Sections (Linker and Libraries Guide)

Linker and Libraries Guide

Sections

An object file's section header table helps you locate all of the sections of the file. The section header table is an array of Elf32_Shdr or Elf64_Shdr structures, as described below. A section header table index is a subscript into this array. The ELF header's e_shoff member gives the byte offset from the beginning of the file to the section header table; e_shnum tells how many entries the section header table contains; e_shentsize gives 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) and 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 0.

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.

Table 7–11 ELF Special Section Indexes


Name	Value
`SHN_UNDEF`	`0`
`SHN_LORESERVE`	`0xff00`
`SHN_LOPROC`	`0xff00`
`SHN_BEFORE`	`0xff00`
`SHN_AFTER`	`0xff01`
`SHN_HIPROC`	`0xff1f`
`SHN_LOOS`	`0xff20`
`SHN_HIOS`	`0xff3f`
`SHN_ABS`	`0xfff1`
`SHN_COMMON`	`0xfff2`
`SHN_XINDEX`	`0xffff`
`SHN_HIRESERVE`	`0xffff`

Note –

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 says a file has 6 entries in the section header table, they 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_BEFORE, SHN_AFTER: Provide for initial and final section ordering in conjunction with the SHF_LINK_ORDER and SHF_ORDERED section flags, listed in Table 7–14.
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 it 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 it. 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, defined in 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;

The elements of this structure are:

sh_name: The name of the section. Its value is an index into the section header string table section giving the location of a null-terminated string. Section names and their descriptions are listed in Table 7–16.
sh_type: Categorizes the section's contents and semantics. Section types and their descriptions are listed in Table 7–12.
sh_flags: Sections support 1-bit flags that describe miscellaneous attributes. Flag definitions are listed in Table 7–14.
sh_addr: If the section is to appear 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 0.
sh_offset: The byte offset from the beginning of the file to the first byte in the section. Section type SHT_NOBITS occupies no space in the file. Its sh_offset member locates the conceptual placement 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 it occupies no space in the file.
sh_link: A section header table index link, whose interpretation depends on the section type. Table 7–15 describes the values.
sh_info: Extra information, whose interpretation depends on the section type. Table 7–15 describes the values.
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. That is, 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 0 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.

Table 7–12 ELF Section Types, sh_type


Name	Value
`SHT_NULL`	`0`
`SHT_PROGBITS`	`1`
`SHT_SYMTAB`	`2`
`SHT_STRTAB`	`3`
`SHT_RELA`	`4`
`SHT_HASH`	`5`
`SHT_DYNAMIC`	`6`
`SHT_NOTE`	`7`
`SHT_NOBITS`	`8`
`SHT_REL`	`9`
`SHT_SHLIB`	`10`
`SHT_DYNSYM`	`11`
`SHT_INIT_ARRAY`	`14`
`SHT_FINI_ARRAY`	`15`
`SHT_PREINIT_ARRAY`	`16`
`SHT_GROUP`	`17`
`SHT_SYMTAB_SHNDX`	`18`
`SHT_LOOS`	`0x60000000`
`SHT_SUNW_move`	`0x6ffffffa`
`SHT_SUNW_COMDAT`	`0x6ffffffb`
`SHT_SUNW_syminfo`	`0x6ffffffc`
`SHT_SUNW_verdef`	`0x6ffffffd`
`SHT_SUNW_verneed`	`0x6ffffffe`
`SHT_SUNW_versym`	`0x6fffffff`
`SHT_HIOS`	`0x6fffffff`
`SHT_LOPROC`	`0x70000000`
`SHT_HIPROC`	`0x7fffffff`
`SHT_LOUSER`	`0x80000000`
`SHT_HIUSER`	`0xffffffff`

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, it can contain many symbols 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 may appear only in relocatable objects. See Group Section for details.

SHT_SYMTAB_SHNDX

Identifies a section containing extended section indexes, that is 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. There is one value 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_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_LOUSER

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

SHT_HIUSER

Specifies the upper boundary of the range of indexes 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.

Table 7–13 ELF Section Header Table Entry: Index 0


Name	Value	Note
`sh_name`	`0`	No name
`sh_type`	`SHT_NULL`	Inactive
`sh_flags`	`0`	No flags
`sh_addr`	`0`	No address
`sh_offset`	`0`	No file offset
`sh_size`	`0`	No size
`sh_link`	`SHN_UNDEF`	No link information
`sh_info`	`0`	No auxiliary information
`sh_addralign`	`0`	No alignment
`sh_entsize`	`0`	No entries

A section header's sh_flags member holds 1-bit flags that describe the section's attributes:

Table 7–14 ELF Section Attribute Flags


Name	Value
`SHF_WRITE`	`0x1`
`SHF_ALLOC`	`0x2`
`SHF_EXECINSTR`	`0x4`
`SHF_MERGE`	`0x10`
`SHF_STRINGS`	`0x20`
`SHF_INFO_LINK`	`0x40`
`SHF_LINK_ORDER`	`0x80`
`SHF_OS_NONCONFORMING`	`0x100`
`SHF_GROUP`	`0x200`
`SHF_TLS`	`0x400`
`SHF_MASKOS`	`0x0ff00000`
`SHF_ORDERED`	`0x40000000`
`SHF_EXCLUDE`	`0x80000000`
`SHF_MASKPROC`	`0xf0000000`

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 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 may 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 7–11) 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 will be contiguous and have the same relative ordering as they 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 link-editor will reject the object file containing this section with an error.

SHF_GROUP

This section is a member, perhaps the only one, 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 contained in relocatable objects. See Group Section for details.

SHF_TLS

This section holds thread-local storage, meaning that each separate execution flow has its own distinct instance of this data. See Thread-Local Storage Section for details.

SHF_MASKOS

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

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 will be 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 7–11) 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 will be contiguous and have the same relative ordering as they did 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 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.

Table 7–15 ELF sh_link and sh_info Interpretation


`sh_type`	`sh_link`	`sh_info`
`SHT_DYNAMIC`	The section header index of the associated string table.	`0`
`SHT_HASH`	The section header index of the associated symbol table.	`0`
`SHT_REL` `SHT_RELA`	The section header index of the associated symbol table.	The section header index of the section to which the relocation applies. See also Table 7–16 and Relocation Sections.
`SHT_SYMTAB` `SHT_DYNSYM`	The section header index of the associated string table.	One greater than the symbol table index of the last local symbol (binding `STB_LOCAL`).
`SHT_GROUP`	The section header index of the associated symbol table.	The symbol table index of an entry in the associated symbol table. The name of the specified symbol table entry provides a signature for the section group.
`SHT_SYMTAB_SHNDX`	The section header index of the associated symbol table.	`0`
`SHT_SUNW_move`	The section header index of the associated symbol table.	`0`
`SHT_SUNW_COMDAT`	`0`	`0`
`SHT_SUNW_syminfo`	The section header index of the associated symbol table.	The section header index of the associated `.dynamic` section.
`SHT_SUNW_verdef`	The section header index of the associated string table.	The number of version definitions within the section.
`SHT_SUNW_verneed`	The section header index of the associated string table.	The number of version dependencies within the section.
`SHT_SUNW_versym`	The section header index of the associated symbol table.	`0`