Dynamic section

If an object file participates in dynamic linking, its program header table will have an element of type PT_DYNAMIC. This segment contains the .dynamic section. A special symbol, _DYNAMIC, labels the section, which contains an array of the following structures.

   typedef struct {
   	Elf32_Sword	d_tag;
   	union {
   		Elf32_Word	d_val;
   		Elf32_Addr	d_ptr;
   	} d_un;
   } Elf32_Dyn;
   

   extern Elf32_Dyn	_DYNAMIC[];

Figure 8-15 Dynamic Structure

For each object with this type, d_tag controls the interpretation of d_un.

d_val

These Elf32_Word objects represent integer values with various interpretations.

d_ptr

These Elf32_Addr objects represent program virtual addresses. As mentioned previously, a file's virtual addresses might not match the memory virtual addresses during execution. When interpreting addresses contained in the dynamic structure, the dynamic linker computes actual addresses, based on the original file value and the memory base address. For consistency, files do not contain relocation entries to correct addresses in the dynamic structure.

Table 8-18 Dynamic array tags, d_tag

 --------------------------------------------------------------------
 Name          Value        d_un          Executable    Shared object
 --------------------------------------------------------------------
 DT_NULL       0            ignored       mandatory     mandatory
 DT_NEEDED     1            d_val         optional      optional
 DT_PLTRELSZ   2            d_val         optional      optional
 DT_PLTGOT     3            d_ptr         optional      optional
 DT_HASH       4            d_ptr         mandatory     mandatory
 DT_STRTAB     5            d_ptr         mandatory     mandatory
 DT_SYMTAB     6            d_ptr         mandatory     mandatory
 DT_RELA       7            d_ptr         mandatory     optional
 DT_RELASZ     8            d_val         mandatory     optional
 DT_RELAENT    9            d_val         mandatory     optional
 DT_STRSZ      10           d_val         mandatory     mandatory
 DT_SYMENT     11           d_val         mandatory     mandatory
 DT_INIT       12           d_ptr         optional      optional
 DT_FINI       13           d_ptr         optional      optional
 DT_SONAME     14           d_val         ignored       optional
 DT_RPATH      15           d_val         optional      ignored
 DT_SYMBOLIC   16           ignored       ignored       optional
 DT_REL        17           d_ptr         mandatory     optional
 DT_RELSZ      18           d_val         mandatory     optional
 DT_RELENT     19           d_val         mandatory     optional
 DT_PLTREL     20           d_val         optional      optional
 DT_DEBUG      21           d_ptr         optional      ignored
 DT_TEXTREL    22           ignored       optional      optional
 DT_JMPREL     23           d_ptr         optional      optional
 DT_LOPROC     0x70000000   unspecified   unspecified   unspecified
 DT_HIPROC     0x7fffffff   unspecified   unspecified   unspecified

DT_NULL

An entry with a DT_NULL tag marks the end of the _DYNAMIC array.

DT_NEEDED

This element holds the string table offset of a null-terminated string, giving the name of a needed library. The offset is an index into the table recorded in the DT_STRTAB entry. See ``Shared object dependencies'' for more information about these names. The dynamic array may contain multiple entries with this type. These entries' relative order is significant, though their relation to entries of other types is not.

DT_PLTRELSZ

This element holds the total size, in bytes, of the relocation entries associated with the procedure linkage table. If an entry of type DT_JMPREL is present, a DT_PLTRELSZ must accompany it.

DT_PLTGOT

This element holds an address associated with the procedure linkage table and/or the global offset table. Semantics are processor specific.

DT_HASH

This element holds the address of the symbol hash table, described in ``Hash table''.

DT_STRTAB

This element holds the address of the string table, described in the first part of this chapter. Symbol names, library names, and other strings reside in this table.

DT_SYMTAB

This element holds the address of the symbol table, described in the first part of this chapter, with Elf32_Sym entries for the 32-bit class of files.

DT_RELA

This element holds the address of a relocation table, described in the first part of this chapter. Entries in the table have explicit addends, such as Elf32_Rela for the 32-bit file class. An object file may have multiple relocation sections. When building the relocation table for an executable or shared object file, the link editor catenates those sections to form a single table. Although the sections remain independent in the object file, the dynamic linker sees a single table. When the dynamic linker creates the process image for an executable file or adds a shared object to the process image, it reads the relocation table and performs the associated actions. If this element is present, the dynamic structure must also have DT_RELASZ and DT_RELAENT elements. When relocation is ``mandatory'' for a file, either DT_RELA or DT_REL may occur (both are permitted but not required).

DT_RELASZ

This element holds the total size, in bytes, of the DT_RELA relocation table.

DT_RELAENT

This element holds the size, in bytes, of the DT_RELA relocation entry.

DT_STRSZ

This element holds the size, in bytes, of the string table.

DT_SYMENT

This element holds the size, in bytes, of a symbol table entry.

DT_INIT

This element holds the address of the initialization function. See ``Initialization and termination functions'' for more information.

DT_FINI

This element holds the address of the termination function. See ``Initialization and termination functions'' for more information.

DT_SONAME

This element holds the string table offset of a null-terminated string, giving the name of the shared object. The offset is an index into the table recorded in the DT_STRTAB entry.

---

NOTE: See ``Shared object dependencies'' for more information about these names.

---

DT_RPATH

This element holds the string table offset of a null-terminated search library search path string. The offset is an index into the table recorded in the DT_STRTAB entry.

DT_SYMBOLIC

This element's presence in a shared object library alters the dynamic linker's symbol resolution algorithm for references within the library. Instead of starting a symbol search with the executable file, the dynamic linker starts from the shared object itself. If the shared object fails to supply the referenced symbol, the dynamic linker then searches the executable file and other shared objects as usual.

DT_REL

This element is similar to DT_RELA, except its table has implicit addends, such as Elf32_Rel for the 32-bit file class. If this element is present, the dynamic structure must also have DT_RELSZ and DT_RELENT elements.

DT_RELSZ

This element holds the total size, in bytes, of the DT_REL relocation table.

DT_RELENT

This element holds the size, in bytes, of the DT_REL relocation entry.

DT_PLTREL

This member specifies the type of relocation entry to which the procedure linkage table refers. The d_val member holds DT_REL or DT_RELA, as appropriate. All relocations in a procedure linkage table must use the same relocation.

DT_DEBUG

This member is used for debugging.

DT_TEXTREL

This member's absence signifies that no relocation entry should cause a modification to a non-writable segment, as specified by the segment permissions in the program header table. If this member is present, one or more relocation entries might request modifications to a non-writable segment, and the dynamic linker can prepare accordingly.

DT_JMPREL

If present, this entry's d_ptr member holds the address of relocation entries associated solely with the procedure linkage table. Separating these relocation entries lets the dynamic linker ignore them during process initialization, if lazy binding is enabled. If this entry is present, the related entries of types DT_PLTRELSZ and DT_PLTREL must also be present.

DT_LOPROC through DT_HIPROC

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