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 (defined in sys/link.h):

typedef struct {
        Elf32_Sword d_tag;
        union {
                Elf32_Word      d_val;
                Elf32_Addr      d_ptr;
                Elf32_Off       d_off;
        } d_un;
} Elf32_Dyn;

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 runtime 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.

The following table summarizes the tag requirements for executable and shared object files. If a tag is marked mandatory, then the dynamic linking array must have an entry of that type. Likewise, optional means an entry for the tag may appear but is not required.

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 dependency. 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.

DT_HASH

This element points to the symbol hash table, described in "Hash Table". This hash table refers to the symbol table indicated by the DT_SYMTAB element.

DT_STRTAB

This element holds the address of the string table, described in the first part of this chapter. Symbol names, dependency names, and other strings required by the runtime linker 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 runtime linker sees a single table. When the runtime 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, discussed in "Initialization and Termination Functions" later.

DT_FINI

This element holds the address of the termination function, discussed in "Initialization and Termination Functions" later.

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. 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, discussed in "Shared Objects with Dependencies". 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 runtime linker's symbol resolution algorithm for references within the library. Instead of starting a symbol search with the executable file, the runtime linker starts from the shared object itself. If the shared object fails to supply the referenced symbol, the runtime 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 runtime linker can prepare accordingly.

DT_FLAGS_1

If present, this entry's d_val member holds various state flags. See Table 7-35.

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 runtime 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_VERDEF

Holds the address of the version definition table, described in the first part of this chapter, with Elf32_Verdef entries for the 32-bit class of files. See section "Version Definition Section"for more information. Elements within these entries contain indexes into the table recorded in the DT_STRTAB entry.

DT_VERDEFNUM

This element specifies the number of entries in the version definition table.

DT_VERNEED

Holds the address of the version dependency table, described in the first part of this chapter, with Elf32_Verneed entries for the 32-bit class of files. See section "Version Dependency Section" for more information. Elements within these entries contain indexes into the table recorded in the DT_STRTAB entry.

DT_VERNEEDNUM

This element specifies the number of entries in the version dependency table.

DT_AUXILIARY

Holds the string table offset of a null-terminated string that names an object. The offset is an index into the table recorded in the DT_STRTAB entry. Symbols in the auxiliary object will be used in preference to the symbols within this object.

DT_FILTER

Holds the string table offset of a null-terminated string that names an object. The offset is an index into the table recorded in the DT_STRTAB entry. The symbol table of this object acts as a filter for the symbol table of the named object.

DT_LOPROC - DT_HIPROC

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

The following dynamic state flags are presently available:

DF_1_NOW

When the object is loaded all relocation processing is completed, see "When Relocations are Performed". This state is recorded in the object using the link-editors' -z now option.

DF_1_GROUP

Indicates that the object is a member of a group, see "Symbol Lookup". This state is recorded in the object using the link-editors' -B group option.

DF_1_NODELETE

Indicates that the object can not be deleted from a process. Thus if the object is loaded in a process, either directly or as a dependency, with dlopen(3X), it can not be unloaded with dlclose(3X). This state is recorded in the object using the link-editors' -z nodelete option.

DF_1_LOADFLTR

This state is only meaningful for filters (See "Shared Objects as Filters"). When the filter is loaded all associated filtees are immediately processed, see "Filtee Processing". This state is recorded in the object using the link-editors' -z loadfltr option.

DF_1_INITFIRST

When the object is loaded its initialization section is run before any other objects loaded with it, see "Initialization and Termination Routines". This specialized state is intended for libthread.so.1. This state is recorded in the object using the link-editors' -z initfirst option.

DF_1_NOOPEN

Indicates that the object can not be added to a running process with dlopen(3X). This state is recorded in the object using the link-editors' -z nodlopen option.

Except for the DT_NULL element at the end of the dynamic array and the relative order of DT_NEEDED elements, entries may appear in any order. Tag values not appearing in the table are reserved.