ELF(5) | File Formats Manual | ELF(5)

# NAME

`elf` — format of ELF executable binary 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 libraries.

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

Applications which wish to process ELF binary files for their native architecture only should include `<elf.h>` in their source code. These applications should need to refer to all the types and structures by their generic names “Elf_xxx” and to the macros by “ELF_xxx”. Applications written this way can be compiled on any architecture, regardless of whether the host is 32-bit or 64-bit.

Should an application need to process ELF files of an unknown architecture, then the application needs to explicitly use either “Elf32_xxx” or “Elf64_xxx” type and structure names. Likewise, the macros need to be identified by “ELF32_xxx” or “ELF64_xxx”.

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 32-bit architectures:

    Elf32_Addr	Unsigned 32-bit program address
    Elf32_Half	Unsigned 16-bit field
    Elf32_Lword	Unsigned 64-bit field
    Elf32_Off	Unsigned 32-bit file offset
    Elf32_Sword	Signed 32-bit field or integer
    Elf32_Word	Unsigned 32-bit field or integer

And the following types are used for 64-bit architectures:

    Elf64_Addr	Unsigned 64-bit program address
    Elf64_Half	Unsigned 16-bit field
    Elf64_Lword	Unsigned 64-bit field
    Elf64_Off	Unsigned 64-bit file offset
    Elf64_Sword	Signed 32-bit field
    Elf64_Sxword	Signed 64-bit field or integer
    Elf64_Word	Unsigned 32-bit field
    Elf64_Xword	Unsigned 64-bit field or integer

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

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

    typedef struct {
            unsigned char   e_ident[EI_NIDENT];
            Elf32_Half      e_type;
            Elf32_Half      e_machine;
            Elf32_Word      e_version;
            Elf32_Addr      e_entry;
            Elf32_Off       e_phoff;
            Elf32_Off       e_shoff;
            Elf32_Word      e_flags;
            Elf32_Half      e_ehsize;
            Elf32_Half      e_phentsize;
            Elf32_Half      e_phnum;
            Elf32_Half      e_shentsize;
            Elf32_Half      e_shnum;
            Elf32_Half      e_shstrndx;
    } Elf32_Ehdr;

    typedef struct {
    	unsigned char   e_ident[EI_NIDENT];
    	Elf64_Half      e_type;
    	Elf64_Half      e_machine;
    	Elf64_Word      e_version;
    	Elf64_Addr      e_entry;
    	Elf64_Off       e_phoff;
    	Elf64_Off       e_shoff;
    	Elf64_Word      e_flags;
    	Elf64_Half      e_ehsize;
    	Elf64_Half      e_phentsize;
    	Elf64_Half      e_phnum;
    	Elf64_Half      e_shentsize;
    	Elf64_Half      e_shnum;
    	Elf64_Half      e_shstrndx;
    } Elf64_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`.
  `EI_MAG1`
    The second byte of the magic number. It must be filled with `ELFMAG1`.
  `EI_MAG2`
    The third byte of the magic number. It must be filled with `ELFMAG2`.
  `EI_MAG3`
    The fourth byte of the magic number. It must be filled with `ELFMAG3`.
  `EI_CLASS`
    The fifth byte identifies the architecture for this binary:

    `ELFCLASSNONE`
      This class is invalid.
    `ELFCLASS32`
      This defines the 32-bit architecture. It supports machines with files and virtual address spaces up to 4 Gigabytes.
    `ELFCLASS64`
      This defines the 64-bit architecture.
  `EI_DATA`
    The sixth byte specifies the data encoding of the processor-specific data in the file. Currently these encodings are supported:

    `ELFDATANONE`
      Unknown data format.
    `ELFDATA2LSB`
      Two's complement, little-endian.
    `ELFDATA2MSB`
      Two's complement, big-endian.
  `EI_VERSION`
    The version number of the ELF specification:

    `EV_NONE`
      Invalid version.
    `EV_CURRENT`
      Current version.
  `EI_OSABI`
    This byte identifies the OS- or ABI-specific ELF extensions used by this object. 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. The following values are currently defined:

