4.4 Symbol Tables

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Note that nm does not list entry points in a DLL, unless there is a symbol table associated with it. By default, nm lists the symbols in file in alphabetical order by name and provides the following information on each:. Not all of these symbol types are available on all systems.

For instance, not all systems support the ability to binary symbols table different segment information. This is the default, since this implementation does not suppress any output. If you specified -A and file is not a library, the format is.

If you did not specify -Athe format is. If you did not also binary symbols table the -t option, nm displays value and size in hexadecimal. If you did not specify -A and the command line contains more than one fileor file is a library, nm displays a line preceding the list of symbols for each specified file or each object file in a specified library.

If file is a library, this line has the following format:. The format shall be one of door xfor decimal, octal, or hexadecimal, respectively. If this option is not used, numbers are displayed in decimal. If a file does not contain a symbol table, nm displays a warning and goes to the next file, but this is not binary symbols table an error. Windows Server R2. On Windows systems, data D, d and uninitialized data B, b are not currently recognized binary symbols table executable files, as this involves trying to binary symbols table segment types.

All such symbols come out as text T, t symbols. Windows executable files, which normally end binary symbols table. A symbol table may be created from a. By default, nm lists the symbols in file in alphabetical order by name and provides the following information on each: File or object name if you specified -A Symbol name Symbol type: If you specified -A and file is not a library, the format is file: If you did not specify -Athe format is name type value size If you did not also specify the -t option, nm displays value and size in hexadecimal.

If file is a library, this line has the following format: If file is not a library, the format is simply file: NOTE On Windows systems, data D, d and uninitialized data B, b are not currently recognized in executable files, as this involves trying to guess segment types.

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In computer science , a symbol table is an implementation of a data structure used by a language translator such as a compiler or interpreter , where each identifier a.

A symbol table may only exist during the translation process [ further explanation needed ] , 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.

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. 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? 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.

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?

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.

Consider the following program written in C:. In addition, the symbol table will also contain entries generated by the compiler for intermediate expression values e. 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.

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. The Python programming language includes extensive support for creating and manipulating 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 [2]. Both the LISP and the Scheme programming languages allow arbitrary, generic properties to be associated with each symbol. 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.

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