engineering

Symbol table

Symbol table is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Symbol table rather than just read about it. In short: In computer science, a symbol table is a data structure where each identifier or symbol (referring to variables, constants, procedures, functions, etc.) in a program's code is associated with the necessary information for its functioning. This may come in two main forms: With object file and executables, the "code" is machine code, and the "information" is typically the memory address of the entity referred to by th…

Key takeaways

Symbol table belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Symbol table to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Symbol table from memory before moving on to harder problems.

Reference excerpt

In computer science, a symbol table is a data structure where each identifier or symbol (referring to variables, constants, procedures, functions, etc.) in a program's code is associated with the necessary information for its functioning. This may come in two main forms:

With object file and executables, the "code" is machine code, and the "information" is typically the memory address of the entity referred to by the symbol. With language translators such as a compiler or interpreters, the "code" is generally a source code, and the "information" refers to the entry's declaration or appearance in the source. (Interpreters also need a way to find the entry's "value" in the table, often an address pointing to another data structure.) With intermediate code files, the "information" may be intermediate between the two above types. A symbol table of the second type usually only exists in memory during the translation process. A symbol table of the first type is embedded in the output of the translation, such as in an ABI object file for later use. For example, it might be used when loading a dynamic library, during an interactive debugging session, or as a resource for formatting a diagnostic report during or after execution of a program.

Object files and executables The minimum information contained in a symbol table used by a machine-code file includes the symbol's name and its location or address. For a compiler targeting a platform with a concept of relocatability, it will also contain relocatability attributes (absolute, relocatable, etc.) and needed relocation information for relocatable symbols. Symbol tables for high-level programming languages may 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. Machine-code symbol tables are optimized for compactness and is typically a flat array of structs. (Conversion into a "smarter" data structure, if at all needed, can happen in the computer kernel once it loads the file.)

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. Usually all undefined external symbols will be searched for in one or more object libraries. If a module is found that defines that symbol it is linked together with the first object file, and any undefined external identifiers are added to the list of identifiers to be looked up. This process continues until all external references have been resolved. It is an error if one or more remains unresolved at the end of the process. 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: C Consider the following program written in C:

The object file would consist of at least two things: an address of the function foo and a way indicating usage of bar.

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. This format uses a sorted memory address field, a "symbol type" field, and a symbol identifier (called "Name"). The symbol table may be read through the nm utility. The symbol types in the SysV ABI (and nm's output) indicate the nature of each entry in the symbol table. Each symbol type is represented by a single character. For example, symbol table entries representing initialized data are denoted by the character "d" and symbol table entries for functions have the symbol type "t" (because executable code is located in the text section of an object file). Additionally, the capitalization of the symbol type indicates the type of linkage: lower-case letters indicate the symbol is local and upper-case indicates external (global) linkage. By compiling the above code (foo.c) to an object file foo.o using gcc -c foo.c, we can use nm foo.o to examine its symbol table:

U bar 0000000000000000 T foo

The above output indicates that "bar" is present as a global undefined symbol (U), awaiting resolution by the linker, and that "foo" is present as a global text symbol (T) at offset 0, which can be verified by objdump -d foo.o:

foo.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 <foo>: 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: 48 83 ec 20 sub $0x20,%rsp (...)

A more in-depth example is available at nm (Unix) § nm output sample.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Symbol table

Start with the simplest possible case. Write down what Symbol table claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Symbol table before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Symbol table ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Symbol table

In research: Symbol table appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Symbol table in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Symbol table is common in secondary-school and first-year university syllabi. It links to neighbouring topics Compiler structures, so understanding it makes those chapters shorter.
In everyday life: Look for Symbol table outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Symbol table in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Symbol table means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Symbol table out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Symbol table in simple terms?

Why does Symbol table matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Symbol table?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Symbol table.