elf

ELF(5) File Formats 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 describe the rest of the particularities of the
   file.

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

Basic types

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

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

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

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

ELF header (Ehdr)

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

       #define EI_NIDENT 16

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

   The fields have the following meanings:

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

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

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

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

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

          EI_CLASS
                 The fifth byte identifies the architecture for this binary:

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

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

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

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

                 EV_NONE       Invalid version.
                 EV_CURRENT    Current version.

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

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

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

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

          EI_NIDENT
                 The size of the e_ident array.

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

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

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

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

   e_version
          This member identifies the file version:

          EV_NONE         Invalid version
          EV_CURRENT      Current version

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

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

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

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

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

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

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

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

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

          If the index of section name string table section is larger than or equal to SHN_LORESERVE (0xff00), this member holds SHN_XINDEX (0xffff) and the real  index  of  the  section
          name  string table section is held in the sh_link member of the initial entry in section header table.  Otherwise, the sh_link member of the initial entry in section header ta
          ble 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 execu‐
   tion.  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 seg
                    ment type may not occur more than once in a file.  Moreover, it may occur only if the program header table is part of the memory image  of  the  program.   If  it  is
                    present, it must precede any loadable segment entry.

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

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

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

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

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

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

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

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

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

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

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

Section header (Shdr)

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

   A section header table index is a subscript into this array.  Some section header table indices are reserved: the initial entry and the indices  between  SHN_LORESERVE  and  SHN_HIRE‐
   SERVE.  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 af‐
          fected 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  ta‐
          ble does not contain entries for the reserved indices.

   The section header has the following structure:

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

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

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

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

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

          SHT_NULL
                 This value marks the section header as inactive.  It does not have an associated section.  Other members of the section header have undefined values.

          SHT_PROGBITS
                 This section holds information defined by the program, whose format and meaning are determined solely by the program.

          SHT_SYMTAB
                 This section holds a symbol table.  Typically, SHT_SYMTAB provides symbols for link editing, though it may also be used for dynamic linking.  As a complete symbol table,
                 it may contain many symbols unnecessary for dynamic linking.  An object file can also contain a SHT_DYNSYM section.

          SHT_STRTAB
                 This section holds a string table.  An object file may have multiple string table sections.

          SHT_RELA
                 This  section  holds relocation entries with explicit addends, such as type Elf32_Rela for the 32-bit class of object files.  An object may have multiple relocation 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 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 ap
                 plication, 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
          nonzero size, but it occupies no space in the file.

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

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

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

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

   Various sections hold program and control information:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

String and symbol tables

   String table sections hold null-terminated character sequences, commonly called strings.  The object file uses these strings to represent symbol and section names.  One  references  a
   string  as  an  index  into the string table section.  The first byte, which is index zero, is defined to hold a null byte ('\0').  Similarly, a string table's last byte is defined to
   hold a null byte, ensuring null termination for all strings.

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

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

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

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

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

   st_value
          This member gives the value of the associated symbol.

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

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

          STT_NOTYPE
                 The symbol's type is not defined.

          STT_OBJECT
                 The symbol is associated with a data object.

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

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

          STT_FILE
                 By  convention, the symbol's name gives the name of the source file associated with the object file.  A file symbol has STB_LOCAL bindings, its section index is SHN_ABS,
                 and it precedes the other STB_LOCAL symbols of the file, if it is present.

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

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

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

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

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

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

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

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

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

   st_other
          This member defines the symbol visibility.

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

          There are macros for extracting the visibility type:

          ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

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

Relocation entries (Rel & Rela)

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

   Relocation structures that do not need an addend:

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

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

   Relocation structures that need an addend:

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

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

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

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

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

Dynamic tags (Dyn)

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

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

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

   d_tag  This member may have any of the following values:

          DT_NULL     Marks end of dynamic section

          DT_NEEDED   String table offset to name of a needed library

          DT_PLTRELSZ Size in bytes of PLT relocation entries

          DT_PLTGOT   Address of PLT and/or GOT

          DT_HASH     Address of symbol hash table

          DT_STRTAB   Address of string table

          DT_SYMTAB   Address of symbol table

          DT_RELA     Address of Rela relocation table

          DT_RELASZ   Size in bytes of the Rela relocation table

          DT_RELAENT  Size in bytes of a Rela relocation table entry

          DT_STRSZ    Size in bytes of string table

          DT_SYMENT   Size in bytes of a symbol table entry

          DT_INIT     Address of the initialization function

          DT_FINI     Address of the termination function

          DT_SONAME   String table offset to name of shared object

          DT_RPATH    String table offset to search path for direct and indirect library dependencies

          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 search path for direct library dependencies

          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 ad
          dress.  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 exten
   sions.  For example, the GNU tool chain uses ELF notes to pass information from the linker to the C library.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

          n_name = GNU
               Extensions used by the GNU tool chain.

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

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

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

                      [0]  number of entries
                      [1]  bit mask of enabled entries

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

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

               NT_GNU_GOLD_VERSION
                      The desc contains the GNU Gold linker version used.

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

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

NOTES

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

   The extensions for e_phnum, e_shnum, and e_shstrndx respectively are Linux extensions.  Sun, BSD, and AMD64 also support them; for further information, look under SEE ALSO.

SEE ALSO

   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 man-pages 6.9.1 2024-06-15 ELF(5)