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.
Contents
Background
A symbol table may only exist during the translation process, or it may be embedded in the output of that process, 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.
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.
A common data structure used to implement symbol tables is the hash table. It also simplifies the classification of literals in tabular format.
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 computer. 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 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.
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.
Example
Consider the following program written in C:
A C compiler that parses this code will contain at least the following symbol table entries:
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.
Another table format
An alternative format is given by the symbol table below. This example illustrates the format of 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.