---------------------------------------------------------------------------- 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 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. A section header has the following structure. 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; Figure 8-4 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. ----------------------------------------------------------------------- NOTE: See ``String table'' for more information. ----------------------------------------------------------------------- sh_type This member categorizes the section's contents and semantics. Section types and their descriptions appear in Table 8-4, ``Section types, sh_type''. sh_flags Sections support 1-bit flags that describe miscellaneous attributes. Flag definitions appear in Table 8-4, ``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 Table 8-4, ``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. Table 8-4, ``Section types, sh_type'' describes the values. sh_info This member holds extra information, whose interpretation depends on the section type. Table 8-4, ``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. A section header's sh_type member specifies the section's semantics. Table 8-4 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. ----------------------------------------------------------------------- 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