    `ELFOSABI_SYSV`
      UNIX System V ABI.
    `ELFOSABI_HPUX`
      HP-UX operating system ABI.
    `ELFOSABI_NETBSD`
      NetBSD operating system ABI.
    `ELFOSABI_LINUX`
      GNU/Linux operating system ABI.
    `ELFOSABI_HURD`
      GNU/Hurd operating system ABI.
    `ELFOSABI_86OPEN`
      86Open Common IA32 ABI.
    `ELFOSABI_SOLARIS`
      Solaris operating system ABI.
    `ELFOSABI_MONTEREY`
      Monterey project ABI.
    `ELFOSABI_IRIX`
      IRIX operating system ABI.
    `ELFOSABI_FREEBSD`
      FreeBSD operating system ABI.
    `ELFOSABI_TRU64`
      TRU64 UNIX operating system ABI.
    `ELFOSABI_MODESTO`
      Novell Modesto operating system ABI.
    `ELFOSABI_OPENBSD`
      OpenBSD operating system ABI.
    `ELFOSABI_ARM`
      ARM architecture ABI.
    `ELFOSABI_STANDALONE`
      Stand-alone (embedded) ABI.
  `EI_ABIVERSION`
    This byte identifies the version of the ABI to which the object is targeted. This field is used to distinguish among incompatible versions of an ABI. The interpretation of this version number is dependent on the ABI identified by the EI_OSABI field.
  `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:

  `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_486`
    Intel 80486.
  `EM_860`
    Intel 80860.
  `EM_MIPS`
    MIPS RS3000 (big-endian only).
  `EM_MIPS_RS4_BE`
    MIPS RS4000 (big-endian only).
  `EM_SPARC64`
    SPARC v9 64-bit (unofficial).
  `EM_PARISC`
    HPPA.
  `EM_SPARC32PLUS`
    SPARC with enhanced instruction set.
  `EM_PPC`
    PowerPC.
  `EM_PPC64`
    PowerPC 64-bit.
  `EM_ARM`
    Advanced RISC Machines ARM.
  `EM_ALPHA`
    Compaq [DEC] Alpha.
  `EM_SH`
    Hitachi/Renesas Super-H.
  `EM_SPARCV9`
    SPARC v9 64-bit.
  `EM_IA_64`
    Intel IA-64.
  `EM_AMD64`
    AMD64.
  `EM_VAX`
    DEC Vax.
  `EM_AARCH64`
    ARM 64-bit.
  `EM_RISCV`
    RISC-V.
  `EM_ALPHA_EXP`
    Compaq [DEC] Alpha with enhanced instruction set.
`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.
`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.
`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`.

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. As with the ELF executable header, the program header also has different versions depending on the architecture:

    typedef struct {
            Elf32_Word      p_type;
            Elf32_Off       p_offset;
            Elf32_Addr      p_vaddr;
            Elf32_Addr      p_paddr;
            Elf32_Word      p_filesz;
            Elf32_Word      p_memsz;
            Elf32_Word      p_flags;
            Elf32_Word      p_align;
    } Elf32_Phdr;

    typedef struct {
            Elf64_Word      p_type;
            Elf64_Word      p_flags;
            Elf64_Off       p_offset;
            Elf64_Addr      p_vaddr;
            Elf64_Addr      p_paddr;
            Elf64_Xword     p_filesz;
            Elf64_Xword     p_memsz;
            Elf64_Xword     p_align;
    } Elf64_Phdr;

