes 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.

          This section holds version control information.  This sec-
          tion is of type SHT_PROGBITS.  No attribute types are used.

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

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          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.

          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.

          This section holds information for symbolic debugging.  The
          contents are unspecified.  This section is of type
          SHT_PROGBITS.  No attribute types are used.

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

          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.

          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.

          This section holds the dynamic linking symbol table.  This
          section is of type SHT_DYNSYM.  The attribute used is
          SHF_ALLOC.

          This section holds executable instructions that contribute
          to the process termination code.  When a program exits nor-
          mally the system arranges to execute the code in this sec-
          tion.  This section is of type SHT_PROGBITS.  The attributes
          used are SHF_ALLOC and SHF_EXECINSTR.

          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.

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          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.

          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.

          This section holds the global offset table.  This section is
          of type SHT_PROGBITS.  The attributes are processor-
          specific.

          This section holds a symbol hash table.  This section is of
          type SHT_HASH.  The attribute used is SHF_ALLOC.

          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 sec-
          tion before calling the main program entry point.  This sec-
          tion is of type SHT_PROGBITS.  The attributes used are
          SHF_ALLOC and SHF_EXECINSTR.

          This section holds the pathname of a program interpreter.
          If the file has a loadable segment that includes the sec-
          tion, the section's attributes will include the SHF_ALLOC
          bit.  Otherwise, that bit will be off.  This section is of
          type SHT_PROGBITS.

          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.

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

          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.

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     ELF(5)                    (2020-12-21)                     ELF(5)

          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.

          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.

          OpenBSD native executables usually contain this section to
          identify themselves so the kernel can bypass any compatibil-
          ity ELF binary emulation tests when loading the file.

          This section holds the procedure linkage table.  This sec-
          tion is of type SHT_PROGBITS.  The attributes are
          processor-specific.

          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 conven-
          tion, "NAME" is supplied by the section to which the reloca-
          tions apply.  Thus a relocation section for .text normally
          would have the name .rel.text.  This section is of type
          SHT_REL.

          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 conven-
          tion, "NAME" is supplied by the section to which the reloca-
          tions apply.  Thus a relocation section for .text normally
          would have the name .rela.text.  This section is of type
          SHT_RELA.

          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.

          This section holds read-only data that typically contributes
          to a nonwritable segment in the process image.  This section

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          is of type SHT_PROGBITS.  The attribute used is SHF_ALLOC.

          This section holds section names.  This section is of type
          SHT_STRTAB.  No attribute types are used.

          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 sec-
          tion is of type SHT_STRTAB.

          This section holds a symbol table.  If the file has a load-
          able 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.

          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 sec-
          tion.  The first byte, which is index zero, is defined to
          hold a null byte (aq0aq).  Similarly, a string table's last
          byte is defined to hold a null byte, ensuring null termina-
          tion 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;

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                  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.

          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.

          This member gives the value of the associated symbol.

          Many symbols have associated sizes.  This member holds zero
          if the symbol has no size or an unknown size.

          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.

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          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),
               Convert a binding and a type into an st_info value.

          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:

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          ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

          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 con-
          tents, 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;

          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 stor-
          age unit affected by the relocation.

          This member gives both the symbol table index with respect
          to which the relocation must be made and the type of

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          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, respec-
          tively, to the entry's r_info member.

          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[];

          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

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          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 (dep-
                      recated)

          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 reloca-
                      tion entries should apply to a nonwritable seg-
                      ment

          DT_JMPREL   Address of relocation entries associated solely
                      with the PLT

          DT_BIND_NOW Instruct dynamic linker to process all reloca-
                      tions before transferring control to the exe-
                      cutable

          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

          This member represents integer values with various interpre-
          tations.

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          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 def-
          initions 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

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          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;

          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.

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

          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 operat-
               ing 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 fall-
               back 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

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     ELF(5)                    (2020-12-21)                     ELF(5)

               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) pro-
                                    grammable 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:

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

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

                    +o word 0: number of entries
                    +o word 1: bit mask of enabled entries

                    Then follow variable-length entries, one byte fol-
                    lowed by a null-terminated hwcap name string.  The

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                    byte gives the bit number to test if enabled, (1U
                    << bit) & bit mask.

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

               NT_GNU_GOLD_VERSION
                    The desc contains the GNU Gold linker version
                    used.

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

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

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

          The extensions for e_phnum, e_shnum, and e_shstrndx respec-
          tively are Linux extensions.  Sun, BSD and AMD64 also sup-
          port them; for further information, look under SEE ALSO.

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

          Hewlett-Packard, Elf-64 Object File Format.

          Santa Cruz Operation, System V Application Binary Interface.

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

          Sun Microsystems, Linker and Libraries Guide.

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

     COLOPHON
          This page is part of release 5.10 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/.

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