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

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

SYNOPSIS
       #include <elf.h>

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

       An executable file using the ELF file format consists of an ELF header, followed by a program header table or a section header table, or both.  The ELF header is
       always at offset zero of the file.  The program header table and the section header table's offset in the file are defined in the ELF header.  The two tables deā€
       scribe 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 conā€
       tain explicit padding to ensure 4-byte alignment for 4-byte objects, to force structure sizes to a multiple of 4, and so on.

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

           #define EI_NIDENT 16

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

       The fields have the following meanings:

       e_ident
              This array of bytes specifies how to interpret the file, independent of the processor or the file's remaining contents.  Within this array  everything  is
              named by macros, which start with the prefix EI_ and may contain values which start with the prefix ELF.  The following macros are defined:

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

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

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

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

              EI_CLASS
                     The fifth byte identifies the architecture for this binary:

                     ELFCLASSNONE  This class is invalid.
                     ELFCLASS32    This defines the 32-bit architecture.  It supports machines with files and virtual address spaces up to 4 Gigabytes.
                     ELFCLASS64    This defines the 64-bit architecture.

              EI_DATA
                     The sixth byte specifies the data encoding of the processor-specific data in the file.  Currently, these encodings are supported:

                       ELFDATANONE   Unknown data format.
                       ELFDATA2LSB   Two's complement, little-endian.
                       ELFDATA2MSB   Two's complement, big-endian.

              EI_VERSION
                     The seventh byte is the version number of the ELF specification:

                     EV_NONE       Invalid version.
                     EV_CURRENT    Current version.

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

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

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

              EI_PAD Start of padding.  These bytes are reserved and set to zero.  Programs which read them should ignore them.  The value for EI_PAD will change in the
                     future if currently unused bytes are given meanings.

              EI_NIDENT
                     The size of the e_ident array.

       e_type This member of the structure identifies the object file type:

              ET_NONE         An unknown type.
              ET_REL          A relocatable file.
              ET_EXEC         An executable file.
              ET_DYN          A shared object.
              ET_CORE         A core file.

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

              EM_NONE         An unknown machine
              EM_M32          AT&T WE 32100
              EM_SPARC        Sun Microsystems SPARC
              EM_386          Intel 80386
              EM_68K          Motorola 68000
              EM_88K          Motorola 88000
              EM_860          Intel 80860
              EM_MIPS         MIPS RS3000 (big-endian only)
              EM_PARISC       HP/PA
              EM_SPARC32PLUS  SPARC with enhanced instruction set
              EM_PPC          PowerPC
              EM_PPC64        PowerPC 64-bit
              EM_S390         IBM S/390
              EM_ARM          Advanced RISC Machines
              EM_SH           Renesas SuperH
              EM_SPARCV9      SPARC v9 64-bit
              EM_IA_64        Intel Itanium
              EM_X86_64       AMD x86-64
              EM_VAX          DEC Vax

       e_version
              This member identifies the file version:

              EV_NONE         Invalid version
              EV_CURRENT      Current version

       e_entry
              This  member gives the virtual address to which the system first transfers control, thus starting the process.  If the file has no associated entry point,
              this member holds zero.

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

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

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

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

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

       e_phnum
              This member holds the number of entries in the program header table.  Thus the product of e_phentsize and e_phnum gives the table's size in bytes.   If  a
              file has no program header, e_phnum holds the value zero.

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

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

       e_shentsize
              This member holds a sections header's size in bytes.  A section header is one entry in the section header table; all entries are the same size.

       e_shnum
              This member holds the number of entries in the section header table.  Thus the product of e_shentsize and e_shnum gives the section header table's size in
              bytes.  If a file has no section header table, e_shnum holds the value of zero.

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

       e_shstrndx
              This member holds the section header table index of the entry associated with the section name string table.  If the file has no section name  string  taā€
              ble, this member holds the value SHN_UNDEF.

