Coding Guidelines

Privoxy Developer Manual
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# 4. Coding Guidelines

## 4.1. Introduction

This set of standards is designed to make our lives easier. It is developed with the simple goal of helping us keep the "new and improved Privoxy" consistent and reliable. Thus making maintenance easier and increasing chances of success of the project.

And that of course comes back to us as individuals. If we can increase our development and product efficiencies then we can solve more of the request for changes/improvements and in general feel good about ourselves. ;->

## 4.2. Using Comments

### 4.2.1. Comment, Comment, Comment

Explanation:

Comment as much as possible without commenting the obvious. For example do not comment "variable_a is equal to variable_b". Instead explain why variable_a should be equal to the variable_b. Just because a person can read code does not mean they will understand why or what is being done. A reader may spend a lot more time figuring out what is going on when a simple comment or explanation would have prevented the extra research. Please help your fellow Privoxy developers out!

The comments will also help justify the intent of the code. If the comment describes something different than what the code is doing then maybe a programming error is occurring.

Example:

    /* if page size greater than 1k ... */
    if (page_length() > 1024)
    {
        ... "block" the page up ...
    }

    /* if page size is small, send it in blocks */
    if (page_length() > 1024)
    {
        ... "block" the page up ...
    }

    This demonstrates 2 cases of "what not to do".  The first is a
    "syntax comment".  The second is a comment that does not fit what
    is actually being done.

### 4.2.2. Use blocks for comments

Explanation:

Comments can help or they can clutter. They help when they are differentiated from the code they describe. One line comments do not offer effective separation between the comment and the code. Block identifiers do, by surrounding the code with a clear, definable pattern.

Example:

    /*********************************************************************
     * This will stand out clearly in your code!
     *********************************************************************/
    if (this_variable == that_variable)
    {
       do_something_very_important();
    }

    /* unfortunately, this may not */
    if (this_variable == that_variable)
    {
       do_something_very_important();
    }

    if (this_variable == that_variable) /* this may not either */
    {
       do_something_very_important();
    }

Exception:

If you are trying to add a small logic comment and do not wish to "disrupt" the flow of the code, feel free to use a 1 line comment which is NOT on the same line as the code.

### 4.2.3. Keep Comments on their own line

Explanation:

It goes back to the question of readability. If the comment is on the same line as the code it will be harder to read than the comment that is on its own line.

There are three exceptions to this rule, which should be violated freely and often: during the definition of variables, at the end of closing braces, when used to comment parameters.

Example:

    /*********************************************************************
     * This will stand out clearly in your code,
     * But the second example won't.
     *********************************************************************/
    if (this_variable == this_variable)
    {
       do_something_very_important();
    }

    if (this_variable == this_variable) /*can you see me?*/
    {
       do_something_very_important(); /*not easily*/
    }

    /*********************************************************************
     * But, the encouraged exceptions:
     *********************************************************************/
    int urls_read     = 0;     /* # of urls read + rejected */
    int urls_rejected = 0;     /* # of urls rejected */

    if (1 == X)
    {
       do_something_very_important();
    }

    short do_something_very_important(
       short firstparam,   /* represents something */
       short nextparam     /* represents something else */ )
    {
       ...code here...

    }   /* -END- do_something_very_important */

### 4.2.4. Comment each logical step

Explanation:

Logical steps should be commented to help others follow the intent of the written code and comments will make the code more readable.

If you have 25 lines of code without a comment, you should probably go back into it to see where you forgot to put one.

Most "for", "while", "do", etc... loops _probably_ need a comment. After all, these are usually major logic containers.

### 4.2.5. Comment All Functions Thoroughly

Explanation:

A reader of the code should be able to look at the comments just prior to the beginning of a function and discern the reason for its existence and the consequences of using it. The reader should not have to read through the code to determine if a given function is safe for a desired use. The proper information thoroughly presented at the introduction of a function not only saves time for subsequent maintenance or debugging, it more importantly aids in code reuse by allowing a user to determine the safety and applicability of any function for the problem at hand. As a result of such benefits, all functions should contain the information presented in the addendum section of this document.

