File Format

Object files participate in both program linking and program execution. For convenience and efficiency, the object file format provides parallel views of a file's contents, reflecting the differing needs of these activities. The following figure shows an object file's organization.

Figure 7–1 Object File Format

An ELF header resides at the beginning of an object file and holds a road map describing the file's organization.

Only the ELF header has a fixed position in the file. The flexibility of the ELF format requires no specified order for header tables, sections or segments. However, this figure is typical of the layout used in the Solaris OS.

Sections represent the smallest indivisible units that can be processed within an ELF file. Segments are a collection of sections. Segments represent the smallest individual units that can be mapped to a memory image by exec(2) or by the runtime linker.

Sections hold the bulk of object file information for the linking view. This data includes instructions, data, symbol table, and relocation information. Descriptions of sections appear in the first part of this chapter. The second part of this chapter discusses segments and the program execution view of the file.

A program header table, if present, tells the system how to create a process image. Files used to generate a process image, executable files and shared objects, must have a program header table. Relocatable object files do not need a program header table.

A section header table contains information describing the file's sections. Every section has an entry in the table. Each entry gives information such as the section name and section size. Files that are used in link-editing must have a section header table.

Data Representation

The object file format supports various processors with 8-bit bytes, 32–bit architectures and 64–bit architectures. Nevertheless, the data representation is intended to be extensible to larger, or smaller, architectures. Table 7–1 and Table 7–2 list the 32–bit data types and 64–bit data types.

Object files represent some control data with a machine-independent format. This format provides for the common identification and interpretation of object files. The remaining data in an object file use the encoding of the target processor, regardless of the machine on which the file was created.

All data structures that the object file format defines follow the natural size and alignment guidelines for the relevant class. Data structures can contain explicit padding to ensure 4-byte alignment for 4-byte objects, to force structure sizes to a multiple of 4, and so forth. Data also have suitable alignment from the beginning of the file. Thus, for example, a structure containing an Elf32_Addr member is aligned on a 4-byte boundary within the file. Similarly, a structure containing an Elf64_Addr member is aligned on an 8–byte boundary.

For portability, ELF uses no bit-fields.

ELF Header

Some control structures within object files can grow because the ELF header contains their actual sizes. If the object file format does change, a program can encounter control structures that are larger or smaller than expected. Programs might therefore ignore extra information. The treatment of missing information depends on context and is specified if and when extensions are defined.

The ELF header has the following structure. See sys/elf.h.

#define EI_NIDENT       16
 
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;

e_ident

The initial bytes mark the file as an object file. These bytes provide machine-independent data with which to decode and interpret the file's contents. Complete descriptions appear in ELF Identification.

e_type

Identifies the object file type, as listed in the following table.

Name	Value	Meaning
ET_NONE	0	No file type
ET_REL	1	Relocatable file
ET_EXEC	2	Executable file
ET_DYN	3	Shared object file
ET_CORE	4	Core file
ET_LOPROC	0xff00	Processor-specific
ET_HIPROC	0xffff	Processor-specific

Although the core file contents are unspecified, type ET_CORE is reserved to mark the file. Values from ET_LOPROC through ET_HIPROC (inclusive) are reserved for processor-specific semantics. Other values are reserved for future use.

e_machine

Specifies the required architecture for an individual file. Relevant architectures are listed in the following table.

Name	Value	Meaning
EM_NONE	0	No machine
EM_SPARC	2	SPARC
EM_386	3	Intel 80386
EM_SPARC32PLUS	18	Sun SPARC 32+
EM_SPARCV9	43	SPARC V9
EM_AMD64	62	AMD 64

Other values are reserved for future use. Processor-specific ELF names are distinguished by using the machine name. For example, the flags defined for e_flags use the prefix EF_. A flag that is named WIDGET for the EM_XYZ machine would be called EF_XYZ_WIDGET.

e_version

Identifies the object file version, as listed in the following table.

Name	Value	Meaning
EV_NONE	0	Invalid version
EV_CURRENT	>=1	Current version

The value 1 signifies the original file format. The value of EV_CURRENT changes as necessary to reflect the current version number.

e_entry

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

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

e_shoff

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

e_flags

Processor-specific flags associated with the file. Flag names take the form EF_machine_flag. This member is presently zero for x86. The SPARC flags are listed in the following table.

Name	Value	Meaning
EF_SPARC_EXT_MASK	0xffff00	Vendor Extension mask
EF_SPARC_32PLUS	0x000100	Generic V8+ features
EF_SPARC_SUN_US1	0x000200	Sun UltraSPARCTM 1 Extensions
EF_SPARC_HAL_R1	0x000400	HAL R1 Extensions
EF_SPARC_SUN_US3	0x000800	Sun UltraSPARC 3 Extensions
EF_SPARCV9_MM	0x3	Mask for Memory Model
EF_SPARCV9_TSO	0x0	Total Store Ordering
EF_SPARCV9_PSO	0x1	Partial Store Ordering
EF_SPARCV9_RMO	0x2	Relaxed Memory Ordering

e_ehsize

The ELF header's size in bytes.

e_phentsize

The size in bytes of one entry in the file's program header table. All entries are the same size.

e_phnum

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

If the number of program headers is greater than or equal to PN_XNUM (0xffff), this member has the value PN_XNUM (0xffff). The actual number of program header table entries is contained in the sh_info field of the section header at index 0. Otherwise, the sh_info member of the initial section header entry contains the value zero. See Table 7–6 and Table 7–7.

e_shentsize

A section header's size in bytes. A section header is one entry in the section header table. All entries are the same size.

e_shnum

The number of entries in the section header table. 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 zero.

If the number of sections is greater than or equal to SHN_LORESERVE (0xff00), e_shnum has the value zero. The actual number of section header table entries is contained in the sh_size field of the section header at index 0. Otherwise, the sh_size member of the initial section header entry contains the value zero. See Table 7–6 and Table 7–7.

e_shstrndx

The section header table index of the entry that is 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 section name string table section index is greater than or equal to SHN_LORESERVE (0xff00), this member has the value SHN_XINDEX (0xffff) and the actual index of the section name string table section is contained in the sh_link field of the section header at index 0. Otherwise, the sh_link member of the initial section header entry contains the value zero. See Table 7–6 and Table 7–7.

ELF Identification

ELF provides an object file framework to support multiple processors, multiple data encoding, and multiple classes of machines. To support this object file family, the initial bytes of the file specify how to interpret the file. These bytes are independent of the processor on which the inquiry is made and independent of the file's remaining contents.

The initial bytes of an ELF header and an object file correspond to the e_ident member.

These indexes access bytes that hold the following values.

EI_MAG0 - EI_MAG3

A 4–byte magic number, identifying the file as an ELF object file, as listed in the following table.

Name	Value	Position
ELFMAG0	0x7f	e_ident[EI_MAG0]
ELFMAG1	'E'	e_ident[EI_MAG1]
ELFMAG2	'L'	e_ident[EI_MAG2]
ELFMAG3	'F'	e_ident[EI_MAG3]

EI_CLASS

Byte e_ident[EI_CLASS] identifies the file's class, or capacity, as listed in the following table.

Name	Value	Meaning
ELFCLASSNONE	0	Invalid class
ELFCLASS32	1	32–bit objects
ELFCLASS64	2	64–bit objects

The file format is designed to be portable among machines of various sizes, without imposing the sizes of the largest machine on the smallest. The class of the file defines the basic types used by the data structures of the object file container. The data that is contained in object file sections can follow a different programming model.

Class ELFCLASS32 supports machines with files and virtual address spaces up to 4 gigabytes. This class uses the basic types that are defined in Table 7–1.

Class ELFCLASS64 is reserved for 64–bit architectures such as 64–bit SPARC and x64. This class uses the basic types that are defined in Table 7–2.

EI_DATA

Byte e_ident[EI_DATA] specifies the data encoding of the processor-specific data in the object file, as listed in the following table.

Name	Value	Meaning
ELFDATANONE	0	Invalid data encoding
ELFDATA2LSB	1	See Figure 7–2.
ELFDATA2MSB	2	See Figure 7–3.

More information on these encodings appears in the section Data Encoding. Other values are reserved for future use.

EI_VERSION

Byte e_ident[EI_VERSION] specifies the ELF header version number. Currently, this value must be EV_CURRENT.

EI_OSABI

Byte e_ident[EI_OSABI] identifies the operating system together with the ABI to which the object is targeted. Some fields in other ELF structures have flags and values that have operating system or ABI specific meanings. The interpretation of those fields is determined by the value of this byte.

EI_ABIVERSION

Byte e_ident[EI_ABIVERSION] 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. If no values are specified for the EI_OSABI field for the processor, or no version values are specified for the ABI determined by a particular value of the EI_OSABI byte, the value zero is used to indicate unspecified.

EI_PAD

This value marks the beginning of the unused bytes in e_ident. These bytes are reserved and are set to zero. Programs that read object files should ignore these values.

Data Encoding

A file's data encoding specifies how to interpret the integer types in a file. Class ELFCLASS32 files and class ELFCLASS64 files use integers that occupy 1, 2, 4, and 8 bytes to represent offsets, addresses and other information. Under the defined encodings, objects are represented as described by the figures that follow. Byte numbers appear in the upper left corners.

