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 normally 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.

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_XINDEX

This value is an escape value. It indicates that the actual section header index is too large to fit in the containing field and is to be found in another location (specific to the structure where it appears).

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. 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 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.

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_INIT_ARRAY

This section contains an array of pointers to initialization functions, as described in ``Initialization and Termination Functions'' in Chapter 5. Each pointer in the array is taken as a parameterless procedure with a void return.

SHT_FINI_ARRAY

This section contains an array of pointers to termination functions, as described in ``Initialization and Termination Functions'' in Chapter 5. Each pointer in the array is taken as a parameterless procedure with a void return.

SHT_PREINIT_ARRAY

This section contains an array of pointers to functions that are invoked before all other initialization functions, as described in ``Initialization and Termination Functions'' in Chapter 5. Each pointer in the array is taken as a parameterless procedure with a void return.

SHT_GROUP

This section defines a section group. A section group is a set of sections that are related and that must be treated specially by the linker (see below for further details). Sections of type SHT_GROUP may appear only in relocatable objects (objects with the ELF header e_type member set to ET_REL). The section header table entry for a group section must appear in the section header table before the entries for any of the sections that are members of the group.

SHT_SYMTAB_SHNDX

This section is associated with a section of type SHT_SYMTAB and is required if any of the section header indexes referenced by that symbol table contain the escape value SHN_XINDEX. The section is an array of Elf32_Word values. Each value corresponds one to one with a symbol table entry and appear in the same order as those entries. 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 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.

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.

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_MERGE

The data in the section 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.

Each element in the section is compared against other elements in sections with the same name, type and flags. Elements that would have identical values at program run-time may be merged. Relocations referencing elements of such sections must be resolved to the merged locations of the referenced values. Note that any relocatable values, including values that would result in run-time relocations, must be analyzed to determine whether the run-time values would actually be identical. An ABI-conforming object file may not depend on specific elements being merged, and an ABI-conforming link editor may choose not to merge specific elements.

SHF_STRINGS

The data elements in the section consist of null-terminated character strings. The size of each character is specified in the section header's sh_entsize field.

SHF_INFO_LINK

The sh_info field of this section header holds a section header table index.

SHF_LINK_ORDER

This flag adds special ordering requirements for link editors. 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, it must appear in the same relative order with respect to those sections, as the linked-to section appears with respect to sections the linked-to section is combined with.

---

A typical use of this flag is to build a table that references text or data sections in address 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 a link editor processing this section does not recognize those values, then the link editor should 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 may be set only for sections contained in relocatable objects (objects with the ELF header e_type member set to ET_REL). See below for further details.

SHF_TLS

This section holds Thread-Local Storage, meaning that each separate execution flow has its own distinct instance of this data. Implementations need not support this flag.

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.

Rules for Linking Unrecognized Sections

If the section's sh_flags bits include the attribute SHF_OS_NONCONFORMING, then the section requires special knowledge to be correctly processed, and the link editor should reject the object containing the section with an error.

Unrecognized sections that do not have the SHF_OS_NONCONFORMING attribute, are combined in a two-phase process. As the link editor combines sections using this process, it must honor the alignment constraints of the input sections (asserted by the sh_addralign field), padding between sections with zero bytes, if necessary, and producing a combination with the maximum alignment constraint of its component input sections.

1. In the first phase, input sections that match in name, type and attribute flags should be concatenated into single sections. The concatenation order should satisfy the requirements of any known input section attributes (e.g, SHF_MERGE and SHF_LINK_ORDER). When not otherwise constrained, sections should be emitted in input order.
2. In the second phase, sections should be assigned to segments or other units based on their attribute flags. Sections of each particular unrecognized type should be assigned to the same unit unless prevented by incompatible flags, and within a unit, sections of the same unrecognized type should be placed together if possible.

Non OS-specific processing (e.g. relocation) should be applied to unrecognized section types. An output section header table, if present, should contain entries for unknown sections. Any unrecognized section attribute flags should be removed.

Section Groups

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, as described above: 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 0. The name of the section (sh_name) is not specified.

The referenced signature symbol is not restricted. Its containing symbol table section need not be a member of the group, for example.

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 flags are currently defined:

GRP_COMDAT

This is a COMDAT group. It may 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 may be retained by the linker, and the members of the remaining groups must be discarded.

GRP_MASKOS

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

GRP_MASKPROC

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

The section header indices in the SHT_GROUP section identify the sections that make up the group. Each such section must have the SHF_GROUP flag set in its sh_flags section header member. If the linker decides to remove the section group, it must remove 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 must be followed:

• A symbol table entry with STB_GLOBAL or STB_WEAK binding that is defined relative to one of a group's sections, and that is contained in a symbol table section that is not part of the group, must be converted to an undefined symbol (its section index must be changed to SHN_UNDEF) if the group members are discarded. References to this symbol table entry from outside the group are allowed.

• A symbol table entry with STB_LOCAL binding that is defined relative to one of a group's sections, and that is contained in a symbol table section that is not part of the group, must be discarded if the group members are discarded. References to this symbol table entry from outside the group are not allowed.

• An undefined symbol that is referenced only from one or more sections that are part of a particular group, and that is contained in a symbol table section that is not part of the group, is not removed when the group members are discarded. In other words, the undefined symbol is not removed even if no references to that symbol remain.

• There may not be non-symbol references to the sections comprising a group from outside the group, for example, use of a group member's section header index in an sh_link or sh_info member.

Special Sections

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

.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.

.fini_array

This section holds an array of function pointers that contributes to a single termination array for the executable or shared object containing the 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).

.init_array

This section holds an array of function pointers that contributes to a single initialization array for the executable or shared object containing the section.

.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.

.preinit_array

This section holds an array of function pointers that contributes to a single pre-initialization array for the executable or shared object containing the section.

.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.

.symtab_shndx

This section holds the special symbol table section index array, as described above. The section's attributes will include the SHF_ALLOC bit if the associated symbol table section does; otherwise that bit will be 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. Implementations need not support thread-local storage.

.tdata

This section holds initialized thread-local data that contributes to the program's memory image. A copy of its contents is instantiated by the system for each new execution flow. Implementations need not support thread-local storage.

.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.