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

   Program header (Phdr)
       An  executable or shared object file's program header table is an array of structures, each describing a segment or other information the system needs to prepare
       the program for execution.  An object file segment contains one or more sections.  Program headers are meaningful only for executable and shared object files.  A
       file  specifies its own program header size with the ELF header's e_phentsize and e_phnum members.  The ELF program header is described by the type Elf32_Phdr or
       Elf64_Phdr depending on the architecture:

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

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

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

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

                 PT_NULL
                        The array element is unused and the other members' values are undefined.  This lets the program header have ignored entries.

                 PT_LOAD
                        The array element specifies a loadable segment, described by p_filesz and p_memsz.  The bytes from the file are mapped to the beginning  of  the
                        memory  segment.   If the segment's memory size p_memsz is larger than the file size p_filesz, the "extra" bytes are defined to hold the value 0
                        and to follow the segment's initialized area.  The file size may not be larger than the memory size.  Loadable segment entries  in  the  program
                        header table appear in ascending order, sorted on the p_vaddr member.

                 PT_DYNAMIC
                        The array element specifies dynamic linking information.

                 PT_INTERP
                        The  array  element  specifies the location and size of a null-terminated pathname to invoke as an interpreter.  This segment type is meaningful
                        only for executable files (though it may occur for shared objects).  However it may not occur more than once in a file.  If it  is  present,  it
                        must precede any loadable segment entry.

                 PT_NOTE
                        The array element specifies the location of notes (ElfN_Nhdr).

                 PT_SHLIB
                        This segment type is reserved but has unspecified semantics.  Programs that contain an array element of this type do not conform to the ABI.

                 PT_PHDR
                        The  array  element, if present, specifies the location and size of the program header table itself, both in the file and in the memory image of
                        the program.  This segment type may not occur more than once in a file.  Moreover, it may occur only if the program header table is part of  the
                        memory image of the program.  If it is present, it must precede any loadable segment entry.

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

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

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

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

       p_paddr
              On  systems  for which physical addressing is relevant, this member is reserved for the segment's physical address.  Under BSD this member is not used and
              must be zero.

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

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

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

              PF_X   An executable segment.
              PF_W   A writable segment.
              PF_R   A readable segment.

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

       p_align
              This member holds the value to which the segments are aligned in memory and in the file.  Loadable process segments must have congruent values for p_vaddr
              and  p_offset,  modulo  the  page size.  Values of zero and one mean no alignment is required.  Otherwise, p_align should be a positive, integral power of
              two, and p_vaddr should equal p_offset, modulo p_align.

   Section header (Shdr)
       A file's section header table lets one locate all the file's sections.  The section header table is an array of Elf32_Shdr or  Elf64_Shdr  structures.   The  ELF
       header's e_shoff member gives the byte offset from the beginning of the file to the section header table.  e_shnum holds the number of entries the section header
       table contains.  e_shentsize holds the size in bytes of each entry.

       A section header table index is a subscript into this array.  Some section header table indices are reserved: the initial entry and the indices between SHN_LOREā€
       SERVE  and  SHN_HIRESERVE.   The initial entry is used in ELF extensions for e_phnum, e_shnum, and e_shstrndx; in other cases, each field in the initial entry is
       set to zero.  An object file does not have sections for these special indices:

       SHN_UNDEF
              This value marks an undefined, missing, irrelevant, or otherwise meaningless section reference.

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

       SHN_LOPROC, SHN_HIPROC
              Values greater in the inclusive range [SHN_LOPROC, SHN_HIPROC] are reserved for processor-specific semantics.

       SHN_ABS
              This value specifies the absolute value for the corresponding reference.  For example, a symbol defined relative to section number SHN_ABS has an absolute
              value and is not affected by relocation.

       SHN_COMMON
              Symbols defined relative to this section are common symbols, such as FORTRAN COMMON or unallocated C external variables.

       SHN_HIRESERVE
              This  value  specifies  the upper bound of the range of reserved indices.  The system reserves indices between SHN_LORESERVE and SHN_HIRESERVE, inclusive.
              The section header table does not contain entries for the reserved indices.

       The section header has the following structure:

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

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

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

       sh_name
              This member specifies the name of the section.  Its value is an index into the section header string table section, giving the location of  a  null-termiā€
              nated 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 valā€
                     ues.