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

`p_type`
  This member of the Phdr struct tells what kind of segment this array element describes or how to interpret the array element's information.
  `PT_NULL`
    The array element is unused and the other members' values are undefined. This lets the program header have ignored entries.
  `PT_LOAD`
    The array element specifies a loadable segment, described by p_filesz and p_memsz. The bytes from the file are mapped to the beginning of the memory segment. If the 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 the location and size of the dynamic section, both in the file and in the memory image of the program. This segment type may not occur more than once in a file and may only occur if the dynamic section is part of the memory image of the program.
  `PT_INTERP`
    The array element specifies the location and size of a null-terminated path name 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 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 and may only occur 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_TLS`
    The array element specifies the location and size of the thread-local storage for this file. Each thread in a process loading this file will have the segment's memory size (p_memsz) allocated for it, where the bytes up to the segment's file size (p_filesz) will be initialized with the data in this segment and the remaining “extra” bytes will be set to zero. This segment type may not occur more than once in a file and may only occur if the thread-local storage is part of the memory image of the program.
  `PT_GNU_EH_FRAME`
    The array element specifies the location and size of the GNU exception frame header, both in the file and in the memory image of the program. This segment type may not occur more than once in a file and may only occur if the GNU exception frame header is part of the memory image of the program.
  `PT_GNU_RELRO`
    The array element specifies the location and size of a part of the memory image of the program that should be made read-only once all immediate relocation processing for the file has been performed. This segment type may not occur more than once in a file.
  `PT_OPENBSD_RANDOMIZE`
    The array element specifies the location and size of a part of the memory image of the program that must be filled with random data before any code in the object is executed. The memory region specified by a segment of this type may overlap the region specified by a `PT_GNU_RELRO` segment, in which case the intersection will be filled with random data before being marked read-only. This segment type may occur more than once in a file, but a limit on the total number of bytes in the segments for an object of no less than 65536 bytes may be imposed.
  `PT_OPENBSD_WXNEEDED`
    The array element specifies that a process executing this file may need to be able to map or protect memory regions as simultaneously executable and writable. If the system is unable or unwilling to permit that for this executable then it may fail immediately. This segment type is meaningful only for executable files and is ignored in other objects.
  `PT_LOOS`
    This value up to and including `PT_HIOS` is reserved for operating system-specific semantics.
  `PT_HIOS`
    This value down to and including `PT_LOOS` is reserved for operating system-specific semantics.
  `PT_LOPROC`
    This value up to and including `PT_HIPROC` is reserved for processor-specific semantics.
  `PT_HIPROC`
    This value down to and including `PT_LOPROC` is reserved for processor-specific semantics.
`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 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. 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. For example, a symbol “defined” relative to section number `SHN_UNDEF` is an undefined symbol.
`SHN_LORESERVE`
  This value specifies the lower bound of the range of reserved indices.
`SHN_LOPROC`
  This value up to and including `SHN_HIPROC` is reserved for processor-specific semantics.
`SHN_HIPROC`
  This value down to and including `SHN_LOPROC` is 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 {
    	Elf32_Word      sh_name;
    	Elf32_Word      sh_type;
    	Elf32_Word      sh_flags;
    	Elf32_Addr      sh_addr;
    	Elf32_Off       sh_offset;
    	Elf32_Word      sh_size;
    	Elf32_Word      sh_link;
    	Elf32_Word      sh_info;
    	Elf32_Word      sh_addralign;
    	Elf32_Word      sh_entsize;
    } Elf32_Shdr;

    typedef struct {
    	Elf64_Word      sh_name;
    	Elf64_Word      sh_type;
    	Elf64_Xword     sh_flags;
    	Elf64_Addr      sh_addr;
    	Elf64_Off       sh_offset;
    	Elf64_Xword     sh_size;
    	Elf64_Word      sh_link;
    	Elf64_Word      sh_info;
    	Elf64_Xword     sh_addralign;
    	Elf64_Xword     sh_entsize;
    } Elf64_Shdr;

