Symbol table

Developer's Topics
Chapter 8, ELF object files

Symbol table

An object file's symbol table holds information needed to locate and relocate a program's symbolic definitions and references. A symbol table index is a subscript into this array. Index 0 both designates the first entry in the table and serves as the undefined symbol index.

 ---------------------------------------------------------------------
 Name         Value
 ---------------------------------------------------------------------
 STN_UNDEF    0

A symbol table entry has the following format.

   typedef struct {
   	Elf32_Word	st_name;
   	Elf32_Addr	st_value;
   	Elf32_Word	st_size;
   	unsigned char	st_info;
   	unsigned char	st_other;
   	Elf32_Half	st_shndx;
   } Elf32_Sym;

Figure 8-6 Symbol table entry

st_name

This member holds an index into the object file's symbol string table, which holds the character representations of the symbol names. If the value is non-zero, it represents a string table index that gives the symbol name. Otherwise, the symbol table entry has no name.

NOTE: External C symbols have the same names in C and object files' symbol tables.

st_value

This member gives the value of the associated symbol. Depending on the context, this may be an absolute value, or an address.

st_size

Many symbols have associated sizes. For example, a data object's size is the number of bytes contained in the object. This member holds 0 if the symbol has no size or an unknown size.

st_info

This member specifies the symbol's type and binding attributes. A list of the values and meanings appears below. The following code shows how to manipulate the values.

#define ELF32_ST_BIND(i)   ((i)>>4)
#define ELF32_ST_TYPE(i)   ((i)&0xf)
#define ELF32_ST_INFO(b,t) (((b)<<4)+((t)&0xf))

st_other

This member currently holds 0 and has no defined meaning.

st_shndx

Every symbol table entry is defined in relation to some section. This member holds the relevant section header table index. Some section indexes indicate special meanings.

A symbol's binding determines the linkage visibility and behavior.

Table 8-10 Symbol binding, ELF32_ST_BIND

 ------------------------------------------------------------------
 Name                      Value
 ------------------------------------------------------------------
 STB_LOCAL                 0
 STB_GLOBAL                1
 STB_WEAK                  2
 STB_LOPROC                13
 STB_HIPROC                15

STB_LOCAL: Local symbols are not visible outside the object file containing their definition. Local symbols of the same name may exist in multiple files without interfering with each other.
STB_GLOBAL: Global symbols are visible to all object files being combined. One file's definition of a global symbol will satisfy another file's undefined reference to the same global symbol.
STB_WEAK: Weak symbols resemble global symbols, but their definitions have lower precedence.
STB_LOPROC through STB_HIPROC: Values in this inclusive range are reserved for processor-specific semantics.

Global and weak symbols differ in two major ways, as described in ``Multiply defined symbols'' in the Programming Tools Guide.

When the link editor combines several relocatable object files, it does not allow multiple definitions of STB_GLOBAL symbols with the same name. On the other hand, if a defined global symbol exists, the appearance of a weak symbol with the same name will not cause an error. The link editor honors the global definition and ignores the weak ones. Similarly, if a common symbol exists (for example, a symbol whose st_shndx field holds SHN_COMMON), the appearance of a weak symbol with the same name will not cause an error. The link editor honors the common definition and ignores the weak one.
When the link editor searches archive libraries, it extracts archive members that contain definitions of undefined global symbols. The member's definition may be either a global or a weak symbol. The link editor does not extract archive members to resolve undefined weak symbols. Unresolved weak symbols have a zero value.

In each symbol table, all symbols with STB_LOCAL binding precede the weak and global symbols. As described in ``Section header'', a symbol table section's sh_info section header member holds the symbol table index for the first non-local symbol.

A symbol's type provides a general classification for the associated entity.

Table 8-11 Symbol types, ELF32_ST_TYPE

 ------------------------------------------------------------------
 Name                      Value
 ------------------------------------------------------------------
 STT_NOTYPE                0
 STT_OBJECT                1
 STT_FUNC                  2
 STT_SECTION               3
 STT_FILE                  4
 STT_LOPROC                13
 STT_HIPROC                15

STT_NOTYPE: The symbol's type is not specified.
STT_OBJECT: The symbol is associated with a data object, such as a variable, an array, and so forth.
STT_FUNC: The symbol is associated with a function or other executable code.
STT_SECTION: The symbol is associated with a section. Symbol table entries of this type exist primarily for relocation and normally have STB_LOCAL binding.
STT_FILE: Conventionally, the symbol's name gives the name of the source file associated with the object file. A file symbol has STB_LOCAL binding, its section index is SHN_ABS, and it precedes the other STB_LOCAL symbols for the file, if it is present.
STT_LOPROC through STT_HIPROC: Values in this inclusive range are reserved for processor-specific semantics.

Function symbols (those with type STT_FUNC) in shared object files have special significance. When another object file references a function from a shared object, the link editor automatically creates a procedure linkage table entry for the referenced symbol. Shared object symbols with types other than STT_FUNC will not be referenced automatically through the procedure linkage table.

If a symbol's value refers to a specific location within a section, its section index member, st_shndx, holds an index into the section header table. As the section moves during relocation, the symbol's value changes as well, and references to the symbol continue to ``point'' to the same location in the program. Some special section index values give other semantics.

SHN_ABS: The symbol has an absolute value that will not change because of relocation.
SHN_COMMON: The symbol labels a common block that has not yet been allocated. The symbol's value gives alignment constraints, similar to a section's sh_addralign member. The link editor will allocate the storage for the symbol at an address that is a multiple of st_value. The symbol's size tells how many bytes are required.
SHN_UNDEF: This section table index means the symbol is undefined. When the link editor combines this object file with another that defines the indicated symbol, this file's references to the symbol will be linked to the actual definition.

The symbol table entry for index 0 (STN_UNDEF) is reserved and holds the following values:

Table 8-12 Symbol table entry: index 0

 ------------------------------------------------------------------
 Name         Value           Note
 ------------------------------------------------------------------
 st_name      0               No name
 st_value     0               Zero value
 st_size      0               No size
 st_info      0               No type, local binding
 st_other     0
 st_shndx     SHN_UNDEF       No section