              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 mulā€
                     tiple relocation sections.

              SHT_HASH
                     This section holds a symbol hash table.  An object participating in dynamic linking must contain a symbol hash table.  An object file may have only
                     one hash table.

              SHT_DYNAMIC
                     This section holds information for dynamic linking.  An object file may have only one dynamic section.

              SHT_NOTE
                     This section holds notes (ElfN_Nhdr).

              SHT_NOBITS
                     A  section of this type occupies no space in the file but otherwise resembles SHT_PROGBITS.  Although this section contains no bytes, the sh_offset
                     member contains the conceptual file offset.

              SHT_REL
                     This section holds relocation offsets without explicit addends, such as type Elf32_Rel for the 32-bit class of object files.  An  object  file  may
                     have multiple relocation sections.

              SHT_SHLIB
                     This section is reserved but has unspecified semantics.

              SHT_DYNSYM
                     This section holds a minimal set of dynamic linking symbols.  An object file can also contain a SHT_SYMTAB section.

              SHT_LOPROC, SHT_HIPROC
                     Values in the inclusive range [SHT_LOPROC, SHT_HIPROC] are reserved for processor-specific semantics.

              SHT_LOUSER
                     This value specifies the lower bound of the range of indices reserved for application programs.

              SHT_HIUSER
                     This  value  specifies  the upper bound of the range of indices reserved for application programs.  Section types between SHT_LOUSER and SHT_HIUSER
                     may be used by the application, without conflicting with current or future system-defined section types.

       sh_flags
              Sections support one-bit flags that describe miscellaneous attributes.  If a flag bit is set in sh_flags, the attribute is "on" for the  section.   Otherā€
              wise, the attribute is "off" or does not apply.  Undefined attributes are set to zero.

              SHF_WRITE
                     This section contains data that should be writable during process execution.

              SHF_ALLOC
                     This  section occupies memory during process execution.  Some control sections do not reside in the memory image of an object file.  This attribute
                     is off for those sections.

              SHF_EXECINSTR
                     This section contains executable machine instructions.

              SHF_MASKPROC
                     All bits included in this mask are reserved for processor-specific semantics.

       sh_addr
              If this section appears in the memory image of a process, this member holds the address at which the section's first byte should reside.   Otherwise,  the
              member contains zero.

       sh_offset
              This  member's  value  holds  the  byte offset from the beginning of the file to the first byte in the section.  One section type, SHT_NOBITS, occupies no
              space in the file, and its sh_offset member locates the conceptual placement in the file.

       sh_size
              This member holds the section's size in bytes.  Unless the section type is SHT_NOBITS, the section occupies sh_size bytes in the file.  A section of  type
              SHT_NOBITS may have a nonzero size, but it occupies no space in the file.

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

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

       sh_addralign
              Some  sections  have  address alignment constraints.  If a section holds a doubleword, the system must ensure doubleword alignment for the entire section.
              That is, the value of sh_addr must be congruent to zero, modulo the value of sh_addralign.  Only zero and positive integral powers  of  two  are  allowed.
              The value 0 or 1 means that the section has no alignment constraints.

       sh_entsize
              Some  sections hold a table of fixed-sized entries, such as a symbol table.  For such a section, this member gives the size in bytes for each entry.  This
              member contains zero if the section does not hold a table of fixed-size entries.

       Various sections hold program and control information:

       .bss   This section holds uninitialized data that contributes to the program's memory image.  By definition, the system initializes the data with zeros when  the
              program begins to run.  This section is of type SHT_NOBITS.  The attribute types are SHF_ALLOC and SHF_WRITE.

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

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

       .data  This section holds initialized data that contribute to the program's memory image.  This section is of type SHT_PROGBITS.  The attribute types are SHF_ALā€
              LOC 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_ALā€
              LOC and SHF_WRITE.

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

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

       .dynamic
              This  section holds dynamic linking information.  The section's attributes will include the SHF_ALLOC bit.  Whether the SHF_WRITE bit is set is processor-
              specific.  This section is of type SHT_DYNAMIC.  See the attributes above.

