Sections

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 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.

Some section header table indexes are reserved; an object file will not have sections for these special indexes.

Name	Value
SHN_UNDEF	0
SHN_LORESERVE	0xff00
SHN_LOPROC	0xff00
SHN_HIPROC	0xff1f
SHN_LOOS	0xff20
SHN_HIOS	0xff3f
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.

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_LOOS through SHN_HIOS

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

Figure 4-8: Section Header


typedef struct {
       Elf32_Word      sh_name;
       Elf32_Word      sh_type;
       Elf32_Word      sh_flags;
       Elf32_Addr      sh_addr;
       Elf32_Off       sh_offset;
       Elf32_Word      sh_size;
       Elf32_Word      sh_link;
       Elf32_Word      sh_info;
       Elf32_Word      sh_addralign;
       Elf32_Word      sh_entsize;
} Elf32_Shdr;

typedef struct {
       Elf64_Word      sh_name;
       Elf64_Word      sh_type;
       Elf64_Xword     sh_flags;
       Elf64_Addr      sh_addr;
       Elf64_Off       sh_offset;
       Elf64_Xword     sh_size;
       Elf64_Word      sh_link;
       Elf64_Word      sh_info;
       Elf64_Xword     sh_addralign;
       Elf64_Xword     sh_entsize;
} Elf64_Shdr;

sh_name

This member specifies the name of the section. Its value is an index into the section header string table section [see ``String Table'' below], giving the location of a null-terminated string.

sh_type

This member categorizes the section's contents and semantics. Section types and their descriptions appear below.

sh_flags

Sections support 1-bit flags that describe miscellaneous attributes. Flag definitions appear below.

sh_addr

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 below, 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. A table below describes the values.

sh_info

This member holds extra information, whose interpretation depends on the section type. A table below describes the values.

sh_addralign

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.

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_LOOS	0x60000000
SHT_HIOS	0x6fffffff
SHT_LOPROC	0x70000000
SHT_HIPROC	0x7fffffff
SHT_LOUSER	0x80000000
SHT_HIUSER	0xffffffff

SHT_NULL

This value marks the section header as inactive; it does not have an associated section. Other members of the section header have undefined values.

SHT_PROGBITS

The section holds information defined by the program, whose format and meaning are determined solely by the program.

SHT_SYMTAB 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. See ``Symbol Table'' below for details.

SHT_STRTAB

The section holds a string table. An object file may have multiple string table sections. See ``String Table'' below 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 or type Elf64_Rela for the 64-bit class of object files. An object file may have multiple relocation sections. ``Relocation'' below 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 Chapter 5 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 Chapter 5 for details.

SHT_NOTE

The section holds information that marks the file in some way. See ``Note Section'' in Chapter 5 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 or type Elf64_Rel for the 64-bit class of object files. An object file may have multiple relocation sections. See ``Relocation'' below for details.

SHT_SHLIB

This section type is reserved but has unspecified semantics.

SHT_LOOS through SHT_HIOS

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

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 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 the following table; other values are reserved.

Name	Value
SHF_WRITE	0x1
SHF_ALLOC	0x2
SHF_EXECINSTR	0x4
SHF_MASKOS	0x0f000000
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_MASKOS

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

SHF_MASKPROC

All bits included in this mask are reserved for processor-specific semantics. If meanings are specified, the processor supplement explains them.

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

sh_type	sh_link	sh_info
SHT_DYNAMIC	The section header index of the string table used by entries in the section.	0
SHT_HASH	The section header index of the symbol table to which the hash table applies.	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.
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).

Special Sections

The following table shows sections that are used by the system and have the indicated types and attributes.

