Code Manipulation

Diomidis Spinellis
Department of Management Science and Technology
Athens University of Economics and Business
Athens, Greece
dds@aueb.gr

Typical Tasks

Identify the declarations
Locate definitions
Go through the places where an entity is used.
List deviations from coding standards.
Discover related code
Find related comments
Check for common errors.
View the code structure.
Understand how the code interacts with its environment

Regular Expressions

A regular expression allows the declarative specification of complex strings.
Regular expressions consist of:
- letters (that represent themselves) and
- special symbols
- Backslash escapes special symbols

Regular Expression Symbols

^	Beginning of a line
$	End of a line
.	Any character
Expression?	The expression zero or one times
Expression*	The expression zero or more times
Expression+	The expression one or more times
Expression{n}	The expression n times
Expression{n,}	The expression at least n times
Expression{n,m}	The expression at least n but no more than m times
Expression1\|Expression2	The expression1 or the expression2
(Expression)	The expression within the brackets
\1 \2 ... \n	The content of the nth bracket

Character Classes

[abc]	One of a, b, or c
[a-z]	A letter from a to z
[^abc]	Any letter appart from a, b, and c.
\t	The character tab
\n	The character newline
\r	The character carriage return
\a	The character alert
\f	The character form feed
\e	The character escape
\cx	The character control-x (a-z)
\d	Digit
\D	Non-digit
\s	Space
\S	Non-space

The Editor as a Code Browser

Regular expression searches
Tag facility
Outlining and folding
Bird's eye view with a 10% zoom factor

Code Searching with grep

Simple wildcard search
Search for definitions
Using a word's root
Locating files: grep on the output of ls
File list: grep -l
Use the result of a file list vi `grep -l ...`
Stream edit the grep result to create scripts
Postprocess with grep (-v) to eliminate noise (malloc / xmalloc)
Use sort -u to eliminate duplicates
Use fgrep to search for a fixed list of strings
Use find and xargs to obtain filename lists

Locating File Differences

Uses:
- Differences between versions
- Examining code modifications
- Verifying test results
The diff program
Obtaining a context diff (-c)
Ignoring blanks (-w)

Using the Compiler

Generate warning messages.
Tweak the code to generate error messages.
Generate program listings.
Obtain the preprocessor output.
Examine the generated symbolic (assembly) code.
Work through the final object code

Compiler Warning Messages

Expressions associated with portability problems
Implicit type casts and conversions between different but compatible types
The use of a constant in place of a conditional expression in if or while statements
Functions that do not return a value although they should
Variable declarations that mask other variables with the same name
Missing enumeration members or extraneous values in a switch statement
Unknown #pragma options
Unreferenced variables, structure members, or labels
Failing to initialize a const declared object

(Depending on the language, some of the above may be errors)

The Compiler as a Code-Reading Tool

Change the name of a definition and read the errors
Post-process error messages to create scripts
Use intermediate files (preprocessing, symbolic code)
Examine the object code using nm (Unix) or dumpbin (Windows)

Assembly Can be Useful

Read symbolic code to

To understand how an implementation-defined operation is actually performed
To see how compiler optimizations affect the resulting code
To convince yourself that the compiler correctly translates a piece of code, or (rarely) to find a compiler bug
To verify that hardware-specific code written in C actually performs the operations you expect
To learn how a particular construct can be written in symbolic code

Code Browsers

Code browsers typicall offer the following facilities:

Definitions: Locate where an entity is defined.
References: Go through all places where an entity is used.
A call graph: List the functions called by a given function.
A caller graph: List the functions calling a given function.
A file outline: List the functions, variables, types, and macros defined in a file.

Code Browsers in OO

In OO languages given a class you can find:

Where the class is defined
The locations where the class is referenced
The classes derived from this class
The base classes for this class
Its public, protected, and private methods and fields

Refactoring

Rename and move (various)
Change organization (classes, interfaces, inheritance)
Local variables into class fields
Code into method
Change method signature
Create getters and setters

(Live demo in Eclipse.)

Beautifiers

Fix code that is written truly inconsistently without following any formatting standards
Adopt orphaned code for maintenance
Create a temporary version of the code to help you decipher it
Integrate code under the common roof of a larger project

For printing use a pretty-printer, like listings (LaTeX), vgrind (troff).

Cdecl: Decoding C Declarations

Convert C declarations into English
Convert English into a C declaration

Example:

void (*signal(int sig, void (*func)(int)))(int);

declare signal as function that expects (sig as int, func as pointer to
function that expects (int) returning void) returning pointer to function
that expects (int) returning void;

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