Sections

An object file's section header table helps you locate all 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 does not have sections for these special indexes.

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. That is, 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. The contents of the initial entry are specified later in this section.

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SHN_LORESERVE

This value specifies the lower boundary of the range of reserved indexes.

SHN_LOPROC - SHN_HIPROC

Values in this inclusive range are reserved for processor-specific semantics.

SHN_BEFORE, SHN_AFTER

These values provide for initial and final section ordering in conjunction with the SHF_ORDERED section flag (see Table 7-14).

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. These symbols are sometimes referred to as tentative.

SHN_HIRESERVE

This value specifies the upper boundary 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. That is, 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 can exist that do not have a section.

• Each section occupies one contiguous (possibly empty) sequence of bytes within a file.

• Sections in a file can not overlap. No byte in a file resides in more than one section.

• An object file can 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.

A section header has the following structure (defined in sys/elf.h):

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") giving the location of a null-terminated string. Section names and their descriptions are in Table 7-16.

sh_type

This member categorizes the section's contents and semantics. Section types and their descriptions are in Table 7-12.

sh_flags

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

This member gives the byte offset from the beginning of the file to the first byte in the section. 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 can have a nonzero 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 7-15 describes the values.

sh_info

This member holds 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:

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

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

SHT_SYMTAB, SHT_DYNSYM

These sections hold 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

These sections hold a string table. An object file can have multiple string table sections. See "String Table" for details.

SHT_RELA

This section holds 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

This section holds 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

This section holds information for dynamic linking. Currently, an object file can have only one dynamic section, but this restriction might be relaxed in the future. See "Dynamic Section" for details.

SHT_NOTE

This section holds information that marks the file in some way. See "Note Section" 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

This section holds 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

This section type is reserved but has unspecified semantics. Programs that contain a section of this type do not conform to the ABI.

SHT_SUNW_COMDAT

This section contains data to handle partially initialized symbols.

SHT_SUNW_move

This section contains data to handle partially initialized symbols.

SHT_SUNW_syminfo

This section contains a table which contains additional symbol information.

SHT_SUNW_verdef

The section contains definitions of fine-grained versions defined by this file.

SHT_SUNW_verneed

This section contains descriptions of fine-grained dependencies required for the execution of an image.

SHT_SUNW_versym

This section contains a table describing the relationship of symbols to the version definitions offered by the file.

SHT_LOPROC - SHT_HIPROC

Values in this inclusive range are reserved for processor-specific semantics.

SHT_LOUSER

This value specifies the lower boundary of the range of indexes reserved for application programs.

SHT_HIUSER

This value 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. This entry holds the following:

A section header's sh_flags member holds 1-bit flags that describe the section's attributes:

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.

SHF_WRITE

This section contains data that should be writable during process execution.

SHF_ALLOC

This 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

This section contains executable machine instructions.

SHF_ORDERED

This section requires ordering in relation to other sections of the same type. Ordered sections are combined within the section pointed to by the sh_link entry. The sh_link entry of an ordered section can point to itself.

If the sh_info entry of the ordered section is a valid section within the same input file, the ordered section will be sorted based on the relative ordering within the output file of the section pointed to by the sh_info entry. The special sh_info values SHN_BEFORE and SHN_AFTER (see Table 7-11) imply that the sorted section is to precede or follow, respectively, all other sections in the set being ordered. Input file link-line order is preserved if multiple sections in an ordered set have one of these special values.

In the absence of the sh_info ordering information, sections from a single input file combined within one section of the output file will be contiguous and have the same relative ordering as they did in the input file. The contributions from multiple input files will appear in link-line order.

SHF_EXCLUDE

This section is excluded from input to the link-edit of an executable or shared object. This flag is ignored if the SHF_ALLOC flag is also set, or if relocations exist against the section.

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.