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 7-17.
- 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_SUNW_move | 0x6ffffffa |
SHT_SUNW_COMDAT | 0x6ffffffb |
SHT_SUNW_syminfo | 0x6ffffffc |
SHT_SUNW_verdef | 0x6ffffffd |
SHT_SUNW_verneed | 0x6ffffffe |
SHT_SUNW_versym | 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" for details.
- SHT_STRTAB, SHT_DYNSTR
Identifies a string table. An object file can have multiple string table sections. See "String Table" 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" for details.
- SHT_HASH
Identifies a symbol hash table. All dynamically linked object files 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" 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" 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_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_move
Identifies data to handle partially initialized symbols. See "Move Section" for details.
- SHT_SUNW_syminfo
Identifies additional symbol information. See "Syminfo Table" 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_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.