       .dynstr
              This section holds strings needed for dynamic linking, most commonly the strings that represent the names associated with symbol table entries.  This secā€
              tion 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_ALā€
              LOC.

       .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 exeā€
              cute 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 inā€
              clude the SHF_ALLOC bit.  Otherwise, that bit will be off.  This section is of type SHT_PROGBITS.

       .line  This section holds line number information for symbolic debugging, which describes the correspondence between the program source  and  the  machine  code.
              The contents are unspecified.  This section is of type SHT_PROGBITS.  No attribute types are used.

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

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

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

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

       .note.openbsd.ident
              OpenBSD  native executables usually contain this section to identify themselves so the kernel can bypass any compatibility ELF binary emulation tests when
              loading the file.

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

       .relNAME
              This section holds relocation information as described below.  If the file has a loadable segment that includes relocation, the section's attributes  will
              include  the SHF_ALLOC bit.  Otherwise, the bit will be off.  By convention, "NAME" is supplied by the section to which the relocations apply.  Thus a reā€
              location section for .text normally would have the name .rel.text.  This section is of type SHT_REL.

       .relaNAME
              This section holds relocation information as described below.  If the file has a loadable segment that includes relocation, the section's attributes  will
              include  the SHF_ALLOC bit.  Otherwise, the bit will be off.  By convention, "NAME" is supplied by the section to which the relocations apply.  Thus a reā€
              location section for .text normally would have the name .rela.text.  This section is of type SHT_RELA.

       .rodata
              This section holds read-only data that typically contributes to a nonwritable segment in the process image.  This section is of  type  SHT_PROGBITS.   The
              attribute used is SHF_ALLOC.

       .rodata1
              This  section  holds  read-only data that typically contributes to a nonwritable segment in the process image.  This section is of type SHT_PROGBITS.  The
              attribute used is SHF_ALLOC.

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

       .strtab
              This section holds strings, most commonly the strings that represent the names associated with symbol table entries.  If the file has a  loadable  segment
              that  includes the symbol string table, the section's attributes will include the SHF_ALLOC bit.  Otherwise, the bit will be off.  This section is of type
              SHT_STRTAB.

       .symtab
              This section holds a symbol table.  If the file has a loadable segment that includes the symbol table, the section's attributes will include the SHF_ALLOC
              bit.  Otherwise, the bit will be off.  This section is of type SHT_SYMTAB.

       .text  This  section  holds  the  "text", or executable instructions, of a program.  This section is of type SHT_PROGBITS.  The attributes used are SHF_ALLOC and
              SHF_EXECINSTR.

   String and symbol tables
       String table sections hold null-terminated character sequences, commonly called strings.  The object file uses these strings  to  represent  symbol  and  section
       names.   One  references  a string as an index into the string table section.  The first byte, which is index zero, is defined to hold a null byte ('\0').  Simiā€
       larly, 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  subā€
       script into this array.

           typedef struct {
               uint32_t      st_name;
               Elf32_Addr    st_value;
               uint32_t      st_size;
               unsigned char st_info;
               unsigned char st_other;
               uint16_t      st_shndx;
           } Elf32_Sym;

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

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

       st_name
              This  member  holds  an index into the object file's symbol string table, which holds character representations of the symbol names.  If the value is nonā€
              zero, it represents a string table index that gives the symbol name.  Otherwise, the symbol has no name.

       st_value
              This member gives the value of the associated symbol.

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

       st_info
              This member specifies the symbol's type and binding attributes:

              STT_NOTYPE
                     The symbol's type is not defined.

              STT_OBJECT
                     The symbol is associated with a data object.

              STT_FUNC
                     The symbol is associated with a function or other executable code.

              STT_SECTION
                     The symbol is associated with a section.  Symbol table entries of this type exist primarily for relocation and normally have STB_LOCAL bindings.

              STT_FILE
                     By convention, the symbol's name gives the name of the source file associated with the object file.  A file symbol has STB_LOCAL bindings, its secā€
                     tion 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 refā€
                     erence to the same symbol.

