An object file's section header table lets one locate all
the file's sections. The section header table is an array
of Elf32_Shdr structures as described in
Table 8-3, ``Special section indexes''.
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.
Some section header table indexes are reserved;
an object file will not have sections for these special indexes.
Table 8-3 Special section indexes
-------------------------------------------------------
Name Value
-------------------------------------------------------
SHN_UNDEF 0
SHN_LORESERVE 0xff00
SHN_LOPROC 0xff00
SHN_HIPROC 0xff1f
SHN_ABS 0xfff1
SHN_COMMON 0xfff2
SHN_HIRESERVE 0xffff
SHN_UNDEF
This value marks an undefined, missing, irrelevant, or
otherwise meaningless section reference. For example, a
symbol ``defined'' relative to section number
SHN_UNDEF is an undefined symbol.
NOTE:
Although index 0 is reserved as the undefined value, the
section header table contains an entry for index 0. 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.
SHN_LORESERVE
This value specifies the lower bound of the
range of reserved indexes.
SHN_LOPROC through SHN_HIPROC
Values in this inclusive range are reserved for
processor-specific semantics.
SHN_ABS
This value specifies absolute values for the corresponding
reference. For example, symbols defined relative to
section number SHN_ABS have absolute values and
are not affected by relocation.
SHN_COMMON
Symbols defined relative to this section are common
symbols, such as FORTRAN, COMMON or
unallocated C external variables.
SHN_HIRESERVE
This value specifies the upper bound of the range of
reserved 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, object files' sections
satisfy several conditions.
Every section in an object file has exactly one section
header describing it. Section headers may exist that do
not have a section.
Each section occupies one contiguous (possibly empty)
sequence of bytes within a file.
Sections in a file may not overlap. No byte in a file
resides in more than one section.
An object file may 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.
This member specifies the name of the section. Its value
is an index into the section header string table section,
giving the location of a null-terminated string.
NOTE:
See
``String table''
for more information.
If the section will 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
This member's value gives the byte offset from the
beginning of the file to the first byte in the section.
One section type, SHT_NOBITS, described in
Table 8-4,
occupies no space in the file, and its sh_offset
member locates the conceptual placement in the file.
sh_size
This member gives the section's size in bytes. Unless the
section type is SHT_NOBITS, the section occupies
sh_size bytes in the file. A section of type
SHT_NOBITS may have a non-zero size, but it
occupies no space in the file.
sh_link
This member holds a section header table index link,
whose interpretation depends on the section type.
Table 8-4, ``Section types, sh_type''
describes the values.
Some sections have address alignment constraints.
For example, if a section holds a doubleword, the system
must ensure doubleword alignment for the entire section.
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.
This value marks the section header as inactive;
it does not have an associated section.
Other members of the section header have undefined values.
SHT_PROGBITS
The section holds information defined by the program,
whose format and meaning are determined solely by the program.
SHT_SYMTAB and SHT_DYNSYM
These sections hold a symbol table. Currently, an object
file may have only one section of each type, but this
restriction may be relaxed in the future. Typically,
SHT_SYMTAB provides symbols for link editing,
though it may also be used for dynamic linking. As a
complete symbol table, it may contain many symbols
unnecessary for dynamic linking. Consequently, an object
file may also contain a SHT_DYNSYM section, which
holds a minimal set of dynamic linking symbols, to save
space.
NOTE:
See
``Symbol table''
for details.
SHT_STRTAB
The section holds a string table.
An object file may have multiple string table sections.
NOTE:
See
``String table''
for details.
SHT_RELA
The section holds relocation entries with explicit addends,
such as type Elf32_Rela for the 32-bit class of
object files. An object file may have multiple relocation
sections.
NOTE:
See
``Relocation''
for details.
SHT_HASH
The section holds a symbol hash table. Currently, an
object file may have only one hash table, but this
restriction may be relaxed in the future.
See
``Hash table''
in the second part of this chapter for details.
SHT_DYNAMIC
The section holds information for dynamic linking.
Currently, an object file may have only one dynamic
section, but this restriction may be relaxed in the
future.
See
``Dynamic section''
in the second part of this
chapter for details.
SHT_NOTE
The section holds information that marks the file in some way.
See
``Note section''
in the second part of this chapter for details.
SHT_NOBITS
A section of this type occupies no space in the file but
otherwise resembles SHT_PROGBITS. Although this
section contains no bytes, the sh_offset member
contains the conceptual file offset.
SHT_REL
The section holds relocation entries
without explicit addends, such as type
Elf32_Rel for the 32-bit class of object files.
An object file may have multiple relocation sections.
NOTE:
See
``Relocation''
for details.
SHT_SHLIB
This section type is reserved but has unspecified semantics.
SHT_LOPROC through SHT_HIPROC
Values in this inclusive range
are reserved for processor-specific semantics.
SHT_LOUSER
This value specifies the lower bound of the range of
indexes reserved for application programs.
SHT_HIUSER
This value specifies the upper bound of the range of
indexes reserved for application programs. Section types
between SHT_LOUSER and SHT_HIUSER
may be used by the application, without conflicting with
current or future system-defined section types.
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. This entry holds the following.
Table 8-5 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.
Defined values appear in
Table 8-6, ``Section attribute flags, sh_flags'';
other values are reserved.
Table 8-6 Section attribute flags, sh_flags
---------------------------------------------------------------------
Name Value
---------------------------------------------------------------------
SHF_WRITE 0x1
SHF_ALLOC 0x2
SHF_EXECINSTR 0x4
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 set to zero.
SHF_WRITE
The section contains data that should be writable during
process execution.
SHF_ALLOC
The section occupies memory during process execution.
Some control sections do not reside in the memory image
of an object file; this attribute is off for those sections.
SHF_EXECINSTR
The section contains executable machine instructions.
SHF_MASKPROC
All bits included in this mask
are reserved for processor-specific semantics.
Two members in the section header,
sh_link and sh_info,
hold special information, depending on section type.
Table 8-7 sh_link and sh_info interpretation
---------------------------------------------------------------------
sh_type sh_link sh_info
---------------------------------------------------------------------
SHT_DYNAMIC The section header index 0
of the string table used
by entries in the
section.
---------------------------------------------------------------------
SHT_HASH The section header index 0
of the symbol table to
which the hash table
applies.
---------------------------------------------------------------------
SHT_REL The section header index The section header index
SHT_RELA of the associated symbol of the section to which
table. the relocation applies.
---------------------------------------------------------------------
SHT_SYMTAB The section header index One greater than the
SHT_DYNSYM of the associated string symbol table index of the
table. last local symbol
(binding STB_LOCAL).
---------------------------------------------------------------------
other SHN_UNDEF 0