ELFDATA2LSB encoding specifies 2's complement values, with the least significant byte occupying the lowest address. This encoding if often referred to informally as little endian.

Figure 7–2 Data Encoding ELFDATA2LSB

ELFDATA2MSB encoding specifies 2's complement values, with the most significant byte occupying the lowest address. This encoding if often referred to informally as big endian.

Figure 7–3 Data Encoding ELFDATA2MSB

Sections

An object file's section header table allows you to locate all of the sections of the file. The section header table is an array of Elf32_Shdr or Elf64_Shdr structures. A section header table index is a subscript into this array. The ELF header's e_shoff member indicates the byte offset from the beginning of the file to the section header table. The e_shnum member indicates how many entries that the section header table contains. The e_shentsize member indicates the size in bytes of each entry.

If the number of sections is greater than or equal to SHN_LORESERVE (0xff00), e_shnum has the value SHN_UNDEF (0). The actual number of section header table entries is contained in the sh_size field of the section header at index 0. Otherwise, the sh_size member of the initial entry contains the value zero.

Some section header table indexes are reserved in contexts where index size is restricted. For example, the st_shndx member of a symbol table entry and the e_shnum and e_shstrndx members of the ELF header. In such contexts, the reserved values do not represent actual sections in the object file. Also in such contexts, an escape value indicates that the actual section index is to be found elsewhere, in a larger field.

Although index 0 is reserved as the undefined value, the section header table contains an entry for index 0. That is, if the e_shnum member of the ELF header indicates a file has 6 entries in the section header table, the sections have the indexes 0 through 5. The contents of the initial entry are specified later in this section.

SHN_UNDEF

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

The lower boundary of the range of reserved indexes.

SHN_LOPROC - SHN_HIPROC

Values in this inclusive range are reserved for processor-specific semantics.

SHN_LOOS - SHN_HIOS

Values in this inclusive range are reserved for operating system-specific semantics.

SHN_LOSUNW - SHN_HISUNW

Values in this inclusive range are reserved for Sun-specific semantics.

SHN_SUNW_IGNORE

This section index provides a temporary symbol definition within relocatable objects. Reserved for internal use by dtrace(1M).

SHN_BEFORE, SHN_AFTER

Provide for initial and final section ordering in conjunction with the SHF_LINK_ORDER and SHF_ORDERED section flags. See Table 7–8.

SHN_AMD64_LCOMMON

x64 specific common block label. This label is similar to SHN_COMMON, but provides for identifying a large common block.

SHN_ABS

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. These symbols are sometimes referred to as tentative.

SHN_XINDEX

An escape value indicating that the actual section header index is too large to fit in the containing field. The header section index is found in another location specific to the structure where the section index appears.

SHN_HIRESERVE

The upper boundary of the range of reserved indexes. The system reserves indexes between SHN_LORESERVE and SHN_HIRESERVE, inclusive. The values do not reference the section header table. The section header table does not contain entries for the reserved indexes.

Sections contain all information in an object file except the ELF header, the program header table, and the section header table. Moreover, the sections in object files satisfy several conditions.

• Every section in an object file has exactly one section header describing the section. Section headers can exist that do not have a section.

• Each section occupies one contiguous, possibly empty, sequence of bytes within a file.

• Sections in a file cannot overlap. No byte in a file resides in more than one section.

• An object file can have inactive space. The various headers and the sections might not cover every byte in an object file. The contents of the inactive data are unspecified.

A section header has the following structure. See sys/elf.h.

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

The name of the section. This members value is an index into the section header string table section giving the location of a null-terminated string. Section names and their descriptions are listed in Table 7–10.

sh_type

Categorizes the section's contents and semantics. Section types and their descriptions are listed in Table 7–5.

sh_flags

Sections support 1-bit flags that describe miscellaneous attributes. Flag definitions are listed in Table 7–8.

sh_addr

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

sh_offset

The byte offset from the beginning of the file to the first byte in the section. For a SHT_NOBITS section, this member indicates the conceptual offset in the file, as the section occupies no space in the file.

sh_size

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 can have a nonzero size, but the section occupies no space in the file.

sh_link

A section header table index link, whose interpretation depends on the section type. Table 7–9 describes the values.

sh_info

Extra information, whose interpretation depends on the section type. Table 7–9 describes the values. If the sh_flags field for this section header includes the attribute SHF_INFO_LINK, then this member represents a section header table index.

sh_addralign

Some sections have address alignment constraints. For example, if a section holds a double-word, the system must ensure double-word alignment for the entire section. In this case, the value of sh_addr must be congruent to 0, modulo the value of sh_addralign. Currently, only 0 and positive integral powers of two are allowed. Values 0 and 1 mean the section has no alignment constraints.

sh_entsize

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

A section header's sh_type member specifies the section's semantics, as shown in the following table.

SHT_NULL

Identifies the section header as inactive. This section header does not have an associated section. Other members of the section header have undefined values.

SHT_PROGBITS

Identifies information defined by the program, whose format and meaning are determined solely by the program.

SHT_SYMTAB, SHT_DYNSYM

Identifies a symbol table. Typically, a SHT_SYMTAB section provides symbols for link-editing. As a complete symbol table, the table can contain many symbols that are unnecessary for dynamic linking. Consequently, an object file can also contain a SHT_DYNSYM section, which holds a minimal set of dynamic linking symbols, to save space.

See Symbol Table Section for details.

SHT_STRTAB, SHT_DYNSTR

Identifies a string table. An object file can have multiple string table sections. See String Table Section for details.

SHT_RELA

Identifies relocation entries with explicit addends, such as type Elf32_Rela for the 32–bit class of object files. An object file can have multiple relocation sections. See Relocation Sections for details.

SHT_HASH

Identifies a symbol hash table. A dynamically linked object file must contain a symbol hash table. Currently, an object file can have only one hash table, but this restriction might be relaxed in the future. See Hash Table Section for details.

SHT_DYNAMIC

Identifies information for dynamic linking. Currently, an object file can have only one dynamic section. See Dynamic Section for details.

SHT_NOTE

Identifies information that marks the file in some way. See Note Section for details.

SHT_NOBITS

Identifies a section that 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

Identifies relocation entries without explicit addends, such as type Elf32_Rel for the 32–bit class of object files. An object file can have multiple relocation sections. See Relocation Sections for details.

SHT_SHLIB

Identifies a reserved section which has unspecified semantics. Programs that contain a section of this type do not conform to the ABI.

SHT_INIT_ARRAY

Identifies a section containing an array of pointers to initialization functions. Each pointer in the array is taken as a parameterless procedure with a void return. See Initialization and Termination Sections for details.

SHT_FINI_ARRAY

Identifies a section containing an array of pointers to termination functions. Each pointer in the array is taken as a parameterless procedure with a void return. See Initialization and Termination Sections for details.

SHT_PREINIT_ARRAY

Identifies a section containing an array of pointers to functions that are invoked before all other initialization functions. Each pointer in the array is taken as a parameterless procedure with a void return. See Initialization and Termination Sections for details.

SHT_GROUP

Identifies a section group. A section group identifies a set of related sections that must be treated as a unit by the link-editor. Sections of type SHT_GROUP can appear only in relocatable objects. See Group Section for details.

SHT_SYMTAB_SHNDX

Identifies a section containing extended section indexes, that are associated with a symbol table. If any section header indexes referenced by a symbol table, contain the escape value SHN_XINDEX, an associated SHT_SYMTAB_SHNDX is required.

The SHT_SYMTAB_SHNDX section is an array of Elf32_Word values. This array contains one entry for every entry in the associated symbol table entry. The values represent the section header indexes against which the symbol table entries are defined. Only if corresponding symbol table entry's st_shndx field contains the escape value SHN_XINDEX will the matching Elf32_Word hold the actual section header index. Otherwise, the entry must be SHN_UNDEF (0).

SHT_LOOS – SHT_HIOS

Values in this inclusive range are reserved for operating system-specific semantics.

SHT_LOSUNW – SHT_HISUNW

Values in this inclusive range are reserved for Solaris OS semantics.

SHT_SUNW_dof

Reserved for internal use by dtrace(1M).

SHT_SUNW_cap

Specifies hardware and software capability requirements. See Hardware and Software Capabilities Section for details.

SHT_SUNW_SIGNATURE

Identifies module verification signature.

SHT_SUNW_ANNOTATE

The processing of an annotate section follows all of the default rules for processing a section. The only exception occurs if the annotate section is in non-allocatable memory. If the section header flag SHF_ALLOC is not set, the link-editor silently ignores any unsatisfied relocations against this section.

SHT_SUNW_DEBUGSTR, SHT_SUNW_DEBUG

Identifies debugging information. Sections of this type are stripped from the object using the link-editor's -s option, or after the link-edit using strip(1).

SHT_SUNW_move

Identifies data to handle partially initialized symbols. See Move Section for details.

SHT_SUNW_COMDAT

Identifies a section that allows multiple copies of the same data to be reduced to a single copy. See COMDAT Section for details.