Name	Type	Attributes
.bss	SHT_NOBITS	SHF_ALLOC+SHF_WRITE
.comment	SHT_PROGBITS	none
.data	SHT_PROGBITS	SHF_ALLOC+SHF_WRITE
.data1	SHT_PROGBITS	SHF_ALLOC+SHF_WRITE
.debug	SHT_PROGBITS	none
.dynamic	SHT_DYNAMIC	see below
.dynstr	SHT_STRTAB	SHF_ALLOC
.dynsym	SHT_DYNSYM	SHF_ALLOC
.fini	SHT_PROGBITS	SHF_ALLOC+SHF_EXECINSTR
.got	SHT_PROGBITS	see below
.hash	SHT_HASH	SHF_ALLOC
.init	SHT_PROGBITS	SHF_ALLOC+SHF_EXECINSTR
.interp	SHT_PROGBITS	see below
.line	SHT_PROGBITS	none
.note	SHT_NOTE	none
.plt	SHT_PROGBITS	see below
.relname	SHT_REL	see below
.relaname	SHT_RELA	see below
.rodata	SHT_PROGBITS	SHF_ALLOC
.rodata1	SHT_PROGBITS	SHF_ALLOC
.shstrtab	SHT_STRTAB	none
.strtab	SHT_STRTAB	see below
.symtab	SHT_SYMTAB	see below
.text	SHT_PROGBITS	SHF_ALLOC+SHF_EXECINSTR

.bss

This section holds 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

This section holds version control information.

.data and .data1

These sections hold initialized data that contribute to the program's memory image.

.debug

This section holds information for symbolic debugging. The contents are unspecified. All section names with the prefix .debug are reserved for future use in the ABI.

.dynamic

This section holds dynamic linking information. The section's attributes will include the SHF_ALLOC bit. Whether the SHF_WRITE bit is set is processor specific. See Chapter 5 for more information.

.dynstr

This section holds strings needed for dynamic linking, most commonly the strings that represent the names associated with symbol table entries. See Chapter 5 for more information.

.dynsym

This section holds the dynamic linking symbol table, as described in ``Symbol Table''. See Chapter 5 for more information.

.fini

This section holds executable instructions that contribute to the process termination code. That is, when a program exits normally, the system arranges to execute the code in this section.

.got

This section holds the global offset table. See ``Coding Examples'' in Chapter 3, ``Special Sections'' in Chapter 4, and ``Global Offset Table'' in Chapter 5 of the processor supplement for more information.

.hash

This section holds a symbol hash table. See ``Hash Table'' in Chapter 5 for more information.

.init

This section holds executable instructions that contribute to the process initialization code. When a program starts to run, the system arranges to execute the code in this section before calling the main program entry point (called main for C programs).

.interp

This section holds the path name of a program interpreter. If the file has a loadable segment that includes relocation, the sections' attributes will include the SHF_ALLOC bit; otherwise, that bit will be off. See Chapter 5 for more information.

.line

This section holds line number information for symbolic debugging, which describes the correspondence between the source program and the machine code. The contents are unspecified.

.note

This section holds information in the format that ``Note Section''. in Chapter 5 describes.

.plt

This section holds the procedure linkage table. See ``Special Sections'' in Chapter 4 and ``Procedure Linkage Table'' in Chapter 5 of the processor supplement for more information.

.relname and .relaname

These sections hold relocation information, as described in ``Relocation''. If the file has a loadable segment that includes relocation, the sections' attributes will include the SHF_ALLOC bit; otherwise, that bit will be off. Conventionally, name is supplied by the section to which the relocations apply. Thus a relocation section for .text normally would have the name .rel.text or .rela.text.

.rodata and .rodata1

These sections hold read-only data that typically contribute to a non-writable segment in the process image. See ``Program Header'' in Chapter 5 for more information.

.shstrtab

This section holds section names.

.strtab

This section holds strings, most commonly the strings that represent the names associated with symbol table entries. If the file has a loadable segment that includes the symbol string table, the section's attributes will include the SHF_ALLOC bit; otherwise, that bit will be off.

.symtab

This section holds a symbol table, as ``Symbol Table''. in this chapter describes. If the file has a loadable segment that includes the symbol table, the section's attributes will include the SHF_ALLOC bit; otherwise, that bit will be off.

.text

This section holds the ``text,'' or executable instructions, of a program.

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

Section names 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 used for e_machine. For instance .FOO.psect is the psect section defined by the FOO architecture. Existing extensions are called by their historical names.

Pre-existing Extensions

.sdata	.tdesc
.sbss	.lit4
.lit8	.reginfo
.gptab	.liblist
.conflict