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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 exe-
       cutable files, relocatable object files, core files and shared libraries.

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

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

       The following types are used for N-bit architectures (N=32,64, ElfN stands for Elf32 or Elf64,  uintN_t  stands
       for uint32_t or uint64_t):

           ElfN_Addr       Unsigned program address, uintN_t
           ElfN_Off        Unsigned file offset, uintN_t
           ElfN_Section    Unsigned section index, uint16_t
           ElfN_Versym     Unsigned version symbol information, uint16_t
           Elf_Byte        unsigned char
           ElfN_Half       uint16_t
           ElfN_Sword      int32_t
           ElfN_Word       uint32_t
           ElfN_Sxword     int64_t
           ElfN_Xword      uint64_t

       (Note:  The  *BSD  terminology  is  a  bit  different.   There  Elf64_Half is twice as large as Elf32_Half, and
       Elf64Quarter is used for uint16_t.  In order to avoid confusion these types are replaced by  explicit  ones  in
       the below.)

       All  data  structures  that  the file format defines follow the "natural" size and alignment guidelines for the
       relevant class.  If necessary, data structures contain explicit padding to ensure 4-byte alignment  for  4-byte
       objects, to force structure sizes to a multiple of 4, etc.

       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 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 version number of the ELF specification:
                               EV_NONE       Invalid version.
                               EV_CURRENT    Current version.

                   EI_OSABI    This  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.
                               E.g.:

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

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

                   EI_BRAND    Start of architecture identification.

                   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.  E.g.:

                   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_phent-
                   size  and  e_phnum gives the table's size in bytes.  If a file has no program header, e_phnum holds
                   the value zero.

       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_shent-
                   size 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.

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

                   SHN_UNDEF     This  value marks an undefined, missing, irrelevant, or otherwise meaningless section
                                 reference.  For example, a symbol "defined" relative to section number  SHN_UNDEF  is
                                 an undefined symbol.

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

                   SHN_LOPROC    Values greater than or equal to SHN_HIPROC are reserved for processor-specific seman-
                                 tics.

                   SHN_HIPROC    Values less than or equal to SHN_LOPROC are reserved  for  processor-specific  seman-
                                 tics.

                   SHN_ABS       This  value  specifies absolute values for the corresponding reference.  For example,
                                 symbols defined relative to section number SHN_ABS have absolute values and  are  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  between
                                 SHN_LORESERVE  and  SHN_HIRESERVE, inclusive; the values do not reference the section
                                 header table.  That is, the section header table does not  contain  entries  for  the
                                 reserved indices.

       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 Phdr struct tells 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 seg-
                               ment'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 and size for auxiliary information.

                   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 only occur if the pro-
                               gram header table is part of the memory image of the program.  If  it  is  present,  it
                               must precede any loadable segment entry.

                   PT_LOPROC   Values  greater  than  or equal to PT_HIPROC are reserved for processor-specific seman-
                               tics.

                   PT_HIPROC   Values less than or equal to PT_LOPROC 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 bitmask 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_X,  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.

       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  con-
       tains.  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.
       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    Values greater than or equal to SHN_HIPROC are reserved for processor-specific semantics.

       SHN_HIPROC    Values less than or equal to SHN_LOPROC 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 reloca-
                     tion.

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

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

       The section header has the following structure:

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

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

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

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

       sh_type   This member categorizes the section's contents and semantics.

                 SHT_NULL       This value marks the section header as inactive.  It does not have an associated  sec-
                                tion.  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 sec-
                                tions.

                 SHT_RELA       This section holds relocation entries with explicit addends, such as  type  Elf32_Rela
                                for  the  32-bit  class  of object files.  An object may have multiple relocation sec-
                                tions.

                 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 information that marks the file in some way.

                 SHT_NOBITS     A section of this type occupies no space in the file but otherwise resembles SHT_PROG-
                                BITS.  Although this section contains no bytes, the sh_offset member contains the con-
                                ceptual 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     This  value  up  to and including SHT_HIPROC is reserved for processor-specific seman-
                                tics.

                 SHT_HIPROC     This value down to and including SHT_LOPROC is reserved for processor-specific  seman-
                                tics.

                 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 applica-
                                tion, without conflicting with current or future system-defined section types.

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

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

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

                 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 non-zero  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.  Values of zero or one mean the section has no alignment constraints.

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

       Various sections hold program and control information:

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

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

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

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

       .data1    This section holds initialized data that contribute to the program's memory image.  This  section  is
                 of type SHT_PROGBITS.  The attribute types are SHF_ALLOC and SHF_WRITE.