SHT_SUNW_syminfo

Identifies additional symbol information. See Syminfo Table Section for details.

SHT_SUNW_verdef

Identifies fine-grained versions defined by this file. See Version Definition Section for details.

SHT_SUNW_verneed

Identifies fine-grained dependencies required by this file. See Version Dependency Section for details.

SHT_SUNW_versym

Identifies a table describing the relationship of symbols to the version definitions offered by the file. See Version Symbol Section for details.

SHT_LOPROC - SHT_HIPROC

Values in this inclusive range are reserved for processor-specific semantics.

SHT_SPARC_GOTDATA

Identifies SPARC specific data, referenced using GOT-relative addressing. That is, offsets relative to the address assigned to the symbol _GLOBAL_OFFSET_TABLE_. For 64–bit SPARC, data in this section must be bound at link-edit time to locations within {+-} 2^32 bytes of the GOT address.

SHT_AMD64_UNWIND

Identifies x64 specific data, containing unwind function table entries for stack unwinding.

SHT_LOUSER

Specifies the lower boundary of the range of indexes that are reserved for application programs.

SHT_HIUSER

Specifies the upper boundary of the range of indexes that are reserved for application programs. Section types between SHT_LOUSER and SHT_HIUSER can be used by the application without conflicting with current or future system-defined section types.

Other section-type values are reserved. As mentioned before, the section header for index 0 (SHN_UNDEF) exists, even though the index marks undefined section references. The following table shows the values.

Should the number of sections or program headers exceed the ELF header data sizes, elements of section header 0 are used to define extended ELF header attributes. The following table shows the values.

A section header's sh_flags member holds 1-bit flags that describe the section's 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 reserved and are set to zero.

SHF_WRITE

Identifies a section that should be writable during process execution.

SHF_ALLOC

Identifies a section that 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

Identifies a section that contains executable machine instructions.

SHF_MERGE

Identifies a section containing data that can be merged to eliminate duplication. Unless the SHF_STRINGS flag is also set, the data elements in the section are of a uniform size. The size of each element is specified in the section header's sh_entsize field. If the SHF_STRINGS flag is also set, the data elements consist of null-terminated character strings. The size of each character is specified in the section header's sh_entsize field.

SHF_STRINGS

Identifies a section that consists of null-terminated character strings. The size of each character is specified in the section header's sh_entsize field.

SHF_INFO_LINK

This section headers sh_info field holds a section header table index.

SHF_LINK_ORDER

This section adds special ordering requirements to the link-editor. The requirements apply if the sh_link field of this section's header references another section, the linked-to section. If this section is combined with other sections in the output file, the section appears in the same relative order with respect to those sections. Similarly the linked-to section appears with respect to sections the linked-to section is combined with.

The special sh_link values SHN_BEFORE and SHN_AFTER (see Table 7–4) imply that the sorted section is to precede or follow, respectively, all other sections in the set being ordered. Input file link-line order is preserved if multiple sections in an ordered set have one of these special values.

A typical use of this flag is to build a table that references text or data sections in address order.

In the absence of the sh_link ordering information, sections from a single input file combined within one section of the output file are contiguous. These section have the same relative ordering as the sections did in the input file. The contributions from multiple input files appear in link-line order.

SHF_OS_NONCONFORMING

This section requires special OS-specific processing beyond the standard linking rules to avoid incorrect behavior. If this section has either an sh_type value or contains sh_flags bits in the OS-specific ranges for those fields, and the link-editor does not recognize these values, then the object file containing this section is rejected with an error.

SHF_GROUP

This section is a member, perhaps the only member, of a section group. The section must be referenced by a section of type SHT_GROUP. The SHF_GROUP flag can be set only for sections that are contained in relocatable objects. See Group Section for details.

SHF_TLS

This section holds thread-local storage. Each thread within a process has a distinct instance of this data. See Chapter 8, Thread-Local Storage for details.

SHF_MASKOS

All bits that are included in this mask are reserved for operating system-specific semantics.

SHF_AMD64_LARGE

The default compilation model for x64 only provides for 32–bit displacements. This displacement limits the size of sections, and eventually segments, to 2 Gbytes. This attribute flag identifies a section that can hold more than 2 Gbyte. This flag allows the linking of object files that use different code models.

An x64 object file section that does not contain the SHF_AMD64_LARGE attribute flag can be freely referenced by objects using small code models. A section that contains this flag can only be referenced by objects that use larger code models. For example, an x64 medium code model object can refer to data in sections that contain the attribute flag and sections that do not contain the attribute flag. However, an x64 small code model object can only refer to data in a section that does not contain this flag.

SHF_ORDERED

This section requires ordering in relation to other sections of the same type. Ordered sections are combined within the section pointed to by the sh_link entry. The sh_link entry of an ordered section can point to itself.

If the sh_info entry of the ordered section is a valid section within the same input file, the ordered section is sorted based on the relative ordering within the output file of the section pointed to by the sh_info entry.

The special sh_info values SHN_BEFORE and SHN_AFTER (see Table 7–4) imply that the sorted section is to precede or follow, respectively, all other sections in the set being ordered. Input file link-line order is preserved if multiple sections in an ordered set have one of these special values.

In the absence of the sh_info ordering information, sections from a single input file combined within one section of the output file are contiguous. These sections have the same relative ordering as the sections appear in the input file. The contributions from multiple input files appear in link-line order.

SHF_EXCLUDE

This section is excluded from input to the link-edit of an executable or shared object. This flag is ignored if the SHF_ALLOC flag is also set, or if relocations exist against the section.

SHF_MASKPROC

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

Two members in the section header, sh_link and sh_info, hold special information, depending on section type.

Special Sections

Various sections hold program and control information. Sections in the following table are used by the system and have the indicated types and attributes.

.bss

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. The section occupies no file space, as indicated by the section type SHT_NOBITS.

.comment

Comment information, typically contributed by the components of the compilation system. This section can be manipulated by mcs(1).

.data, .data1

Initialized data that contribute to the program's memory image.

.dynamic

Dynamic linking information. See Dynamic Section for details.

.dynstr

Strings needed for dynamic linking, most commonly the strings that represent the names associated with symbol table entries.

.dynsym

Dynamic linking symbol table. See Symbol Table Section for details.

.eh_frame_hdr, .eh_frame

Call frame information used to unwind the stack.

.fini

Executable instructions that contribute to a single termination function for the executable or shared object containing the section. See Initialization and Termination Routines for details.

.finiarray

An array of function pointers that contribute to a single termination array for the executable or shared object containing the section. See Initialization and Termination Routines for details.

.got

The global offset table. See Global Offset Table (Processor-Specific) for details.

.hash

Symbol hash table. See Hash Table Section for details.

.init

Executable instructions that contribute to a single initialization function for the executable or shared object containing the section. See Initialization and Termination Routines for details.

.initarray

An array of function pointers that contributes to a single initialization array for the executable or shared object containing the section. See Initialization and Termination Routines for details.

.interp

The path name of a program interpreter. See Program Interpreter for details.

.lbss

x64 specific uninitialized data. This data is similar to .bss, but provides for a section that is larger than 2 Gbytes.

.ldata, .ldata1

x64 specific initialized data. This data is similar to .data, but provides for a section that is larger than 2 Gbytes.

.lrodata, .lrodata1

x64 specific read-only data. This data is similar to .rodata, but provides for a section that is larger than 2 Gbytes.

.note

Information in the format described in Note Section.

.plt

The procedure linkage table. See Procedure Linkage Table (Processor-Specific) for details.

.preinitarray

An array of function pointers that contribute to a single pre-initialization array for the executable or shared object containing the section. See Initialization and Termination Routines for details.

.rela

Relocations that do not apply to a particular section. One use of this section is for register relocations. See Register Symbols for details.

.relname, .relaname

Relocation information, as Relocation Sections describes. If the file has a loadable segment that includes relocation, the sections' attributes include the SHF_ALLOC bit. Otherwise, that bit is off. Conventionally, name is supplied by the section to which the relocations apply. Thus, a relocation section for .text normally will have the name .rel.text or .rela.text.

.rodata, .rodata1

Read-only data that typically contribute to a non-writable segment in the process image. See Program Header for details.

.shstrtab

Section names.

.strtab

Strings, most commonly the strings that represent the names that are associated with symbol table entries. If the file has a loadable segment that includes the symbol string table, the section's attributes include the SHF_ALLOC bit. Otherwise, that bit is turned off.

.symtab

Symbol table, as Symbol Table Section describes. If the file has a loadable segment that includes the symbol table, the section's attributes include the SHF_ALLOC bit. Otherwise, that bit is turned off.

.symtab_shndx

This section holds the special symbol table section index array, as described by .symtab. The section's attributes include the SHF_ALLOC bit if the associated symbol table section does. Otherwise, that bit is turned off.

.tbss

This section holds uninitialized thread-local data that contribute to the program's memory image. By definition, the system initializes the data with zeros when the data is instantiated for each new execution flow. The section occupies no file space, as indicated by the section type, SHT_NOBITS. See Chapter 8, Thread-Local Storage for details.

