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 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 include both <sys/elf32.h> and <sys/elf64.h> instead of <elf.h>. Furthermore, all types and structures need to be identified by either “Elf32_xxx” or “Elf64_xxx”. The macros need to be identified by “ELF32_xxx” or “ELF64_xxx”.

Whatever the system's architecture is, it will always include <sys/elf_common.h> as well as <sys/elf_generic.h>.

These header files describe the above mentioned headers as C structures and also include structures for dynamic sections, relocation sections and symbol tables.

The following types are being 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

For 64-bit architectures we have the following types:

    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 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 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. 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_ARM
            ARM architecture ABI.
        ELFOSABI_STANDALONE
            Standalone (embedded) ABI.
    EI_ABIVERSION
        This byte identifies the version of the ABI to which the object is targeted. This field is used to distinguish among incompatible versions of an ABI. The interpretation of this version number is dependent on the ABI identified by the EI_OSABI field. Applications conforming to this specification use the value 0.
    EI_PAD
        Start of padding. These bytes are reserved and set to zero. Programs which read them should ignore them. The value for EI_PAD will change in the future if currently unused bytes are given meanings.
    EI_BRAND
        Start of architecture identification.
    EI_NIDENT
        The size of the e_ident array.

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

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

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

    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_PPC
        PowerPC.
    EM_ALPHA
        Compaq [DEC] Alpha.

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. If the file is using extended program header numbering, then the e_phnum member will contain the value PN_XNUM and the actual number of program header table entries will be stored in the sh_info member of the section header at index SHN_UNDEF. The product of e_phentsize and the number of program header table entries gives the program header 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. If the file is using extended section numbering, then the e_shnum member will be zero and the actual section number will be stored in the sh_size member of the section header at index SHN_UNDEF. If a file has no section header table, both the e_shnum and the e_shoff fields of the ELF header will be zero. The product of e_shentsize and the number of sections in the file gives the section header table's size in bytes.
e_shstrndx
    This member holds the section header table index of the entry associated with the section name string table. If extended section numbering is being used, this field will hold the value SHN_XINDEX, and the actual section header table index will be present in the sh_link field of the section header entry at index SHN_UNDEF. 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 dynamic linking information.
    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, if present, specifies the location and size of the program header table itself, both in the file and in the memory image of the program. This segment type may not occur more than once in a file. Moreover, it may only occur if the 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
        This value up to and including PT_HIPROC are reserved for processor-specific semantics.
    PT_HIPROC
        This value down to and including PT_LOPROC are 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.

An 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 are reserved for processor-specific semantics.
SHN_HIPROC
    This value down to and including SHN_LOPROC are reserved for processor-specific semantics.
SHN_ABS
    This value specifies absolute values for the corresponding reference. For example, symbols defined relative to section number SHN_ABS have absolute values and are not affected by relocation.
SHN_COMMON
    Symbols defined relative to this section are common symbols, such as FORTRAN COMMON or unallocated C external variables.
SHN_HIRESERVE
    This value specifies the upper bound of the range of reserved indices. 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
        The 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 SHN_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. All 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 SHN_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 SHN_SYMTAB section.
    SHT_LOPROC
        This value up to and including SHT_HIPROC are reserved for processor-specific semantics.
    SHT_HIPROC
        This value down to and including SHT_LOPROC 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
        The 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
        The section contains executable machine instructions.
    SHF_MASKPROC
        All bits included in this mask are reserved for processor-specific semantics.
    SHF_COMPRESSED
        The section data is compressed.

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

Various sections hold program and control information:
.bss
    (Block Started by Symbol) 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 attributes 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 legacy section holds pointers to initialization routines, executed before calling the main program entry point. This section is of type SHT_PROGBITS. The attributes used are SHF_ALLOC.
.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 legacy section holds pointers to finalization routines, executed when the program exits normally. This section is of type SHT_PROGBITS. The attributes used are SHF_ALLOC.
.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 legacy 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.
.fini_array
    This section holds pointers to finalization routines. When a program exits normally rtld(1) executes the code referenced by this section. This section is of type SHT_FINI_ARRAY. The attributes used are SHF_ALLOC. Refer to NT_FREEBSD_NOINIT_TAG (below) for a description of how initialization and finalization code is invoked.
.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 legacy 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.
.init_array
    This section holds pointers to initialization routines. When a program starts to run rtld(1) executes the code referenced by this section before calling the program entry point. This section is of type SHT_INIT_ARRAY. The attributes used are SHF_ALLOC. Refer to NT_FREEBSD_NOINIT_TAG (below) for a description of how initialization and finalization code is invoked.
.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.
.plt
    This section holds the procedure linkage table. This section is of type SHT_PROGBITS. The attributes are processor-specific.
.relNAME
    This section holds relocation information as described below. If the file has a loadable segment that includes relocation, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. By convention, “NAME” is supplied by the section to which the relocations apply. Thus a relocation section for .text normally would have the name .rel.text. This section is of type SHT_REL.
.relaNAME
    This section holds relocation information as described below. If the file has a loadable segment that includes relocation, the section's attributes will include the SHF_ALLOC bit. Otherwise the bit will be off. By convention, “NAME” is supplied by the section to which the relocations apply. Thus a relocation section for .text normally would have the name .rela.text. This section is of type SHT_RELA.
.rodata
    This section holds read-only data that typically contributes to a non-writable segment in the process image. This section is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.
.rodata1
    This section holds read-only data that typically contributes to a non-writable segment in the process image. This section is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.
.shstrtab
    This section holds section names. This section is of type SHT_STRTAB. No attribute types are used.
.strtab
    This section holds strings, most commonly the strings that represent the names associated with symbol table entries. If the file has a loadable segment that includes the symbol string table, the 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.
.jcr
    This section holds information about Java classes that must be registered. It is obsolete and binaries created for FreeBSD 15 or later do not process it.
.eh_frame
    This section holds information used for C++ exception-handling.

