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

Name Value

STN_UNDEF 0

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;
   
   typedef struct {
   	Elf64_Word	st_name;
   	unsigned char	st_info;
   	unsigned char	st_other;
   	Elf64_Half	st_shndx;
   	Elf64_Addr	st_value;
   	Elf64_Xword	st_size;
   } Elf64_Sym;

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, an address and so on; more details are given below.

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 for both 32 and 64-bit objects.

   #define ELF32_ST_BIND(i)   ((i)>>4)
   #define ELF32_ST_TYPE(i)   ((i)&0xf)
   #define ELF32_ST_INFO(b,t) (((b)<<4)+((t)&0xf))
   
   #define ELF64_ST_BIND(i)   ((i)>>4)
   #define ELF64_ST_TYPE(i)   ((i)&0xf)
   #define ELF64_ST_INFO(b,t) (((b)<<4)+((t)&0xf))

st_other

This member currently specifies a symbol's visibility. A list of the values and meanings appears below. The following code shows how to manipulate the values for both 32 and 64-bit objects. Other bits contain 0 and have no defined meaning.

   #define ELF32_ST_VISIBILITY(o) ((o)&0x3)
   #define ELF32_ST_OTHER(v)      ((v)&0x3)
   
   #define ELF64_ST_VISIBILITY(o) ((o)&0x3)
   #define ELF64_ST_OTHER(v)      ((v)&0x3)

st_shndx

Every symbol table entry is defined in relation to some section. This member holds the relevant section header table index. As the sh_link and sh_info interpretation table and the related text describe, some section indexes indicate special meanings.

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

Name Value

STB_LOCAL 0

STB_GLOBAL 1

STB_WEAK 2

STB_LOOS 10

STB_HIOS 12

STB_LOPROC 13

STB_HIPROC 15

Name	Value
`STB_LOCAL`	0
`STB_GLOBAL`	1
`STB_WEAK`	2
`STB_LOOS`	10
`STB_HIOS`	12
`STB_LOPROC`	13
`STB_HIPROC`	15

Symbol binding

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_LOOS through STB_HIOS: Values in this inclusive range are reserved for operating system-specific semantics.
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.

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.

NOTE: The behavior of weak symbols in areas not specified by this document is implementation defined. Weak symbols are intended primarily for use in system software. Their use in application programs is discouraged.

In each symbol table, all symbols with STB_LOCAL binding precede the weak and global symbols. As described in ``Sections'', 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.

Name Value

STT_NOTYPE 0

STT_OBJECT 1

STT_FUNC 2

STT_SECTION 3

STT_FILE 4

STT_COMMON 5

STT_LOOS 10

STT_HIOS 12

STT_LOPROC 13

STT_HIPROC 15

Name	Value
`STT_NOTYPE`	0
`STT_OBJECT`	1
`STT_FUNC`	2
`STT_SECTION`	3
`STT_FILE`	4
`STT_COMMON`	5
`STT_LOOS`	10
`STT_HIOS`	12
`STT_LOPROC`	13
`STT_HIPROC`	15

Symbol types

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 on.
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_COMMON: The symbol labels an uninitialized common block. This is described in greater detail below.
STT_LOOS through STT_HIOS: Values in this inclusive range are reserved for operating system-specific semantics.
STT_LOPROC through STT_HIPROC: Values in this inclusive range are reserved for processor-specific semantics. If meanings are specified, the processor supplement explains them.

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.

Symbols with type STT_COMMON label uninitialized common blocks. In relocatable objects, these symbols are not allocated and must have the special section index SHN_COMMON. In shared objects and executables these symbols must be allocated to some section in the defining object.

In relocatable objects, symbols with type STT_COMMON are treated just as other symbols with index SHN_COMMON. If the link-editor allocates space for the SHN_COMMON symbol in an output section of the object it is producing, it must preserve the type of the output symbol as STT_COMMON.

When the dynamic linker encounters a reference to a symbol that resolves to a definition of type STT_COMMON, it may (but is not required to) change its symbol resolution rules as follows: instead of binding the reference to the first symbol found with the given name, the dynamic linker searches for the first symbol with that name with type other than STT_COMMON. If no such symbol is found, it looks for the STT_COMMON definition of that name that has the largest size.

A symbol's visibility, although it may be specified in a relocatable object, defines how that symbol may be accessed once it has become part of an executable or shared object.

Name Value

STV_DEFAULT 0

STV_INTERNAL 1

STV_HIDDEN 2

STV_PROTECTED 3

Name	Value
`STV_DEFAULT`	0
`STV_INTERNAL`	1
`STV_HIDDEN`	2
`STV_PROTECTED`	3

Symbol visibility

STV_DEFAULT

The visibility of symbols with the STV_DEFAULT attribute is as specified by the symbol's binding type. That is, global and weak symbols are visible outside of their defining component (executable file or shared object). Local symbols are hidden, as described below. Global and weak symbols are also preemptable, that is, they may by preempted by definitions of the same name in another component.

NOTE: An implementation may restrict the set of global and weak symbols that are externally visible.

STV_PROTECTED

A symbol defined in the current component is protected if it is visible in other components but not preemptable, meaning that any reference to such a symbol from within the defining component must be resolved to the definition in that component, even if there is a definition in another component that would preempt by the default rules. A symbol with STB_LOCAL binding may not have STV_PROTECTED visibility.

STV_HIDDEN

A symbol defined in the current component is hidden if its name is not visible to other components. Such a symbol is necessarily protected. This attribute may be used to control the external interface of a component. Note that an object named by such a symbol may still be referenced from another component if its address is passed outside.

A hidden symbol contained in a relocatable object must be either removed or converted to STB_LOCAL binding by the link-editor when the relocatable object is included in an executable file or shared object.

STV_INTERNAL

The meaning of this visibility attribute may be defined by processor supplements to further constrain hidden symbols. A processor supplement's definition should be such that generic tools can safely treat internal symbols as hidden.

An internal symbol contained in a relocatable object must be either removed or converted to STB_LOCAL binding by the link-editor when the relocatable object is included in an executable file or shared object.

None of the visibility attributes affects resolution of symbols within an executable or shared object during link-editing - such resolution is controlled by the binding type. Once the link-editor has chosen its resolution, these attributes impose two requirements, both based on the fact that references in the code being linked may have been optimized to take advantage of the attributes.

First, all of the non-default visibility attributes, when applied to a symbol reference, imply that a definition to satisfy that reference must be provided within the current executable or shared object. If such a symbol reference has no definition within the component being linked, then the reference must have STB_WEAK binding and is resolved to zero.
Second, if any reference to or definition of a name is a symbol with a non-default visibility attribute, the visibility attribute must be propagated to the resolving symbol in the linked object. If different visibility attributes are specified for distinct references to or definitions of a symbol, the most constraining visibility attribute must be propagated to the resolving symbol in the linked object. The attributes, ordered from least to most constraining, are: STV_PROTECTED, STV_HIDDEN and STV_INTERNAL .

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. Symbols with section index SHN_COMMON can appear only in relocatable objects.
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:

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

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

Symbol table entry: Index 0