### 4.2.6. Comment at the end of braces if the content is more than one screen length

Explanation:

Each closing brace should be followed on the same line by a comment that describes the origination of the brace if the original brace is off of the screen, or otherwise far away from the closing brace. This will simplify the debugging, maintenance, and readability of the code.

As a suggestion , use the following flags to make the comment and its brace more readable:

use following a closing brace: } /* -END- if() or while () or etc... */

Example:

    if (1 == X)
    {
       do_something_very_important();
       ...some long list of commands...
    } /* -END- if x is 1 */

    or:

    if (1 == X)
    {
       do_something_very_important();
       ...some long list of commands...
    } /* -END- if (1 == X) */

## 4.3. Naming Conventions

### 4.3.1. Variable Names

Explanation:

Use all lowercase, and separate words via an underscore ('_'). Do not start an identifier with an underscore. (ANSI C reserves these for use by the compiler and system headers.) Do not use identifiers which are reserved in ANSI C++. (E.g. template, class, true, false, ...). This is in case we ever decide to port Privoxy to C++.

Example:

    int ms_iis5_hack = 0;

Instead of:

    int msiis5hack = 0; int msIis5Hack = 0;

### 4.3.2. Function Names

Explanation:

Use all lowercase, and separate words via an underscore ('_'). Do not start an identifier with an underscore. (ANSI C reserves these for use by the compiler and system headers.) Do not use identifiers which are reserved in ANSI C++. (E.g. template, class, true, false, ...). This is in case we ever decide to port Privoxy to C++.

Example:

    int load_some_file(struct client_state *csp)

Instead of:

    int loadsomefile(struct client_state *csp)
    int loadSomeFile(struct client_state *csp)

### 4.3.3. Header file prototypes

Explanation:

Use a descriptive parameter name in the function prototype in header files. Use the same parameter name in the header file that you use in the c file.

Example:

    (.h) extern int load_aclfile(struct client_state *csp);
    (.c) int load_aclfile(struct client_state *csp)

Instead of:

    (.h) extern int load_aclfile(struct client_state *); or
    (.h) extern int load_aclfile();
    (.c) int load_aclfile(struct client_state *csp)

### 4.3.4. Enumerations, and #defines

Explanation:

Use all capital letters, with underscores between words. Do not start an identifier with an underscore. (ANSI C reserves these for use by the compiler and system headers.)

Example:

    (enumeration) : enum Boolean {FALSE, TRUE};
    (#define) : #define DEFAULT_SIZE 100;

Note: We have a standard naming scheme for #defines that toggle a feature in the preprocessor: FEATURE_>, where > is a short (preferably 1 or 2 word) description.

Example:

    #define FEATURE_FORCE 1

    #ifdef FEATURE_FORCE
    #define FORCE_PREFIX blah
    #endif /* def FEATURE_FORCE */

### 4.3.5. Constants

Explanation:

Spell common words out entirely (do not remove vowels).

Use only widely-known domain acronyms and abbreviations. Capitalize all letters of an acronym.

Use underscore (_) to separate adjacent acronyms and abbreviations. Never terminate a name with an underscore.

Example:

    #define USE_IMAGE_LIST 1

Instead of:

    #define USE_IMG_LST 1 or
    #define _USE_IMAGE_LIST 1 or
    #define USE_IMAGE_LIST_ 1 or
    #define use_image_list 1 or
    #define UseImageList 1

## 4.4. Using Space

### 4.4.1. Put braces on a line by themselves.

Explanation:

The brace needs to be on a line all by itself, not at the end of the statement. Curly braces should line up with the construct that they're associated with. This practice makes it easier to identify the opening and closing braces for a block.

Example:

    if (this == that)
    {
       ...
    }

Instead of:

if (this == that) { ... }

or

if (this == that) { ... }

Note: In the special case that the if-statement is inside a loop, and it is trivial, i.e. it tests for a condition that is obvious from the purpose of the block, one-liners as above may optically preserve the loop structure and make it easier to read.