.tdata, .tdata1

These sections hold initialized thread-local data that contribute to the program's memory image. A copy of its contents is instantiated by the system for each new execution flow. See Chapter 8, Thread-Local Storage for details.

.text

The text or executable instructions of a program.

.SUNW_bss

Partially initialized data for shared objects that contribute to the program's memory image. The data is initialized at runtime. The section occupies no file space, as indicated by the section type SHT_NOBITS.

.SUNW_cap

Hardware and software capability requirements. See Hardware and Software Capabilities Section for details.

.SUNW_heap

The heap of a dynamic executable created from dldump(3C).

.SUNW_move

Additional information for partially initialized data. See Move Section for details.

.SUNW_reloc

Relocation information, as Relocation Sections describes. This section is a concatenation of relocation sections that provides better locality of reference of the individual relocation records. Only the offset of the relocation record is meaningful, thus the section sh_info value is zero.

.SUNW_syminfo

Additional symbol table information. See Syminfo Table Section for details.

.SUNW_version

Versioning information. See Versioning Sections for details.

Section names with a dot (.) prefix are reserved for the system, although applications can use these sections if their existing meanings are satisfactory. Applications can use names without the prefix to avoid conflicts with system sections. The object file format enables you to define sections that are not reserved. An object file can have more than one section with the same name.

Section names that are reserved for a processor architecture are formed by placing an abbreviation of the architecture name ahead of the section name. The name should be taken from the architecture names that are used for e_machine. For example, .Foo.psect is the psect section defined by the FOO architecture.

Existing extensions use their historical names

COMDAT Section

COMDAT sections are uniquely identified by their section name (sh_name). If the link-editor encounters multiple sections of type SHT_SUNW_COMDAT, with the same section name, the first section is retained and the rest discarded. Any relocations that are applied to a discarded SHT_SUNW_COMDAT section are ignored. Any symbols that are defined in a discarded section are removed.

Additionally, the link-editor supports the section naming convention that is used for section reordering when the compiler is invoked with the -xF option. If a function is placed in a SHT_SUNW_COMDAT section that is named .sectname%funcname, the final SHT_SUNW_COMDAT sections that are retained are coalesced into the section that is named .sectname. This method can be used to place SHT_SUNW_COMDAT sections into the .text, .data, or any other section as their final destination.

Group Section

Some sections occur in interrelated groups. For example, an out-of-line definition of an inline function might require additional information besides the section containing executable instructions. This additional information can be a read-only data section containing literals referenced, one or more debugging information sections, or other informational sections.

There can be internal references among group sections. However, these references make no sense if one of the sections were removed, or one of the sections were replaced by a duplicate from another object. Therefore, these groups are included, or these groups are omitted, from the linked object as a unit.

A section of type SHT_GROUP defines such a grouping of sections. The name of a symbol from one of the containing object's symbol tables provides a signature for the section group. The section header of the SHT_GROUP section specifies the identifying symbol entry. The sh_link member contains the section header index of the symbol table section that contains the entry. The sh_info member contains the symbol table index of the identifying entry. The sh_flags member of the section header contains the value zero. The name of the section (sh_name) is not specified.

The section data of a SHT_GROUP section is an array of Elf32_Word entries. The first entry is a flag word. The remaining entries are a sequence of section header indices.

The following flag is currently defined.

GRP_COMDAT

GRP_COMDAT is a COMDAT group. This group can duplicate another COMDAT group in another object file, where duplication is defined as having the same group signature. In such cases, only one of the duplicate groups is retained by the link-editor. The members of the remaining groups are discarded.

The section header indices in the SHT_GROUP section, identify the sections that make up the group. These sections must have the SHF_GROUP flag set in their sh_flags section header member. If the link-editor decides to remove the section group, the link-editor removes all members of the group.

To facilitate removing a group without leaving dangling references and with only minimal processing of the symbol table, the following rules are followed.

• References to the sections comprising a group from sections outside of the group must be made through symbol table entries with STB_GLOBAL or STB_WEAK binding and section index SHN_UNDEF. A definition of the same symbol in the object containing the reference must have a separate symbol table entry from the reference. Sections outside of the group can not reference symbols with STB_LOCAL binding for addresses that are contained in the group's sections, including symbols with type STT_SECTION.

• Non-symbol references to the sections comprising a group are not allowed from outside the group. For example, you cannot use a group member's section header index in an sh_link or sh_info member.

• A symbol table entry defined relative to one of the group's sections can be removed if the group members are discarded. This removal occurs if the symbol table entry is contained in a symbol table section that is not part of the group.

Hardware and Software Capabilities Section

A SHT_SUNW_cap section identifies the hardware and software capabilities of an object. This section contains an array of the following structures. See sys/link.h.

typedef struct {
        Elf32_Word      c_tag;
        union {
                Elf32_Word      c_val;
                Elf32_Addr      c_ptr;
        } c_un;
} Elf32_Cap;

typedef struct {
        Elf64_Xword     c_tag;
        union {
                Elf64_Xword     c_val;
                Elf64_Addr      c_ptr;
        } c_un;
} Elf64_Cap;

For each object with this type, c_tag controls the interpretation of c_un.

c_val

These objects represent integer values with various interpretations.

c_ptr

These objects represent program virtual addresses.

The following capabilities tags exist.

CA_SUNW_NULL

Marks the end of the capabilities array.

CA_SUNW_HW_1

Indicates hardware capability values. The c_val element contains a value that represents the associated hardware capabilities. On SPARC platforms, hardware capabilities are defined in sys/auxv_SPARC.h. On x86 platforms, hardware capabilities are defined in sys/auxv_386.h.

CA_SUNW_SF_1

Indicates software capability values. The c_val element contains a value that represents the associated software capabilities that are defined in sys/elf.h.

Relocatable objects can contain a capabilities section. The link-editor combines any capabilities sections from multiple input relocatable objects into a single capabilities section. The link-editor also allows capabilities to be defined at the time an object is built. See Identifying Hardware and Software Capabilities.

A dynamic object that contains a capabilities section that contains hardware capabilities information, has a PT_SUNWCAP program header associated to the section. This program header allows the runtime linker to validate the object against the hardware capabilities that are available to the process.

Dynamic objects that exploit different hardware capabilities can provide a flexible runtime environment using filters. See Hardware Capability Specific Shared Objects.

Hash Table Section

A hash table consists of Elf32_Word or Elf64_Word objects that provide for symbol table access. The SHT_HASH section provides this hash table. The symbol table to which the hashing is associated is specified in the sh_link entry of the hash table's section header. Labels are used in the following figure to help explain the hash table organization, but these labels are not part of the specification.

Figure 7–4 Symbol Hash Table

The bucket array contains nbucket entries, and the chain array contains nchain entries. Indexes start at 0. Both bucket and chain hold symbol table indexes. Chain table entries parallel the symbol table. The number of symbol table entries should equal nchain, so symbol table indexes also select chain table entries.

A hashing function that accepts a symbol name, returns a value to compute a bucket index. Consequently, if the hashing function returns the value x for some name, bucket [x% nbucket] gives an index y. This index is an index into both the symbol table and the chain table. If the symbol table entry is not the name desired, chain[y] gives the next symbol table entry with the same hash value.

The chain links can be followed until the selected symbol table entry holds the desired name, or the chain entry contains the value STN_UNDEF.

The hash function is as follows.

unsigned long
elf_Hash(const unsigned char *name)
{
    unsigned long h = 0, g;
 
	    while (*name)
	    {
		     h = (h << 4) + *name++;
		     if (g = h & 0xf0000000)
			      h ^= g >> 24;
				   h &= ~g;
	    }
	    return h;
}

Move Section

Typically, within ELF files, initialized data variables are maintained within the object file. If a data variable is very large, and contains only a small number of initialized (nonzero) elements, the entire variable is still maintained in the object file.

Objects that contain large partially initialized data variables, such as FORTRAN COMMON blocks, can result in a significant disk space overhead. The SHT_SUNW_move section provides a mechanism of compressing these data variables. This compression reduces the disk size of the associated object.

The SHT_SUNW_move section contains multiple entries of the type ELF32_Move or Elf64_Move. These entries allow data variables to be defined as tentative items (.bss). These items occupy no space in the object file, but contribute to the object's memory image at runtime. The move records establish how the memory image is initialized with data to construct the complete data variable.

