Linker and Libraries Guide

Symbol Table Section

An object file's symbol table holds information needed to locate and relocate a program's symbolic definitions and symbolic 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. See Table 7–21.

A symbol table entry has the following format. See 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;

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;

st_name

An index into the object file's symbol string table, which holds the character representations of the symbol names. If the value is nonzero, the value represents a string table index that gives the symbol name. Otherwise, the symbol table entry has no name.

st_value

The value of the associated symbol. The value can be an absolute value or an address, depending on the context. See Symbol Values.

st_size

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

st_info

The symbol's type and binding attributes. A list of the values and meanings appears in Table 7–18. The following code shows how to manipulate the values. See sys/elf.h.

#define ELF32_ST_BIND(info)          ((info) >> 4)
#define ELF32_ST_TYPE(info)          ((info) & 0xf)
#define ELF32_ST_INFO(bind, type)    (((bind)<<4)+((type)&0xf))

#define ELF64_ST_BIND(info)          ((info) >> 4)
#define ELF64_ST_TYPE(info)          ((info) & 0xf)
#define ELF64_ST_INFO(bind, type)    (((bind)<<4)+((type)&0xf))

st_other

A symbol's visibility. A list of the values and meanings appears in Table 7–20. The following code shows how to manipulate the values for both 32–bit objects and 64–bit objects. Other bits are set to zero, and have no defined meaning.

#define ELF32_ST_VISIBILITY(o)       ((o)&0x3)
#define ELF64_ST_VISIBILITY(o)       ((o)&0x3)

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 7–4.

If this member contains SHN_XINDEX, then the actual section header index is too large to fit in this field. The actual value is contained in the associated section of type SHT_SYMTAB_SHNDX.

A symbol's binding, determined from its st_info field, determines the linkage visibility and behavior.

Table 7–18 ELF Symbol Binding, ELF32_ST_BIND and ELF64_ST_BIND


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

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

Global symbols and weak symbols differ in two major ways.

When the link-editor combines several relocatable object files, multiple definitions of STB_GLOBAL symbols with the same name are not allowed. However, if a defined global symbol exists, the appearance of a weak symbol with the same name does not cause an error. The link-editor honors the global definition and ignores the weak definitions.

Similarly, if a common symbol exists, 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 definition. A common symbol has the st_shndx field holding SHN_COMMON. See Symbol Resolution.
When the link-editor searches archive libraries, archive members that contain definitions of undefined or tentative global symbols are extracted. The member's definition can 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. This options enables weak references to cause the extraction of archive members.

Note –

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 symbols 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, as determined from its st_info field, provides a general classification for the associated entity.

Table 7–19 ELF Symbol Types, ELF32_ST_TYPE and ELF64_ST_TYPE


Name	Value
`STT_NOTYPE`	`0`
`STT_OBJECT`	`1`
`STT_FUNC`	`2`
`STT_SECTION`	`3`
`STT_FILE`	`4`
`STT_COMMON`	`5`
`STT_TLS`	`6`
`STT_LOOS`	`10`
`STT_HIOS`	`12`
`STT_LOPROC`	`13`
`STT_SPARC_REGISTER`	`13`
`STT_HIPROC`	`15`

STT_NOTYPE

The symbol type is not specified.

STT_OBJECT

This symbol is associated with a data object, such as a variable, an array, and so forth.

STT_FUNC

This symbol is associated with a function or other executable code.

STT_SECTION

This 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 that is associated with the object file. A file symbol has STB_LOCAL binding and a section index of SHN_ABS. This symbol, if present, precedes the other STB_LOCAL symbols for the file.

Symbol index 1 of the SHT_SYMTAB is an STT_FILE symbol representing the object file. Conventionally, this symbol is followed by the files STT_SECTION symbols. These section symbols are then followed by any global symbols that have been reduced to locals.

STT_COMMON

This symbol labels an uninitialized common block. This symbol is treated exactly the same as STT_OBJECT.

STT_TLS

The symbol specifies a thread-local storage entity. When defined, this symbol gives the assigned offset for the symbol, not the actual address.

Thread-local storage relocations can only reference symbols with type STT_TLS. A reference to a symbol of type STT_TLS from an allocatable section, can only be achieved by using special thread-local storage relocations. See Chapter 8, Thread-Local Storage for details. A reference to a symbol of type STT_TLS from a non-allocatable section does not have this restriction.

STT_LOOS - STT_HIOS

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

STT_LOPROC - STT_HIPROC

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

A symbol's visibility is determined from its st_other field. This visibility can be specified in a relocatable object. This visibility defines how that symbol can be accessed once the symbol has become part of an executable or shared object.