`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`
    This value up to and including `SHT_HIPROC` is reserved for processor-specific semantics.
  `SHT_HIPROC`
    This value down to and including `SHT_LOPROC` is 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_TLS`
    This section is for thread-local storage.
  `SHF_MASKPROC`
    All bits included in this mask are reserved for processor-specific semantics.
`sh_addr`
  If this section appears in the memory image of a process, this member holds the address at which the section's first byte should reside. Otherwise, the member contains zero.
`sh_offset`
  This member's value holds the byte offset from the beginning of the file to the first byte in the section. One section type, `SHT_NOBITS`, occupies no space in the file, and its sh_offset member locates the conceptual placement in the file.
`sh_size`
  This member holds the section's size in bytes. Unless the section type is `SHT_NOBITS`, the section occupies sh_size bytes in the file. A section of type `SHT_NOBITS` may have a non-zero size, but it occupies no space in the file.
`sh_link`
  This member holds a section header table index link, whose interpretation depends on the section type.
`sh_info`
  This member holds extra information, whose interpretation depends on the section type.
`sh_addralign`
  Some sections have address alignment constraints. If a section holds a doubleword, the system must ensure doubleword alignment for the entire section. That is, the value of sh_addr must be congruent to zero, modulo the value of sh_addralign. Only zero and positive integral powers of two are allowed. Values of zero or one mean the section has no alignment constraints.
`sh_entsize`
  Some sections hold a table of fixed-sized entries, such as a symbol table. For such a section, this member gives the size in bytes for each entry. This member contains zero if the section does not hold a table of fixed-size entries.

Various sections hold program and control information:

.SUNW_ctf
  This section contains the (un)compressed Compact C-Type Format data describing the object's types and symbols. This section is of type `SHT_PROGBITS`.
.bss
  This section holds uninitialized data that contribute 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`.
.got
  This section holds the global offset table. This section is of type `SHT_PROGBITS`. The attributes are processor-specific.
.hash
  This section holds a symbol hash table. This section is of type `SHT_HASH`. The attribute used is `SHF_ALLOC`.
.init
  This section holds executable instructions that contribute to the process initialization code. When a program starts to run, the system arranges to execute the code in this section before calling the main program entry point. This section is of type `SHT_PROGBITS`. The attributes used are `SHF_ALLOC` and `SHF_EXECINSTR`.
.interp
  This section holds the pathname of a program interpreter. If the file has a loadable segment that includes the section, the section's attributes will include the `SHF_ALLOC` bit. Otherwise, that bit will be off. This section is of type `SHT_PROGBITS`.
.line
  This section holds line number information for symbolic debugging, which describes the correspondence between the program source and the machine code. The contents are unspecified. This section is of type `SHT_PROGBITS`. No attribute types are used.
.note
  This section holds information in the note section format described below. This section is of type `SHT_NOTE`. No attribute types are used. OpenBSD native executables contain a .note.openbsd.ident section to identify themselves.
.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 contribute to a non-writable segment in the process image. This section is of type `SHT_PROGBITS`. The attribute used is `SHF_ALLOC`.
.rodata1
  This section holds read-only data that typically contribute to a non-writable segment in the process image. This section is of type `SHT_PROGBITS`. The attribute used is `SHF_ALLOC`.
.shstrtab
  This section holds section names. This section is of type `SHT_STRTAB`. No attribute types are used.
.strtab
  This section holds strings, most commonly the strings that represent the names associated with symbol table entries. If the file has a loadable segment that includes the symbol string table, the 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`.
.tbss
  This section is the thread-local storage version of .bss, holding uninitialized data that contribute to the program's memory image on a per-thread basis. By definition, the system allocates and initializes the data with zeros for each thread before it first accesses it. This section is of type `SHT_NOBITS`. The attribute types are `SHF_ALLOC`, `SHF_WRITE`, and `SHF_TLS`.
.tdata
  This section is the thread-local storage version of .data, holding initialized data that contribute to the program's memory image on a per-thread basis. The system allocates and initializes the data for each thread before it first accesses it. This section is of type `SHT_PROGBITS`. The attribute types are `SHF_ALLOC`, `SHF_WRITE`, and `SHF_TLS`.
.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 character. Similarly, a string table's last byte is defined to hold a null character, 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 {
    	Elf32_Word      st_name;
    	Elf32_Addr      st_value;
    	Elf32_Word      st_size;
    	unsigned char   st_info;
    	unsigned char   st_other;
    	Elf32_Half      st_shndx;
    } Elf32_Sym;

    typedef struct {
    	Elf64_Word      st_name;
    	unsigned char	st_info;
    	unsigned char	st_other;
    	Elf64_Half   	st_shndx;
    	Elf64_Addr	st_value;
    	Elf64_Xword     st_size;
    } Elf64_Sym;