ELF32_Move and Elf64_Move entries are defined as follows.

typedef struct {
        Elf32_Lword       m_value;
        Elf32_Word        m_info;
        Elf32_Word        m_poffset;
        Elf32_Half        m_repeat;
        Elf32_Half        m_stride;
} Elf32_Move;

#define ELF32_M_SYM(info)       ((info)>>8)
#define ELF32_M_SIZE(info)      ((unsigned char)(info))
#define ELF32_M_INFO(sym, size) (((sym)<<8)+(unsigned char)(size))

typedef struct {
        Elf64_Lword       m_value;
        Elf64_Xword       m_info;
        Elf64_Xword       m_poffset;
        Elf64_Half        m_repeat;
        Elf64_Half        m_stride;
} Elf64_Move;

#define ELF64_M_SYM(info)       ((info)>>8)
#define ELF64_M_SIZE(info)      ((unsigned char)(info))
#define ELF64_M_INFO(sym, size) (((sym)<<8)+(unsigned char)(size))

The elements of these structures are as follows.

m_value

The initialization value, which is the value that is moved into the memory image.

m_info

The symbol table index, with respect to which the initialization is applied, together with the size, in bytes, of the offset being initialized. The lower 8 bits of the member define the size, which can be 1, 2, 4 or 8. The upper bytes define the symbol index.

m_poffset

The offset relative to the associated symbol to which the initialization is applied.

m_repeat

A repetition count.

m_stride

The stride count. This value indicates the number of units that should be skipped when performing a repetitive initialization. A unit is the size of an initialization object as defined by m_info. An m_stride value of zero indicates that the initialization be performed contiguously for units.

The following data definition would traditionally consume 0x8000 bytes within an object file.

typedef struct {
        int     one;
        char    two;
} Data;

Data move[0x1000] = {
        {0, 0},       {1, '1'},     {0, 0},
        {0xf, 'F'},   {0xf, 'F'},   {0, 0},
        {0xe, 'E'},   {0, 0},       {0xe, 'E'}
};

A SHT_SUNW_move section can be used to describe this data. The data item is defined within the .bss section. The non-zero elements of the data item are initialized with the appropriate move entries.

$ elfdump -s data | fgrep move
      [17]  0x00020868 0x00008000  OBJT GLOB 0   .bss       move
$ elfdump -m data

Move Section: .SUNW_move
    symndx offset   size repeat stride   value               with respect to
      [17]      8      4      1      1 0x000000000000000001  move
      [17]     12      4      1      1 0x000000000031000000  move
      [17]     24      4      2      1 0x00000000000000000f  move
      [17]     28      4      2      1 0x000000000046000000  move
      [17]     48      4      1      1 0x00000000000000000e  move
      [17]     52      4      1      1 0x000000000045000000  move
      [17]     64      4      1      1 0x00000000000000000e  move
      [17]     68      4      1      1 0x000000000045000000  move

Move sections that are supplied from relocatable objects are concatenated and output in the object being created by the link-editor. However, the following conditions cause the link-editor to process the move entries. This processing expands the move entry contents into a traditional data item.

• The output file is a static executable.

• The size of the move entries is greater than the size of the symbol into which the move data would be expanded.

• The -z nopartial option is in effect.

Note Section

A vendor or system engineer might need to mark an object file with special information that other programs can check for conformance or compatibility. Sections of type SHT_NOTE and program header elements of type PT_NOTE can be used for this purpose.

The note information in sections and program header elements holds any number of entries, as shown in the following figure. For 64–bit objects and 32–bit objects, each entry is an array of 4-byte words in the format of the target processor. Labels are shown in Figure 7–6 to help explain note information organization, but are not part of the specification.

Figure 7–5 Note Information

namesz and name

The first namesz bytes in name contain a null-terminated character representation of the entry's owner or originator. No formal mechanism exists for avoiding name conflicts. By convention, vendors use their own name, such as “XYZ Computer Company,” as the identifier. If no name is present, namesz contains the value zero. Padding is present, if necessary, to ensure 4-byte alignment for the descriptor. Such padding is not included in namesz.

descsz and desc

The first descsz bytes in desc hold the note descriptor. If no descriptor is present, descsz contains the value zero. Padding is present, if necessary, to ensure 4-byte alignment for the next note entry. Such padding is not included in descsz.

type

Provides the interpretation of the descriptor. Each originator controls its own types. Multiple interpretations of a single type value can exist. A program must recognize both the name and the type to understand a descriptor. Types currently must be nonnegative.

The note segment that is shown in the following figure holds two entries.

Figure 7–6 Example Note Segment

The system reserves note information with no name (namesz == 0) and with a zero-length name (name[0] == '\0'), but currently defines no types. All other names must have at least one non-null character.

Relocation Sections

Relocation is the process of connecting symbolic references with symbolic definitions. For example, when a program calls a function, the associated call instruction must transfer control to the proper destination address at execution. Relocatable files must have information that describes how to modify their section contents. This information allows executable and shared object files to hold the right information for a process's program image. Relocation entries are these data.

Relocation entries can have the following structure. See sys/elf.h.

typedef struct {
        Elf32_Addr      r_offset;
        Elf32_Word      r_info;
} Elf32_Rel;
 
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_Rel;
 
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. Different object files have slightly different interpretations for this member.

For a relocatable file, the value indicates a section offset. The relocation section describes how to modify another section in the file. Relocation offsets designate a storage unit within the second section.

For an executable or shared object, the value indicates the virtual address of the storage unit affected by the relocation. This information makes the relocation entries more useful for the runtime linker.

Although the interpretation of the member changes for different object files to allow efficient access by the relevant programs, the meanings of the relocation types stay the same.

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. For example, a call instruction's relocation entry holds the symbol table index of the function being called. If the index is STN_UNDEF, the undefined symbol index, the relocation uses zero as the symbol value.

Relocation types are processor-specific. A relocation entry's relocation type or symbol table index is the result of applying ELF32_R_TYPE or ELF32_R_SYM, respectively, to the entry's r_info member.

#define ELF32_R_SYM(info)             ((info)>>8)
#define ELF32_R_TYPE(info)            ((unsigned char)(info))
#define ELF32_R_INFO(sym, type)       (((sym)<<8)+(unsigned char)(type))

#define ELF64_R_SYM(info)             ((info)>>32)
#define ELF64_R_TYPE(info)            ((Elf64_Word)(info))
#define ELF64_R_INFO(sym, type)       (((Elf64_Xword)(sym)<<32)+ \ 
                                        (Elf64_Xword)(type))

For 64–bit SPARC Elf64_Rela structures, the r_info field is further broken down into an 8–bit type identifier and a 24–bit type dependent data field. For the existing relocation types, the data field is zero. New relocation types, however, might make use of the data bits.

#define ELF64_R_TYPE_DATA(info)       (((Elf64_Xword)(info)<<32)>>40)
#define ELF64_R_TYPE_ID(info)         (((Elf64_Xword)(info)<<56)>>56)
#define ELF64_R_TYPE_INFO(data, type) (((Elf64_Xword)(data)<<8)+ \ 
                                        (Elf64_Xword)(type))

r_addend

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

Rela entries contain an explicit addend. Entries of type Rel store an implicit addend in the location to be modified. 32–bit SPARC use only Elf32_Rela relocation enteries. 64–bit SPARC and 64–bit x86 use only Elf64_Rela relocation entries. Thus, the r_addend member serves as the relocation addend. x86 uses only Elf32_Rel relocation entries. The field to be relocated holds the addend. In all cases, the addend and the computed result use the same byte order.

A relocation section can reference two other sections: a symbol table, identified by the sh_link section header entry, and a section to modify, identified by the sh_info section header entry. Sections specifies these relationships. A sh_info entry is required when a relocation section exists in a relocatable object, but is optional for executables and shared objects. The relocation offset is sufficient to perform the relocation.

Relocation Types (Processor-Specific)

Relocation entries describe how to alter instruction and data fields in the following figures. Bit numbers appear in the lower box corners.

On the SPARC platform, relocation entries apply to bytes (byte8), half-words (half16), or words.

On 64–bit SPARC and x64, relocations also apply to extended-words (xword64).

On x86, relocation entries apply to words (word32).

word32 specifies a 32–bit field occupying 4 bytes with an arbitrary byte alignment. These values use the same byte order as other word values in the x86 architecture.

In all cases, the r_offset value designates the offset or virtual address of the first byte of the affected storage unit. The relocation type specifies which bits to change and how to calculate their values.

Calculations for the following relocation types assume the actions are transforming a relocatable file into either an executable or a shared object file. Conceptually, the link-editor merges one or more relocatable files to form the output. The link-editor first decides how to combine and locate the input files. The link-editor then updates the symbol values and performs the relocation. Relocations applied to executable or shared object files are similar and accomplish the same result. Descriptions in the tables in this section use the following notation.

A

The addend used to compute the value of the relocatable field.

B

The base address at which a shared object is loaded into memory during execution. Generally, a shared object file is built with a base virtual address of 0. However, the execution address of the shared object is different. See Program Header.

G

The offset into the global offset table at which the address of the relocation entry's symbol resides during execution. See Global Offset Table (Processor-Specific).

GOT

The address of the global offset table. See Global Offset Table (Processor-Specific).

L

The section offset or address of the procedure linkage table entry for a symbol. See Procedure Linkage Table (Processor-Specific).

P

The section offset or address of the storage unit being relocated, computed using r_offset.

S

The value of the symbol whose index resides in the relocation entry.

Z

The size of the symbol whose index resides in the relocation entry.

SPARC: Relocation Types

Field names in the following table tell whether the relocation type checks for overflow. A calculated relocation value can be larger than the intended field, and a relocation type can verify (V) the value fits or truncate (T) the result. As an example, V-simm13 means that the computed value can not have significant, nonzero bits outside the simm13 field.