Table 7–20 ELF Symbol Visibility


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

STV_DEFAULT

The visibility of symbols with the STV_DEFAULT attribute is as specified by the symbol's binding type. Global symbols and weak symbols are visible outside of their defining component, the executable file or shared object. Local symbols are hidden. Global symbols and weak symbols can also be preempted. These symbols can by interposed by definitions of the same name in another component.

STV_PROTECTED

A symbol that is defined in the current component is protected if the symbol is visible in other components, but cannot be preempted. Any reference to such a symbol from within the defining component must be resolved to the definition in that component. This resolution must occur, even if a symbol definition exists in another component that would interpose by the default rules. A symbol with STB_LOCAL binding will not have STV_PROTECTED visibility.

STV_HIDDEN

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

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

STV_INTERNAL

This visibility attribute is currently reserved.

The visibility attributes do not affect the 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 requirements are based on the fact that references in the code being linked might have been optimized to take advantage of the attributes.

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 object being linked. If this type of symbol reference has no definition within the object being linked, then the reference must have STB_WEAK binding. In this case, the reference is resolved to zero.
If any reference to a name, or definition of a name is a symbol with a non-default visibility attribute, the visibility attribute is propagated to the resolving symbol in the object being linked. If different visibility attributes are specified for distinct instances of a symbol, the most constraining visibility attribute is propagated to the resolving symbol in the object being linked. 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, the symbols's 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. References to the symbol continue to point to the same location in the program. Some special section index values give other semantics.

SHN_ABS: This symbol has an absolute value that does not change because of relocation.
SHN_COMMON, and SHN_AMD64_LCOMMON: This 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 allocates 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 indicates that the symbol is undefined. When the link-editor combines this object file with another object that defines the indicated symbol, this file's references to the symbol is bound to the definition.

As mentioned previously, the symbol table entry for index 0 (STN_UNDEF) is reserved. This entry holds the values listed in the following table.

Table 7–21 ELF 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

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. 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.

Symbol Table Layout and Conventions

The symbols in a symbol table are written in the following order.

Index 0 in any symbol table is used to represent undefined symbols. This first entry in a symbol table is always completely zeroed. The symbol type is therefore STT_NOTYPE.
If the symbol table contains any local symbols, the second entry of the symbol table is an STT_FILE symbol giving the name of the file.
Section symbols of type STT_SECTION.
Register symbols of type STT_REGISTER.
Global symbols that have been reduced to local scope.
For each input file that supplies local symbols, a STT_FILE symbol giving the name of the input file, followed by the symbols in question.
The global symbols immediately follow the local symbols in the symbol table. The first global symbol is identified by the symbol table sh_info value. Local and global symbols are always kept separate in this manner, and cannot be mixed together.

Two symbol tables are of special interest in the Solaris OS.

.symtab (SHT_SYMTAB)

This symbol table contains every symbol that describes the associated ELF file. This symbol table is typically non-allocable, and is therefore not available in the memory image of the process.

Global symbols can be eliminated from the .symtab by using a mapfile together with the ELIMINATE keyword. See Defining Additional Symbols with a mapfile. Local symbols can also be eliminated by using the link-editor -z redlocsym option.

.dynsym (SHT_DYNSYM)

This table contains a subset of the symbols from the .symtab table that are needed to support dynamic linking. This symbol table is allocable, and is therefore available in the memory image of the process.

The .dynsym table begins with the standard NULL symbol, followed by the files global symbols. STT_FILE symbols are typically not present in this symbol table. STT_SECTION symbols might be present if required by relocation entries.

Register Symbols

The SPARC architecture supports register symbols, which are symbols that initialize a global register. A symbol table entry for a register symbol contains the entries that are listed in the following table.

Table 7–22 SPARC: ELF Symbol Table Entry: Register Symbol


Field	Meaning
`st_name`	Index into the string table for the name of the symbol, or the value `0` for a scratch register.
`st_value`	Register number. See the ABI manual for integer register assignments.
`st_size`	Unused (`0`).
`st_info`	Bind is typically `STB_GLOBAL`, type must be `STT_SPARC_REGISTER`.
`st_other`	Unused (`0`).
`st_shndx`	`SHN_ABS` if this object initializes this register symbol,`SHN_UNDEF` otherwise.

The register values that are defined for SPARC are listed in the following table.

Table 7–23 SPARC: ELF Register Numbers


Name	Value	Meaning
`STO_SPARC_REGISTER_G2`	`0x2`	`%g2`
`STO_SPARC_REGISTER_G3`	`0x3`	`%g3`

Absence of an entry for a particular global register means that the particular global register is not used at all by the object.