A section with the SHF_COMPRESSED flag set contains a compressed copy of the section data. Compressed section data begins with an Elf64_Chdr or Elf32_Chdr structure which encodes the compression algorithm and some characteristics of the uncompressed data.

    typedef struct {
    	Elf32_Word    ch_type;
    	Elf32_Word    ch_size;
    	Elf32_Word    ch_addralign;
    } Elf32_Chdr;

    typedef struct {
    	Elf64_Word    ch_type;
    	Elf64_Word    ch_reserved;
    	Elf64_Xword   ch_size;
    	Elf64_Xword   ch_addralign;
    } Elf64_Chdr;

ch_type
    The compression algorithm used. A value of ELFCOMPRESS_ZLIB indicates that the data is compressed using zlib(3). A value of ELFCOMPRESS_ZSTD indicates that the data is compressed using Zstandard.
ch_size
    The size, in bytes, of the uncompressed section data. This corresponds to the sh_size field of a section header containing uncompressed data.
ch_addralign
    The address alignment of the uncompressed section data. This corresponds to the sh_addralign field of a section header containing uncompressed data.

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_LOPROC
        This value up to and including STT_HIPROC are reserved for processor-specific semantics.
    STT_HIPROC
        This value down to and including STT_LOPROC 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 file 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 are reserved for processor-specific semantics.
    STB_HIPROC
        This value down to and including STB_LOPROC are 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.

  Note Section
ELF note sections consist of entries with the following format:
Field | Size | Description
namesz | 32 bits | Size of name
descsz | 32 bits | Size of desc
type | 32 bits | OS-dependent note type
name | namesz | Null-terminated originator name
desc | descsz | OS-dependent note data

The name and desc fields are padded to ensure 4-byte alignment. namesz and descsz specify the unpadded length.

FreeBSD defines the following ELF note types (with corresponding interpretation of desc):

NT_FREEBSD_ABI_TAG (Value: 1)
    Indicates the OS ABI version in a form of a 32-bit integer containing expected ABI version (i.e., __FreeBSD_version).

NT_FREEBSD_NOINIT_TAG (Value: 2)
    Indicates that the C startup does not call initialization routines, and thus rtld(1) must do so. desc is ignored.

NT_FREEBSD_ARCH_TAG (Value: 3)
    Contains the MACHINE_ARCH that the executable was built for.

NT_FREEBSD_FEATURE_CTL (Value: 4)
    Contains a bitmask of mitigations and features to enable:

    NT_FREEBSD_FCTL_ASLR_DISABLE (Value: 0x01)
        Request that address randomization (ASLR) not be performed. See security(7).

    NT_FREEBSD_FCTL_PROTMAX_DISABLE (Value: 0x02)
        Request that mmap(2) calls not set PROT_MAX to the initial value of the prot argument.

    NT_FREEBSD_FCTL_STKGAP_DISABLE (Value: 0x04)
        Disable stack gap.

    NT_FREEBSD_FCTL_WXNEEDED (Value: 0x08)
        Indicate that the binary requires mappings that are simultaneously writeable and executable.

    NT_FREEBSD_FCTL_LA48 (Value: 0x10)
        Request 48-bit linear address space on amd64.

    NT_FREEBSD_FCTL_LA57 (Value: 0x40)
        Accept 57-bit linear address space on amd64.

SEE ALSO
as(1), gdb(1) (ports/devel/gdb), ld(1), objdump(1), readelf(1), execve(2), zlib(3), ar(5), 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
The ELF header files made their appearance in FreeBSD 2.2.6. ELF in itself first appeared in AT&T 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.