Section header

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 ``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.

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.

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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.

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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.

• 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.

   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;

Section header

sh_name

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.

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NOTE: See ``String table'' for more information.

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sh_type

This member categorizes the section's contents and semantics. Section types and their descriptions appear in ``Section types, sh_type''.

sh_flags

Sections support 1-bit flags that describe miscellaneous attributes. Flag definitions appear in ``Section types, sh_type''.

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 in ``Section types, sh_type'', 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. ``Section types, sh_type'' describes the values.

sh_info

This member holds extra information, whose interpretation depends on the section type. ``Section types, sh_type'' 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.

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_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.

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NOTE: See ``Symbol table'' for details.

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SHT_STRTAB

The section holds a string table. An object file may have multiple string table sections.

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NOTE: See ``String table'' for details.

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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.

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NOTE: See ``Relocation'' for details.

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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.

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NOTE: See ``Relocation'' for details.

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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.

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 ``Section attribute flags, sh_flags''; other values are reserved.

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.

sh_link and sh_info interpretation

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).
other	SHN_UNDEF	0