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

A symbol table entry has the following format (defined in sys/elf.h):

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;

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 nonzero, it represents a string table index that gives the symbol name. Otherwise, the symbol table entry has no name.

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Note -

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

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st_value

This member gives the value of the associated symbol. Depending on the context, this may be an absolute value, an address, and so forth. See "Symbol Values".

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 (defined in sys/elf.h):

#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. See Table 7-10.

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

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 - STB_HIPROC

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

Global and weak symbols differ in two major ways:

• 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 (that is, a symbol with the st_index field holding SHN_COMMON), the appearance of a weak symbol with the same name does not cause an error. The link-editor uses the common definition and ignores the weak one.

• When the link-editor searches archive libraries (see "Archive Processing") it extracts archive members that contain definitions of undefined or tentative, global symbols. The member's definition may be either a global or a weak symbol.

  The link-editor, by default, does not extract archive members to resolve undefined weak symbols. Unresolved weak symbols have a zero value. The use of -z weakextract overrides this default behavior, and allows weak references to cause the extraction of archive members.

In each symbol table, all symbols with STB_LOCAL binding precede the weak and global symbols. As "Sections" describes, 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.

STT_NOTYPE

The symbol 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. Symbol index 1 of the SHT_SYMTAB is an STT_FILE symbol representing the file itself. Conventionally, this is followed by the files STT_SECTION symbols, and any global symbols that have been reduced to locals (see "Reducing Symbol Scope", and Chapter 5, Versioning for more details).

STT_LOPROC - 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. That is, 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 bound to the actual definition.

As mentioned above, the symbol table entry for index 0 (STN_UNDEF) is reserved; it holds the following:

Symbol Values

Symbol table entries for different object file types have slightly different interpretations for the st_value member.

• In relocatable files, st_value holds alignment constraints for a symbol whose section index is SHN_COMMON.

• In relocatable files, st_value holds a section offset for a defined symbol. That is, st_value is an offset from the beginning of the section that st_shndx identifies.

• In executable and shared object files, st_value holds a virtual address. To make these files' symbols more useful for the runtime linker, the section offset (file interpretation) gives way to a virtual address (memory interpretation) for which the section number is irrelevant.

Although the symbol table values have similar meanings for different object files, the data allow efficient access by the appropriate programs.