Status: developer-discretion.

Example exception:

    while (more lines are read)
    {
       /* Please document what is/is not a comment line here */
       if (it's a comment) continue;

       do_something(line);
    }

### 4.4.2. ALL control statements should have a block

Explanation:

Using braces to make a block will make your code more readable and less prone to error. All control statements should have a block defined.

Example:

    if (this == that)
    {
       do_something();
       do_something_else();
    }

Instead of:

if (this == that) do_something(); do_something_else();

or

if (this == that) do_something();

Note: The first example in "Instead of" will execute in a manner other than that which the developer desired (per indentation). Using code braces would have prevented this "feature". The "explanation" and "exception" from the point above also applies.

### 4.4.3. Do not belabor/blow-up boolean expressions

Example:

    structure->flag = (condition);

Instead of:

if (condition) { structure->flag = 1; } else { structure->flag = 0; }

Note: The former is readable and concise. The later is wordy and inefficient. Please assume that any developer new to the project has at least a "good" knowledge of C/C++. (Hope I do not offend by that last comment ... 8-)

### 4.4.4. Use white space freely because it is free

Explanation:

Make it readable. The notable exception to using white space freely is listed in the next guideline.

Example:

    int first_value   = 0;
    int some_value    = 0;
    int another_value = 0;
    int this_variable = 0;

### 4.4.5. Don't use white space around structure operators

Explanation:

- structure pointer operator ( "->" ) - member operator ( "." ) - functions and parentheses

It is a general coding practice to put pointers, references, and function parentheses next to names. With spaces, the connection between the object and variable/function name is not as clear.

Example:

    a_struct->a_member;
    a_struct.a_member;
    function_name();

Instead of: a_struct -> a_member; a_struct . a_member; function_name ();

### 4.4.6. Make the last brace of a function stand out

Example:

    int function1( ... )
    {
       ...code...
       return(ret_code);

    } /* -END- function1 */

    int function2( ... )
    {
    } /* -END- function2 */

Instead of:

int function1( ... ) { ...code... return(ret_code); } int function2( ... ) { }

Note: Use 1 blank line before the closing brace and 2 lines afterward. This makes the end of function standout to the most casual viewer. Although function comments help separate functions, this is still a good coding practice. In fact, I follow these rules when using blocks in "for", "while", "do" loops, and long if {} statements too. After all whitespace is free!

Status: developer-discretion on the number of blank lines. Enforced is the end of function comments.

### 4.4.7. Use 3 character indentions

Explanation:

If some use 8 character TABs and some use 3 character TABs, the code can look *very* ragged. So use 3 character indentions only. If you like to use TABs, pass your code through a filter such as "expand -t3" before checking in your code.

Example:

    static const char * const url_code_map[256] =
    {
       NULL, ...
    };

    int function1( ... )
    {
       if (1)
       {
          return ALWAYS_TRUE;
       }
       else
       {
          return HOW_DID_YOU_GET_HERE;
       }

       return NEVER_GETS_HERE;

    }

## 4.5. Initializing

### 4.5.1. Initialize all variables

Explanation:

Do not assume that the variables declared will not be used until after they have been assigned a value somewhere else in the code. Remove the chance of accidentally using an unassigned variable.

Example:

    short a_short = 0;
    float a_float  = 0;
    struct *ptr = NULL;

Note: It is much easier to debug a SIGSEGV if the message says you are trying to access memory address 00000000 and not 129FA012; or array_ptr[20] causes a SIGSEV vs. array_ptr[0].

Status: developer-discretion if and only if the variable is assigned a value "shortly after" declaration.

## 4.6. Functions

### 4.6.1. Name functions that return a boolean as a question.

Explanation:

Value should be phrased as a question that would logically be answered as a true or false statement

Example:

    should_we_block_this();
    contains_an_image();
    is_web_page_blank();

### 4.6.2. Always specify a return type for a function.

Explanation:

The default return for a function is an int. To avoid ambiguity, create a return for a function when the return has a purpose, and create a void return type if the function does not need to return anything.