Additional relocations are available for thread-local storage references. These relocations are covered in Chapter 8, Thread-Local Storage.

Some relocation types have semantics beyond simple calculation.

R_SPARC_GOT10

Resembles R_SPARC_LO10, except that the relocation refers to the address of the symbol's GOT entry. Additionally, R_SPARC_GOT10 instructs the link-editor to create a global offset table.

R_SPARC_GOT13

Resembles R_SPARC_13, except that the relocation refers to the address of the symbol's GOT entry. Additionally, R_SPARC_GOT13 instructs the link-editor to create a global offset table.

R_SPARC_GOT22

Resembles R_SPARC_22, except that the relocation refers to the address of the symbol's GOT entry. Additionally, R_SPARC_GOT22 instructs the link-editor to create a global offset table.

R_SPARC_WPLT30

Resembles R_SPARC_WDISP30, except that the relocation refers to the address of the symbol's procedure linkage table entry. Additionally, R_SPARC_WPLT30 instructs the link-editor to create a procedure linkage table.

R_SPARC_COPY

Created by the link-editor for dynamic executables to preserve a read-only text segment. The relocation offset member refers to a location in a writable segment. The symbol table index specifies a symbol that should exist both in the current object file and in a shared object. During execution, the runtime linker copies data associated with the shared object's symbol to the location specified by the offset. See Copy Relocations.

R_SPARC_GLOB_DAT

Resembles R_SPARC_32, except that the relocation sets a GOT entry to the address of the specified symbol. The special relocation type enables you to determine the correspondence between symbols and GOT entries.

R_SPARC_JMP_SLOT

Created by the link-editor for dynamic objects to provide lazy binding. The relocation offset member gives the location of a procedure linkage table entry. The runtime linker modifies the procedure linkage table entry to transfer control to the designated symbol address.

R_SPARC_RELATIVE

Created by the link-editor for dynamic objects. The relocation offset member gives the location within a shared object that contains a value representing a relative address. The runtime linker computes the corresponding virtual address by adding the virtual address at which the shared object is loaded to the relative address. Relocation entries for this type must specify a value of zero for the symbol table index.

R_SPARC_UA32

Resembles R_SPARC_32, except that the relocation refers to an unaligned word. The word to be relocated must be treated as four separate bytes with arbitrary alignment, not as a word aligned according to the architecture requirements.

R_SPARC_LM22

Resembles R_SPARC_HI22, except that the relocation truncates rather than validates.

R_SPARC_PC_LM22

Resembles R_SPARC_PC22, except that the relocation truncates rather than validates.

R_SPARC_HIX22

Used with R_SPARC_LOX10 for executables that are confined to the uppermost 4 gigabytes of the 64–bit address space. Similar to R_SPARC_HI22, but supplies ones complement of linked value.

R_SPARC_LOX10

Used with R_SPARC_HIX22. Similar to R_SPARC_LO10, but always sets bits 10 through 12 of the linked value.

R_SPARC_L44

Used with the R_SPARC_H44 and R_SPARC_M44 relocation types to generate a 44-bit absolute addressing model.

R_SPARC_REGISTER

Used to initialize a register symbol. The relocation offset member contains the register number to be initialized. A corresponding register symbol must exist for this register. The symbol must be of type SHN_ABS.

R_SPARC_GOTDATA_OP_HIX22, R_SPARC_GOTDATA_OP_LOX10, and R_SPARC_GOTDATA_OP

These relocations provide for code transformations.

64-bit SPARC: Relocation Types

The following notation, used in relocation calculation, is unique to 64–bit SPARC.

O

The secondary addend used to compute the value of the relocation field. This addend is extracted from the r_info field by applying the ELF64_R_TYPE_DATA macro.

The relocations that are listed in the following table extend, or alter, the relocations defined for 32–bit SPARC. See SPARC: Relocation Types.

The following relocation type has semantics beyond simple calculation.

R_SPARC_OLO10

Resembles R_SPARC_LO10, except that an extra offset is added to make full use of the 13-bit signed immediate field.

32-bit x86: Relocation Types

The relocations that are listed in the following table are defined for 32–bit x86.

Additional relocations are available for thread-local storage references. These relocations are covered in Chapter 8, Thread-Local Storage.

Some relocation types have semantics beyond simple calculation.

R_386_GOT32

Computes the distance from the base of the GOT to the symbol's GOT entry. The relocation also instructs the link-editor to create a global offset table.

R_386_PLT32

Computes the address of the symbol's procedure linkage table entry and instructs the link-editor to create a procedure linkage table.

R_386_COPY

Created by the link-editor for dynamic executables to preserve a read-only text segment. The relocation offset member refers to a location in a writable segment. The symbol table index specifies a symbol that should exist both in the current object file and in a shared object. During execution, the runtime linker copies data associated with the shared object's symbol to the location specified by the offset. See Copy Relocations.

R_386_GLOB_DAT

Used to set a GOT entry to the address of the specified symbol. The special relocation type enable you to determine the correspondence between symbols and GOT entries.

R_386_JMP_SLOT

Created by the link-editor for dynamic objects to provide lazy binding. The relocation offset member gives the location of a procedure linkage table entry. The runtime linker modifies the procedure linkage table entry to transfer control to the designated symbol address.

R_386_RELATIVE

Created by the link-editor for dynamic objects. The relocation offset member gives the location within a shared object that contains a value representing a relative address. The runtime linker computes the corresponding virtual address by adding the virtual address at which the shared object is loaded to the relative address. Relocation entries for this type must specify a value of zero for the symbol table index.

R_386_GOTOFF

Computes the difference between a symbol's value and the address of the GOT. The relocation also instructs the link-editor to create the global offset table.

R_386_GOTPC

Resembles R_386_PC32, except that it uses the address of the GOT in its calculation. The symbol referenced in this relocation normally is _GLOBAL_OFFSET_TABLE_, which also instructs the link-editor to create the global offset table.

x64: Relocation Types

The relocations that are listed in the following table are defined for x64.

Additional relocations are available for thread-local storage references. These relocations are covered in Chapter 8, Thread-Local Storage.

The special semantics for most of these relocation types are identical to those used for x86. Some relocation types have semantics beyond simple calculation.

R_AMD64_GOTPCREL

This relocations has different semantics from the R_AMD64_GOT32 or equivalent R_386_GOTPC relocation. The x64 architecture provides an addressing mode that is relative to the instruction pointer. Therefore, an address can be loaded from the GOT using a single instruction.

The calculation for the R_AMD64_GOTPCREL relocation provides the difference between the location in the GOT where the symbol's address is given, and the location where the relocation is applied.

R_AMD64_32

The computed value is truncated to 32–bits. The link-editor verifies that the generated value for the relocation zero-extends to the original 64–bit value.

R_AMD64_32S

The computed value is truncated to 32–bits. The link-editor verifies that the generated value for the relocation sign-extends to the original 64–bit value.

R_AMD64_8, R_AMD64_16, R_AMD64_PC16, and R_AMD64_PC8

These relocations are not conformant to the x64 ABI, but are added here for documentation purposes. The R_AMD64_8 relocation truncates the computed value to 8-bits. The R_AMD64_16 relocation truncates the computed value to 16-bits.

String Table Section

String table sections hold null-terminated character sequences, commonly called strings. The object file uses these strings to represent symbol and section names. You reference a string as an index into the string table section.

The first byte, which is index zero, holds a null character. Likewise, a string table's last byte holds a null character, ensuring null termination for all strings. A string whose index is zero specifies either no name or a null name, depending on the context.

An empty string table section is permitted. The section header's sh_size member contains zero. Nonzero indexes are invalid for an empty string table.

A section header's sh_name member holds an index into the section header string table section. The section header string table is designated by the e_shstrndx member of the ELF header. The following figure shows a string table with 25 bytes and the strings associated with various indexes.

Figure 7–7 ELF String Table

The following table shows the strings of the string table that are shown in the preceding figure.

As the example shows, a string table index can refer to any byte in the section. A string can appear more than once. References to substrings can exist. A single string can be referenced multiple times. Unreferenced strings also are allowed.

Symbol Table Section

An object file's symbol table holds information needed to locate and relocate a program's symbolic definitions and symbolic references. A symbol table index is a subscript into this array. Index 0 both designates the first entry in the table and serves as the undefined symbol index. See Table 7–21.

A symbol table entry has the following format. See sys/elf.h.

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

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

st_value

The value of the associated symbol. The value can be an absolute value or an address, depending on the context. See Symbol Values.

st_size

Many symbols have associated sizes. For example, a data object's size is the number of bytes that are contained in the object. This member holds the value zero if the symbol has no size or an unknown size.

st_info

The symbol's type and binding attributes. A list of the values and meanings appears in Table 7–18. The following code shows how to manipulate the values. See sys/elf.h.

#define ELF32_ST_BIND(info)          ((info) >> 4)
#define ELF32_ST_TYPE(info)          ((info) & 0xf)
#define ELF32_ST_INFO(bind, type)    (((bind)<<4)+((type)&0xf))

#define ELF64_ST_BIND(info)          ((info) >> 4)
#define ELF64_ST_TYPE(info)          ((info) & 0xf)
#define ELF64_ST_INFO(bind, type)    (((bind)<<4)+((type)&0xf))

st_other

A symbol's visibility. A list of the values and meanings appears in Table 7–20. The following code shows how to manipulate the values for both 32–bit objects and 64–bit objects. Other bits are set to zero, and have no defined meaning.

