NAME | SYNOPSIS | DESCRIPTION | NOTES | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #include <elf.h>

DESCRIPTION         top

       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.

       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.

       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.

       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 and
                             size for auxiliary information.

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

                 PT_PHDR     The array element, if present, specifies the
                             location and size of the program header table
                             itself, both in the file and in the memory
                             image of the program.  This segment type may
                             not occur more than once in a file.  Moreover,
                             it may 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_X, PF_W, and PF_R.

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

       A file's section header table lets one locate all the file's
       sections.  The section header table is an array of Elf32_Shdr or
       Elf64_Shdr structures.  The ELF header's e_shoff member gives the
       byte offset from the beginning of the file to the section header
       table.  e_shnum holds the number of entries the section header table
       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 information that marks
                                the file in some way.

                 SHT_NOBITS     A section of this type occupies no space in
                                the file but otherwise resembles
                                SHT_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.  Values of zero or one mean the
                 section has no alignment constraints.

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

       Various sections hold program and control information:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       .rodata   This section holds read-only data that typically
                 contributes to a 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 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 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.

       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         top

       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         top

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

       Hewlett-Packard, Elf-64 Object File Format.

       Santa Cruz Operation, System V Application Binary Interface.

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

       Sun Microsystems, Linker and Libraries Guide.

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

COLOPHON         top

       This page is part of release 4.07 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                            2016-07-17                           ELF(5)