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

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

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

       .dynstr   This  section  holds strings needed for dynamic linking, most commonly the strings that represent the
                 names associated with symbol table entries.  This section is of type SHT_STRTAB.  The attribute  type
                 used is SHF_ALLOC.

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

       .fini     This section holds executable instructions that contribute to the process termination code.   When  a
                 program  exits  normally the system arranges to execute the code in this section.  This section is of
                 type SHT_PROGBITS.  The attributes used are SHF_ALLOC and SHF_EXECINSTR.

       .gnu.version
                 This section holds the version symbol table, an array of ElfN_Half elements.  This section is of type
                 SHT_GNU_versym.  The attribute type used is SHF_ALLOC.

       .gnu.version_d
                 This  section  holds the version symbol definitions, a table of ElfN_Verdef structures.  This section
                 is of type SHT_GNU_verdef.  The attribute type used is SHF_ALLOC.

       .gnu.version_r
                 This section holds the version symbol needed elements, a table of ElfN_Verneed structures.  This sec-
                 tion is of type SHT_GNU_versym.  The attribute type used is SHF_ALLOC.

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

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

       .init     This  section holds executable instructions that contribute to the process initialization code.  When
                 a program starts to run the system arranges to execute the code in this section  before  calling  the
                 main  program  entry point.  This section is of type SHT_PROGBITS.  The attributes used are SHF_ALLOC
                 and SHF_EXECINSTR.

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

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

       .note     This section holds information in the "Note Section" format described below.  This section is of type
                 SHT_NOTE.    No   attribute   types   are   used.   OpenBSD  native  executables  usually  contain  a
                 .note.openbsd.ident section to identify themselves, for the kernel to bypass  any  compatibility  ELF
                 binary emulation tests when loading the file.

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

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

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

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

       .rodata   This section holds read-only data that typically contributes to a non-writable 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 non-writable 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  sec-
                 tion'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  ta-
                 ble,  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  table sections hold null-terminated character sequences, commonly called strings.  The object file uses
       these strings to represent symbol and section names.  One references a string as an index into the string table
       section.   The first byte, which is index zero, is defined to hold a null byte ('\0').  Similarly, a string ta-
       ble's last byte is defined to hold a null byte, ensuring null termination for all strings.

       An object file's symbol table holds information needed to locate and relocate a program's symbolic  definitions
       and references.  A symbol table index is a subscript into this array.

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

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

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

       st_name   This  member  holds an index into the object file's symbol string table, which holds character repre-
                 sentations of the symbol names.  If the value is non-zero, it represents a string  table  index  that
                 gives the symbol name.  Otherwise, the symbol table 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 primar-
                             ily 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  This value up to and including STT_HIPROC is reserved for processor-specific semantics.

                 STT_HIPROC  This value down to and including STT_LOPROC is reserved for processor-specific semantics.

                 STB_LOCAL   Local symbols are not visible outside the object file containing their definition.  Local
                             symbols of the same name may exist in multiple files without interfering with each other.

                 STB_GLOBAL  Global symbols are visible to all object files being combined.  One file's definition  of
                             a global symbol will satisfy another file's undefined reference to the same symbol.

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

                 STB_LOPROC  This value up to and including STB_HIPROC is reserved for processor-specific semantics.

                 STB_HIPROC  This value down to and including STB_LOPROC is reserved for processor-specific semantics.

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

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

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

                             ELF32_ST_INFO(bind, type) or 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.
                 STV_INTERNAL    Processor-specific hidden class.
                 STV_HIDDEN      Symbol is unavailable in other modules.
                 STV_PROTECTED   Not preemptible, not exported.

                 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  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 stor-
                   age unit affected by the relocation.

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

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

       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 relocs

                 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 relocs table

                 DT_RELASZ   Size in bytes of Rela table

                 DT_RELAENT  Size in bytes of a Rela 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 relocs table

                 DT_RELSZ    Size in bytes of Rel table

                 DT_RELENT   Size in bytes of a Rel table entry

                 DT_PLTREL   Type of reloc the PLT refers (Rela or Rel)

                 DT_DEBUG    Undefined use for debugging

                 DT_TEXTREL  Absence of this indicates no relocs should apply to a non-writable segment

                 DT_JMPREL   Address of reloc entries solely for the PLT

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

                 DT_RUNPATH  String table offset to library search path

                 DT_LOPROC   Start of processor-specific semantics

                 DT_HIPROC   End of 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
       ELF first appeared in System V.  The ELF format is an adopted standard.

SEE ALSO
       as(1), gdb(1), ld(1), objdump(1), execve(2), core(5)

       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).

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2007-12-28                            ELF(5)