elf

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



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

SYNOPSIS
       #include <elf.h>

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

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

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

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

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

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

       e_version This member identifies the file version:

                 EV_NONE         Invalid version
                 EV_CURRENT      Current version

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                 PT_DYNAMIC  The array element specifies dynamic linking
                             information.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       The section header has the following structure:

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

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

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

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

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

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

                 SHT_NOTE       This section holds notes (ElfN_Nhdr).

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

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

                 SHT_SHLIB      This section is reserved but has unspecified
                                semantics.

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

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

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

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

       sh_flags  Sections support one-bit flags that describe miscellaneous
                 attributes.  If a flag bit is set in sh_flags, the attribute
                 is "on" for the section.  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 sections.

                 SHF_EXECINSTR  This section contains executable machine
                                instructions.

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

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

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

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

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

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

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

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

       Various sections hold program and control information:

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

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

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

       .data     This section holds initialized data that contribute to the
                 program's memory image.  This section is of type
                 SHT_PROGBITS.  The attribute types are SHF_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 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
                 unspecified.  This section is of type SHT_PROGBITS.  No
                 attribute types are used.

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

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

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

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

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

       .plt      This section holds the procedure linkage table.  This section
                 is of type SHT_PROGBITS.  The attributes are processor-
                 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 symbol
                 string table, the section's attributes will include the
                 SHF_ALLOC bit.  Otherwise, the bit will be off.  This section
                 is of type SHT_STRTAB.

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

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

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

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

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

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

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

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

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

       st_other  This member defines the symbol visibility.

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

                 There are macros for extracting the visibility type:

                 ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

       st_shndx  Every symbol table entry is "defined" in relation to some
                 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 library search path
                             (deprecated)

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

                 DT_REL      Address of Rel relocation table

                 DT_RELSZ    Size in bytes of Rel relocation table

                 DT_RELENT   Size in bytes of a Rel table entry

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

                 DT_DEBUG    Undefined use for debugging

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

                 DT_JMPREL   Address of relocation entries associated solely
                             with the PLT

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

                 DT_RUNPATH  String table offset to library search path

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

       d_val     This member represents integer values with various
                 interpretations.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                 Core files (e_type = ET_CORE)
                      Notes used by all core files.  These are highly
                      operating system or architecture specific and often
                      require close coordination with kernels, C 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:

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

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

                             · word 0: number of entries
                             · word 1: bit mask of enabled entries

                             Then follow variable-length entries, one byte
                             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_strndx 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), objdump(1), patchelf(1),
       readelf(1), size(1), strings(1), strip(1), execve(2),
       dl_iterate_phdr(3), core(5), ld.so(8)

       Hewlett-Packard, Elf-64 Object File Format.

       Santa Cruz Operation, System V Application Binary Interface.

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

       Sun Microsystems, Linker and Libraries Guide.

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

COLOPHON
       This page is part of release 5.03 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-05-09                            ELF(5)