`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 non-zero, it represents a string table index that gives the symbol name. Otherwise, the symbol table has no name.
`st_value`
  This member gives the value of the associated symbol.
`st_size`
  Many symbols have associated sizes. This member holds zero if the symbol has no size or an unknown size.
`st_info`
  This member specifies the symbol's type and binding attributes:
  `STT_NOTYPE`
    The symbol's type is not defined.
  `STT_OBJECT`
    The symbol is associated with a data object.
  `STT_FUNC`
    The symbol is associated with a function or other executable code.
  `STT_SECTION`
    The symbol is associated with a section. Symbol table entries of this type exist 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_TLS`
    The symbol is associated with an object in thread-local storage. The symbol's value is its offset in the TLS storage for this file.
  `STT_LOPROC`
    This value up to and including `STT_HIPROC` is reserved for processor-specific semantics.
  `STT_HIPROC`
    This value down to and including `STT_LOPROC` is reserved for processor-specific semantics.
  `STB_LOCAL`
    Local symbols are not visible outside the object file containing their definition. Local symbols of the same name may exist in multiple files without interfering with each other.
  `STB_GLOBAL`
    Global symbols are visible to all object files being combined. One file's definition of a global symbol will satisfy another file's undefined reference to the same symbol.
  `STB_WEAK`
    Weak symbols resemble global symbols, but their definitions have lower precedence.
  `STB_LOPROC`
    This value up to and including `STB_HIPROC` is reserved for processor-specific semantics.
  `STB_HIPROC`
    This value down to and including `STB_LOPROC` is reserved for processor-specific semantics.

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

    `ELF32_ST_BIND`(info)
      or `ELF64_ST_BIND`(info) extract a binding from an st_info value.
    `ELF64_ST_TYPE`(info)
      or `ELF32_ST_TYPE`(info) extract a type from an st_info value.
    `ELF32_ST_INFO`(bind, type)
      or `ELF64_ST_INFO`(bind, type) convert a binding and a type into an st_info value.
`st_other`
  This member currently holds zero and has no defined meaning.
`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' program image. Relocation entries are these data.

Relocation structures that do not need an addend:

    typedef struct {
    	Elf32_Addr      r_offset;
    	Elf32_Word      r_info;
    } Elf32_Rel;

    typedef struct {
    	Elf64_Addr      r_offset;
    	Elf64_Xword     r_info;
    } Elf64_Rel;

Relocation structures that need an addend:

    typedef struct {
    	Elf32_Addr      r_offset;
    	Elf32_Word      r_info;
    	Elf32_Sword     r_addend;
    } Elf32_Rela;

    typedef struct {
    	Elf64_Addr      r_offset;
    	Elf64_Xword     r_info;
    	Elf64_Sxword    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 note section is used to hold vendor-specific information that may be used to help identify a binary's ABI. It should start with an Elf_Note struct, followed by the section name and the section description. The actual note contents follow thereafter.

    typedef struct {
    	Elf32_Word namesz;
    	Elf32_Word descsz;
    	Elf32_Word type;
    } Elf32_Note;

    typedef struct {
    	Elf64_Word namesz;
    	Elf64_Word descsz;
    	Elf64_Word type;
    } Elf64_Note;

`namesz`
  Length of the note name, rounded up to a 4-byte boundary.
`descsz`
  Length of the note description, rounded up to a 4-byte boundary.
`type`
  A vendor-specific note type.

The name and description strings follow the note structure. Each string is aligned on a 4-byte boundary.

# SEE ALSO

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

Hewlett-Packard, Elf-64 Object File Format.

Santa Cruz Operation, System V Application Binary Interface.

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

# HISTORY

OpenBSD ELF support first appeared in OpenBSD 1.2. Starting with OpenBSD 5.4, all supported platforms use it as the native binary file format. ELF in itself first appeared in AT&T System V UNIX. The ELF format is an adopted standard.

# AUTHORS

This manual page was written by Jeroen Ruigrok van der Werven <asmodai@FreeBSD.org> with inspiration from BSDi's BSD/OS `elf` manpage.

March 31, 2022 | Debian