#define ELF32_ST_VISIBILITY(o)       ((o)&0x3)
#define ELF64_ST_VISIBILITY(o)       ((o)&0x3)

st_shndx

Every symbol table entry is defined in relation to some section. This member holds the relevant section header table index. Some section indexes indicate special meanings. See Table 7–4.

If this member contains SHN_XINDEX, then the actual section header index is too large to fit in this field. The actual value is contained in the associated section of type SHT_SYMTAB_SHNDX.

A symbol's binding, determined from its st_info field, determines the linkage visibility and behavior.

STB_LOCAL

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

STB_GLOBAL

Global symbols. These symbols are visible to all object files being combined. One file's definition of a global symbol satisfies another file's undefined reference to the same global symbol.

STB_WEAK

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

STB_LOOS - STB_HIOS

Values in this inclusive range are reserved for operating system-specific semantics.

STB_LOPROC - STB_HIPROC

Values in this inclusive range are reserved for processor-specific semantics.

Global symbols and weak symbols differ in two major ways.

• When the link-editor combines several relocatable object files, multiple definitions of STB_GLOBAL symbols with the same name are not allowed. However, if a defined global symbol exists, the appearance of a weak symbol with the same name does not cause an error. The link-editor honors the global definition and ignores the weak definitions.

  Similarly, if a common symbol exists, the appearance of a weak symbol with the same name does not cause an error. The link-editor uses the common definition and ignores the weak definition. A common symbol has the st_shndx field holding SHN_COMMON. See Symbol Resolution.

• When the link-editor searches archive libraries, archive members that contain definitions of undefined or tentative global symbols are extracted. The member's definition can be either a global or a weak symbol.

  The link-editor, by default, does not extract archive members to resolve undefined weak symbols. Unresolved weak symbols have a zero value. The use of -z weakextract overrides this default behavior. This options enables weak references to cause the extraction of archive members.

Weak symbols are intended primarily for use in system software. Their use in application programs is discouraged.

In each symbol table, all symbols with STB_LOCAL binding precede the weak symbols and global symbols. As Sections describes, a symbol table section's sh_info section header member holds the symbol table index for the first non-local symbol.

A symbol's type, as determined from its st_info field, provides a general classification for the associated entity.

STT_NOTYPE

The symbol type is not specified.

STT_OBJECT

This symbol is associated with a data object, such as a variable, an array, and so forth.

STT_FUNC

This symbol is associated with a function or other executable code.

STT_SECTION

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

STT_FILE

Conventionally, the symbol's name gives the name of the source file that is associated with the object file. A file symbol has STB_LOCAL binding and a section index of SHN_ABS. This symbol, if present, precedes the other STB_LOCAL symbols for the file.

Symbol index 1 of the SHT_SYMTAB is an STT_FILE symbol representing the object file. Conventionally, this symbol is followed by the files STT_SECTION symbols. These section symbols are then followed by any global symbols that have been reduced to locals.

STT_COMMON

This symbol labels an uninitialized common block. This symbol is treated exactly the same as STT_OBJECT.

STT_TLS

The symbol specifies a thread-local storage entity. When defined, this symbol gives the assigned offset for the symbol, not the actual address.

Thread-local storage relocations can only reference symbols with type STT_TLS. A reference to a symbol of type STT_TLS from an allocatable section, can only be achieved by using special thread-local storage relocations. See Chapter 8, Thread-Local Storage for details. A reference to a symbol of type STT_TLS from a non-allocatable section does not have this restriction.

STT_LOOS - STT_HIOS

Values in this inclusive range are reserved for operating system-specific semantics.

STT_LOPROC - STT_HIPROC

Values in this inclusive range are reserved for processor-specific semantics.

A symbol's visibility is determined from its st_other field. This visibility can be specified in a relocatable object. This visibility defines how that symbol can be accessed once the symbol has become part of an executable or shared object.

STV_DEFAULT

The visibility of symbols with the STV_DEFAULT attribute is as specified by the symbol's binding type. Global symbols and weak symbols are visible outside of their defining component, the executable file or shared object. Local symbols are hidden. Global symbols and weak symbols can also be preempted. These symbols can by interposed by definitions of the same name in another component.

STV_PROTECTED

A symbol that is defined in the current component is protected if the symbol is visible in other components, but cannot be preempted. Any reference to such a symbol from within the defining component must be resolved to the definition in that component. This resolution must occur, even if a symbol definition exists in another component that would interpose by the default rules. A symbol with STB_LOCAL binding will not have STV_PROTECTED visibility.

STV_HIDDEN

A symbol that is defined in the current component is hidden if its name is not visible to other components. Such a symbol is necessarily protected. This attribute is used to control the external interface of a component. An object named by such a symbol can still be referenced from another component if its address is passed outside.

A hidden symbol contained in a relocatable object is either removed or converted to STB_LOCAL binding when the object is included in an executable file or shared object.

STV_INTERNAL

This visibility attribute is currently reserved.

The visibility attributes do not affect the resolution of symbols within an executable or shared object during link-editing. Such resolution is controlled by the binding type. Once the link-editor has chosen its resolution, these attributes impose two requirements. Both requirements are based on the fact that references in the code being linked might have been optimized to take advantage of the attributes.

• All of the non-default visibility attributes, when applied to a symbol reference, imply that a definition to satisfy that reference must be provided within the object being linked. If this type of symbol reference has no definition within the object being linked, then the reference must have STB_WEAK binding. In this case, the reference is resolved to zero.

• If any reference to a name, or definition of a name is a symbol with a non-default visibility attribute, the visibility attribute is propagated to the resolving symbol in the object being linked. If different visibility attributes are specified for distinct instances of a symbol, the most constraining visibility attribute is propagated to the resolving symbol in the object being linked. The attributes, ordered from least to most constraining, are STV_PROTECTED, STV_HIDDEN and STV_INTERNAL.

If a symbol's value refers to a specific location within a section, the symbols's section index member, st_shndx, holds an index into the section header table. As the section moves during relocation, the symbol's value changes as well. References to the symbol continue to point to the same location in the program. Some special section index values give other semantics.

SHN_ABS

This symbol has an absolute value that does not change because of relocation.

SHN_COMMON, and SHN_AMD64_LCOMMON

This symbol labels a common block that has not yet been allocated. The symbol's value gives alignment constraints, similar to a section's sh_addralign member. The link-editor allocates the storage for the symbol at an address that is a multiple of st_value. The symbol's size tells how many bytes are required.

SHN_UNDEF

This section table index indicates that the symbol is undefined. When the link-editor combines this object file with another object that defines the indicated symbol, this file's references to the symbol is bound to the definition.

As mentioned previously, the symbol table entry for index 0 (STN_UNDEF) is reserved. This entry holds the values listed in the following table.

Symbol Values

Symbol table entries for different object file types have slightly different interpretations for the st_value member.

• In relocatable files, st_value holds alignment constraints for a symbol whose section index is SHN_COMMON.

• In relocatable files, st_value holds a section offset for a defined symbol. st_value is an offset from the beginning of the section that st_shndx identifies.

• In executable and shared object files, st_value holds a virtual address. To make these files' symbols more useful for the runtime linker, the section offset (file interpretation) gives way to a virtual address (memory interpretation) for which the section number is irrelevant.

Although the symbol table values have similar meanings for different object files, the data allow efficient access by the appropriate programs.

Symbol Table Layout and Conventions

The symbols in a symbol table are written in the following order.

• Index 0 in any symbol table is used to represent undefined symbols. This first entry in a symbol table is always completely zeroed. The symbol type is therefore STT_NOTYPE.

• If the symbol table contains any local symbols, the second entry of the symbol table is an STT_FILE symbol giving the name of the file.

• Section symbols of type STT_SECTION.

• Register symbols of type STT_REGISTER.

• Global symbols that have been reduced to local scope.

• For each input file that supplies local symbols, a STT_FILE symbol giving the name of the input file, followed by the symbols in question.

• The global symbols immediately follow the local symbols in the symbol table. The first global symbol is identified by the symbol table sh_info value. Local and global symbols are always kept separate in this manner, and cannot be mixed together.

Two symbol tables are of special interest in the Solaris OS.

.symtab (SHT_SYMTAB)

This symbol table contains every symbol that describes the associated ELF file. This symbol table is typically non-allocable, and is therefore not available in the memory image of the process.

Global symbols can be eliminated from the .symtab by using a mapfile together with the ELIMINATE keyword. See Defining Additional Symbols with a mapfile. Local symbols can also be eliminated by using the link-editor -z redlocsym option.

.dynsym (SHT_DYNSYM)

This table contains a subset of the symbols from the .symtab table that are needed to support dynamic linking. This symbol table is allocable, and is therefore available in the memory image of the process.

