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 in a program's source code is associated with information relating to its declaration or appearance in the source, such as its type, scope level and sometimes its location.

Implementation[edit]

A common implementation technique is to use a hash table. A compiler may use one large symbol table for all symbols or use separated, hierarchical symbol tables for different scopes. There are also trees, linear lists and self-organizing lists which can be used to implement symbol table.It also simplifies the classification of literals in tabular format.The symbol table is accessed by most phases of a compiler, beginning with the lexical analysis to optimization.

Uses[edit]

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 use these symbol tables to resolve any unresolved references.

A symbol table may only exist during the translation process, or it may be embedded in the output of that process for later exploitation, for example, during an interactive debugging session, or as a resource for formatting a diagnostic report during or after execution of a program.

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.

At that time of accessing variables and allocating memory dynamically, a compiler should perform many works and as such the extended stack model requires the symbol table.[further explanation needed]

Example[edit]

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.

As another example, the symbol table of a small program is listed below. The table itself was generated using the GNU binutils' nm utility. There is one data symbol, (noted by the "D" type), and many functions (self defined as well as from the standard library). The first column is where the symbol is located in the memory, the second is "The symbol type" and the third is the name of the symbol. By passing suitable parameters, the symbol table was made to sort on basis of address.

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

See also[edit]