Symbol table

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In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier (a.k.a. symbol) in a program's source code is associated with information relating to its declaration or appearance in the source. Symbol table stores the information related about the symbol.

Background

A symbol table may only exist in memory during the translation process, or it may be embedded in the output of the translation, such as in an ABI object file for later exploitation. For example, it might be used during an interactive debugging session, or as a resource for formatting a diagnostic report during or after execution of a program.[1]

Description

The minimum information contained in a symbol table used by a translator includes the symbol's name, its relocatability attributes (absolute, relocatable, etc.), and its location or address. For relocatable symbols, some relocation information must be stored. Symbol tables for high-level programming languages store the symbol's type: string, integer, floating-point, etc., its size, and its dimensions and its bounds. Not all of this information is included in the output file, but may be provided for use in debugging. In many cases, the symbol's cross-reference information is stored with or linked to the symbol table. Most compilers print some or all of this information in symbol table and cross-reference listings at the end of translation.

Implementation

Numerous data structures are available for implementing tables. Trees, linear lists and self-organizing lists can all be used to implement a symbol table. The symbol table is accessed by most phases of a compiler, beginning with lexical analysis, and continuing through optimization.

A compiler may use one large symbol table for all symbols or use separated, hierarchical symbol tables for different scopes. For example, in a scoped language such as Algol or PL/I a symbol "p" can be declared separately in several procedures, perhaps with different attributes. The scope of each declaration is the section of the program in which references to "p" resolve to that declaration. Each declaration represents a unique identifier "p". The symbol table must have some means of differentiating references to the different of "p"s.

A common data structure used to implement symbol tables is the hash table. The time for searching in hash tables is independent of the number of elements stored in the table, so it is efficient for a large number of elements. It also simplifies[how?] the classification of literals in tabular format.

As the lexical analyser spends a great proportion of its time looking up the symbol table, this activity has a crucial effect on the overall speed of the compiler. A symbol table must be organised in such a way that entries can be found as quick as possible. Hash tables are used to organise a symbol table, where the keyword or identifier is 'hashed' to produce an array subscript. Collisions are inevitable in a hash table, and a common way of handling them is to store the synonym in the next available free space in the table.

Applications

An object file will contain a symbol table of the identifiers it contains that are externally visible. During the linking of different object files, a linker will identify and resolve[how?] these symbol references.

While reverse engineering an executable, many tools refer to the symbol table to check what addresses have been assigned to global variables and known functions. If the symbol table has been stripped or cleaned out before being converted into an executable, tools will find it harder to determine addresses or understand anything about the program.

Example

Consider the following program written in C:

// Declare an external function
extern double bar(double x);

// Define a public function
double foo(int count)
{
    double  sum = 0.0;

    // Sum all the values bar(1) to bar(count)
    for (int i = 1;  i <= count;  i++)
        sum += bar((double) i);
    return sum;
}

A C compiler that parses this code will contain at least the following symbol table entries:

Symbol name Type Scope
bar function, double extern
x double function parameter
foo function, double global
count int function parameter
sum double block local
i int for-loop statement

In addition, the symbol table will also contain entries generated by the compiler for intermediate expression values (e.g., the expression that casts the i loop variable into a double, and the return value of the call to function bar()), statement labels, and so forth.

Example: SysV ABI

An example of a symbol table can be found in the SysV Application Binary Interface (ABI) specification, which mandates how symbols are to be laid out in a binary file, so that different compilers, linkers and loaders can all consistently find and work with the symbols in a compiled object.

The SysV ABI is implemented in the GNU binutils' nm utility. This format uses a sorted memory address field, a "The symbol type" field, and a symbol identifier (called "Name").

One entry is a data symbol, denoted by the type "D". Many functions, including both user-defined functions and library functions are also present.[further explanation needed]

Example table
Address Type Name
00000020 a T_BIT
00000040 a F_BIT
00000080 a I_BIT
20000004 t irqvec
20000008 t fiqvec
2000000c t InitReset
20000018 T _main
20000024 t End
20000030 T AT91F_US3_CfgPIO_useB
2000005c t AT91F_PIO_CfgPeriph
200000b0 T main
20000120 T AT91F_DBGU_Printk
20000190 t AT91F_US_TxReady
200001c0 t AT91F_US_PutChar
200001f8 T AT91F_SpuriousHandler
20000214 T AT91F_DataAbort
20000230 T AT91F_FetchAbort
2000024c T AT91F_Undef
20000268 T AT91F_UndefHandler
20000284 T AT91F_LowLevelInit
200002e0 t AT91F_DBGU_CfgPIO
2000030c t AT91F_PIO_CfgPeriph
20000360 t AT91F_US_Configure
200003dc t AT91F_US_SetBaudrate
2000041c t AT91F_US_Baudrate
200004ec t AT91F_US_SetTimeguard
2000051c t AT91F_PDC_Open
2000059c t AT91F_PDC_DisableRx
200005c8 t AT91F_PDC_DisableTx
200005f4 t AT91F_PDC_SetNextTx
20000638 t AT91F_PDC_SetNextRx
2000067c t AT91F_PDC_SetTx
200006c0 t AT91F_PDC_SetRx
20000704 t AT91F_PDC_EnableRx
20000730 t AT91F_PDC_EnableTx
2000075c t AT91F_US_EnableTx
20000788 T __aeabi_uidiv
20000788 T __udivsi3
20000884 T __aeabi_uidivmod
2000089c T __aeabi_idiv0
2000089c T __aeabi_ldiv0
2000089c T __div0
200009a0 D _data
200009a0 A _etext
200009a0 D holaamigosh
200009a4 A __bss_end__
200009a4 A __bss_start
200009a4 A __bss_start__
200009a4 A _edata
200009a4 A _end

Example: the Python symbol table

The Python programming language includes extensive support for creating and manipulating symbol tables.[2] Properties that can be queried include whether a given symbol is a free variable or a bound variable, whether it is block scope or global scope, whether it is imported, and what namespace it belongs to.

Example: Dynamic symbol tables

Some programming languages allow the symbol table to be manipulated at run-time, so that symbols can be added at any time. Racket is an example of such a language[3].

Both the LISP and the Scheme programming languages allow arbitrary, generic properties to be associated with each symbol.[4]

The Prolog programming language is essentially a symbol-table manipulation language; symbols are called atoms, and the relationships between symbols can be reasoned over. Similarly, OpenCog provides a dynamic symbol table, called the atomspace, which is used for knowledge representation.

See also

References

  1. ^ Nguyen, Binh (2004). Linux Dictionary. p. 1482. Retrieved Apr 14, 2018.
  2. ^ symtable — Python documentation
  3. ^ Symbols - Racket Documentation
  4. ^ Symbols - Guile Documentation
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