The .dynsym table begins with the standard NULL symbol, followed by the files global symbols. STT_FILE symbols are typically not present in this symbol table. STT_SECTION symbols might be present if required by relocation entries.

Register Symbols

The SPARC architecture supports register symbols, which are symbols that initialize a global register. A symbol table entry for a register symbol contains the entries that are listed in the following table.

The register values that are defined for SPARC are listed in the following table.

Absence of an entry for a particular global register means that the particular global register is not used at all by the object.

Syminfo Table Section

The syminfo section contains multiple entries of the type Elf32_Syminfo or Elf64_Syminfo. The .SUNW_syminfo section contains one entry for every entry in the associated symbol table (sh_link).

If this section is present in an object, additional symbol information is to be found by taking the symbol index from the associated symbol table and using that to find the corresponding Elf32_Syminfo entry or Elf64_Syminfo entry in this section. The associated symbol table and the Syminfo table will always have the same number of entries.

Index 0 is used to store the current version of the Syminfo table, which is SYMINFO_CURRENT. Since symbol table entry 0 is always reserved for the UNDEF symbol table entry, this usage does not pose any conflicts.

An Syminfo entry has the following format. See sys/link.h.

typedef struct {
        Elf32_Half      si_boundto;
        Elf32_Half      si_flags;
} Elf32_Syminfo;

typedef struct {
        Elf64_Half      si_boundto;
        Elf64_Half      si_flags;
} Elf64_Syminfo;

si_boundto

An index to an entry in the .dynamic section, identified by the sh_info field, which augments the Syminfo flags. For example, a DT_NEEDED entry identifies a dynamic object associated with the Syminfo entry. The entries that follow are reserved values for si_boundto.

Name	Value	Meaning
SYMINFO_BT_SELF	0xffff	Symbol bound to self.
SYMINFO_BT_PARENT	0xfffe	Symbol bound to parent. The parent is the first object to cause this dynamic object to be loaded.
SYMINFO_BT_NONE	0xfffd	Symbol has no special symbol binding.

si_flags

This bit-field can have flags set, as shown in the following table.

Name	Value	Meaning
SYMINFO_FLG_DIRECT	0x01	Symbol reference has a direct association to the object containing the definition.
SYMINFO_FLG_COPY	0x04	Symbol definition is the result of a copy-relocation.
SYMINFO_FLG_LAZYLOAD	0x08	Symbol reference is to an object that should be lazily loaded.
SYMINFO_FLG_DIRECTBIND	0x10	Symbol reference should be bound directly to the definition.
SYMINFO_FLG_NOEXTDIRECT	0x20	Do not allow an external reference to directly bind to this symbol definition.

Versioning Sections

Objects created by the link-editor can contain two types of versioning information.

• Version definitions provide associations of global symbols and are implemented using sections of type SHT_SUNW_verdef and SHT_SUNW_versym.

• Version dependencies indicate the version definition requirements from other object dependencies and are implemented using sections of type SHT_SUNW_verneed.

The structures that form these sections are defined in sys/link.h. Sections that contain versioning information are named .SUNW_version.

Version Definition Section

This section is defined by the type SHT_SUNW_verdef. If this section exists, a SHT_SUNW_versym section must also exist. These two structures provide an association of symbols to version definitions within the file. See Creating a Version Definition. Elements of this section have the following structure.

typedef struct {
        Elf32_Half      vd_version;
        Elf32_Half      vd_flags;
        Elf32_Half      vd_ndx;
        Elf32_Half      vd_cnt;
        Elf32_Word      vd_hash;
        Elf32_Word      vd_aux;
        Elf32_Word      vd_next;
} Elf32_Verdef;
 
typedef struct {
        Elf32_Word      vda_name;
        Elf32_Word      vda_next;
} Elf32_Verdaux;

typedef struct {
        Elf64_Half      vd_version;
        Elf64_Half      vd_flags;
        Elf64_Half      vd_ndx;
        Elf64_Half      vd_cnt;
        Elf64_Word      vd_hash;
        Elf64_Word      vd_aux;
        Elf64_Word      vd_next;
} Elf64_Verdef;
 
typedef struct {
        Elf64_Word      vda_name;
        Elf64_Word      vda_next;
} Elf64_Verdaux;

vd_version

This member identifies the version of the structure, as listed in the following table.

Name	Value	Meaning
VER_DEF_NONE	0	Invalid version.
VER_DEF_CURRENT	>=1	Current version.

The value 1 signifies the original section format. Extensions require new versions with higher numbers. The value of VER_DEF_CURRENT changes as necessary to reflect the current version number.

vd_flags

This member holds version definition-specific information, as listed in the following table.

Name	Value	Meaning
VER_FLG_BASE	0x1	Version definition of the file.
VER_FLG_WEAK	0x2	Weak version identifier.

The base version definition is always present when version definitions, or symbol auto-reduction, have been applied to the file. The base version provides a default version for the files reserved symbols. A weak version definition has no symbols associated with the version. See Creating a Weak Version Definition.

vd_ndx

The version index. Each version definition has a unique index that is used to associate SHT_SUNW_versym entries to the appropriate version definition.

vd_cnt

The number of elements in the Elf32_Verdaux array.

vd_hash

The hash value of the version definition name. This value is generated using the same hashing function that is described in Hash Table Section.

vd_aux

The byte offset from the start of this Elf32_Verdef entry to the Elf32_Verdaux array of version definition names. The first element of the array must exist. This element points to the version definition string this structure defines. Additional elements can be present. The number of elements is indicated by the vd_cnt value. These elements represent the dependencies of this version definition. Each of these dependencies will have its own version definition structure.

vd_next

The byte offset from the start of this Elf32_Verdef structure to the next Elf32_Verdef entry.

vda_name

The string table offset to a null-terminated string, giving the name of the version definition.

vda_next

The byte offset from the start of this Elf32_Verdaux entry to the next Elf32_Verdaux entry.

Version Dependency Section

The version dependency section is defined by the type SHT_SUNW_verneed. This section complements the dynamic dependency requirements of the file by indicating the version definitions required from these dependencies. A recording is made in this section only if a dependency contains version definitions. Elements of this section have the following structure.

typedef struct {
        Elf32_Half      vn_version;
        Elf32_Half      vn_cnt;
        Elf32_Word      vn_file;
        Elf32_Word      vn_aux;
        Elf32_Word      vn_next;
} Elf32_Verneed;
 
typedef struct {
        Elf32_Word      vna_hash;
        Elf32_Half      vna_flags;
        Elf32_Half      vna_other;
        Elf32_Word      vna_name;
        Elf32_Word      vna_next;
} Elf32_Vernaux;

typedef struct {
        Elf64_Half      vn_version;
        Elf64_Half      vn_cnt;
        Elf64_Word      vn_file;
        Elf64_Word      vn_aux;
        Elf64_Word      vn_next;
} Elf64_Verneed;
 
typedef struct {
        Elf64_Word      vna_hash;
        Elf64_Half      vna_flags;
        Elf64_Half      vna_other;
        Elf64_Word      vna_name;
        Elf64_Word      vna_next;
} Elf64_Vernaux;

vn_version

This member identifies the version of the structure, as listed in the following table.

Name	Value	Meaning
VER_NEED_NONE	0	Invalid version.
VER_NEED_CURRENT	>=1	Current version.

The value 1 signifies the original section format. Extensions require new versions with higher numbers. The value of VER_NEED_CURRENT changes as necessary to reflect the current version number.

vn_cnt

The number of elements in the Elf32_Vernaux array.

vn_file

The string table offset to a null-terminated string, providing the file name of a version dependency. This name matches one of the .dynamic dependencies found in the file. See Dynamic Section.

vn_aux

The byte offset, from the start of this Elf32_Verneed entry, to the Elf32_Vernaux array of version definitions that are required from the associated file dependency. At least one version dependency must exist. Additional version dependencies can be present, the number being indicated by the vn_cnt value.

vn_next

The byte offset, from the start of this Elf32_Verneed entry, to the next Elf32_Verneed entry.

vna_hash

The hash value of the version dependency name. This value is generated using the same hashing function that is described in Hash Table Section.

vna_flags

Version dependency specific information, as listed in the following table.

Name	Value	Meaning
VER_FLG_WEAK	0x2	Weak version identifier.

A weak version dependency indicates an original binding to a weak version definition.

vna_other

Presently unused.

vna_name

The string table offset to a null-terminated string, giving the name of the version dependency.

vna_next

The byte offset from the start of this Elf32_Vernaux entry to the next Elf32_Vernaux entry.

Version Symbol Section

The version symbol section is defined by the type SHT_SUNW_versym. This section consists of an array of elements of the following structure.

typedef Elf32_Half      Elf32_Versym;
typedef Elf64_Half      Elf64_Versym;

The number of elements of the array must equal the number of symbol table entries that are contained in the associated symbol table. This number is determined by the section's sh_link value. Each element of the array contains a single index that can have the values shown in the following table.

Any index values that are greater than VER_NDX_GLOBAL must correspond to the vd_ndx value of an entry in the SHT_SUNW_verdef section. If no index values that are greater than VER_NDX_GLOBAL exist, then no SHT_SUNW_verdef section need be present.