              STB_WEAK
                     Weak symbols resemble global symbols, but their definitions have lower precedence.

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

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

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

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

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

       st_other
              This member defines the symbol visibility.

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

              There are macros for extracting the visibility type:

              ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

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

   Relocation entries (Rel & Rela)
       Relocation  is  the  process  of  connecting symbolic references with symbolic definitions.  Relocatable files must have information that describes how to modify
       their section contents, thus allowing executable and shared object files to hold the right information for a process's program  image.   Relocation  entries  are
       these data.

       Relocation structures that do not need an addend:

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

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

       Relocation structures that need an addend:

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

           typedef struct {
               Elf64_Addr r_offset;
               uint64_t   r_info;
               int64_t    r_addend;
           } Elf64_Rela;

       r_offset
              This  member  gives  the location at which to apply the relocation action.  For a relocatable file, the value is the byte offset from the beginning of the
              section to the storage unit affected by the relocation.  For an executable file or shared object, the value is the virtual address of the storage unit afā€
              fected by the relocation.

       r_info This member gives both the symbol table index with respect to which the relocation must be made and the type of relocation to apply.  Relocation types are
              processor-specific.  When the text refers to a relocation entry's relocation type or symbol table index, it means the result of applying ELF[32|64]_R_TYPE
              or ELF[32|64]_R_SYM, respectively, to the entry's r_info member.

       r_addend
              This member specifies a constant addend used to compute the value to be stored into the relocatable field.

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

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

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

       d_tag  This member may have any of the following values:

              DT_NULL     Marks end of dynamic section

              DT_NEEDED   String table offset to name of a needed library

              DT_PLTRELSZ Size in bytes of PLT relocation entries

              DT_PLTGOT   Address of PLT and/or GOT

              DT_HASH     Address of symbol hash table

              DT_STRTAB   Address of string table

              DT_SYMTAB   Address of symbol table

              DT_RELA     Address of Rela relocation table

              DT_RELASZ   Size in bytes of the Rela relocation table

              DT_RELAENT  Size in bytes of a Rela relocation table entry

              DT_STRSZ    Size in bytes of string table

              DT_SYMENT   Size in bytes of a symbol table entry

              DT_INIT     Address of the initialization function

              DT_FINI     Address of the termination function

              DT_SONAME   String table offset to name of shared object

              DT_RPATH    String table offset to library search path (deprecated)

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

              DT_REL      Address of Rel relocation table

              DT_RELSZ    Size in bytes of Rel relocation table

              DT_RELENT   Size in bytes of a Rel table entry

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

              DT_DEBUG    Undefined use for debugging

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

              DT_JMPREL   Address of relocation entries associated solely with the PLT

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

              DT_RUNPATH  String table offset to library search path

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

       d_val  This member represents integer values with various interpretations.

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

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

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

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

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

           void *memory, *name, *desc;
           Elf64_Nhdr *note, *next_note;

           /* The buffer is pointing to the start of the section/segment. */
           note = memory;

           /* If the name is defined, it follows the note. */
           name = note->n_namesz == 0 ? NULL : memory + sizeof(*note);

           /* If the descriptor is defined, it follows the name
              (with alignment). */

           desc = note->n_descsz == 0 ? NULL :
                  memory + sizeof(*note) + ALIGN_UP(note->n_namesz, 4);

           /* The next note follows both (with alignment). */
           next_note = memory + sizeof(*note) +
                                ALIGN_UP(note->n_namesz, 4) +
                                ALIGN_UP(note->n_descsz, 4);

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

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

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

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

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

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

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

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

              n_name = GNU
                   Extensions used by the GNU tool chain.

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

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

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

                          ā€¢ word 0: number of entries
                          ā€¢ word 1: bit mask of enabled entries

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

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

                   NT_GNU_GOLD_VERSION
                          The desc contains the GNU Gold linker version used.

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

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

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

       The  extensions for e_phnum, e_shnum, and e_shstrndx respectively are Linux extensions.  Sun, BSD, and AMD64 also support them; for further information, look unā€
       der 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.

Linux                                                                          2021-03-22                                                                         ELF(5)