### 4.6.3. Minimize function calls when iterating by using variables

Explanation:

It is easy to write the following code, and a clear argument can be made that the code is easy to understand:

Example:

    for (size_t cnt = 0; cnt < block_list_length(); cnt++)
    {
       ....
    }

Note: Unfortunately, this makes a function call for each and every iteration. This increases the overhead in the program, because the compiler has to look up the function each time, call it, and return a value. Depending on what occurs in the block_list_length() call, it might even be creating and destroying structures with each iteration, even though in each case it is comparing "cnt" to the same value, over and over. Remember too - even a call to block_list_length() is a function call, with the same overhead.

Instead of using a function call during the iterations, assign the value to a variable, and evaluate using the variable.

Example:

    size_t len = block_list_length();

    for (size_t cnt = 0; cnt < len; cnt++)
    {
       ....
    }

Exceptions: if the value of block_list_length() *may* change or could *potentially* change, then you must code the function call in the for/while loop.

### 4.6.4. Pass and Return by Const Reference

Explanation:

This allows a developer to define a const pointer and call your function. If your function does not have the const keyword, we may not be able to use your function. Consider strcmp, if it were defined as: extern int strcmp(char *s1, char *s2);

I could then not use it to compare argv's in main: int main(int argc, const char *argv[]) { strcmp(argv[0], "privoxy"); }

Both these pointers are *const*! If the c runtime library maintainers do it, we should too.

### 4.6.5. Pass and Return by Value

Explanation:

Most structures cannot fit onto a normal stack entry (i.e. they are not 4 bytes or less). Aka, a function declaration like: int load_aclfile(struct client_state csp)

would not work. So, to be consistent, we should declare all prototypes with "pass by value": int load_aclfile(struct client_state *csp)

### 4.6.6. Names of include files

Explanation:

Your include statements should contain the file name without a path. The path should be listed in the Makefile, using -I as processor directive to search the indicated paths. An exception to this would be for some proprietary software that utilizes a partial path to distinguish their header files from system or other header files.

Example:

    #include <iostream.h>     /* This is not a local include */
    #include "config.h"       /* This IS a local include */

Exception:

    /* This is not a local include, but requires a path element. */
    #include <sys/fileName.h>

Note: Please! do not add "-I." to the Makefile without a _very_ good reason. This duplicates the #include "file.h" behavior.

### 4.6.7. Provide multiple inclusion protection

Explanation:

Prevents compiler and linker errors resulting from redefinition of items.

Wrap each header file with the following syntax to prevent multiple inclusions of the file. Of course, replace PROJECT_H with your file name, with "." Changed to "_", and make it uppercase.

Example:

    #ifndef PROJECT_H_INCLUDED
    #define PROJECT_H_INCLUDED
     ...
    #endif /* ndef PROJECT_H_INCLUDED */

### 4.6.8. Use `extern "C"` when appropriate

Explanation:

If our headers are included from C++, they must declare our functions as `extern "C"`. This has no cost in C, but increases the potential re-usability of our code.

Example:

    #ifdef __cplusplus
    extern "C"
    {
    #endif /* def __cplusplus */

    ... function definitions here ...

    #ifdef __cplusplus
    }
    #endif /* def __cplusplus */

### 4.6.9. Where Possible, Use Forward Struct Declaration Instead of Includes

Explanation:

Useful in headers that include pointers to other struct's. Modifications to excess header files may cause needless compiles.

Example:

    /*********************************************************************
     * We're avoiding an include statement here!
     *********************************************************************/
    struct file_list;
    extern file_list *xyz;

Note: If you declare "file_list xyz;" (without the pointer), then including the proper header file is necessary. If you only want to prototype a pointer, however, the header file is unnecessary.

Status: Use with discretion.

## 4.7. General Coding Practices

### 4.7.1. Turn on warnings

Explanation

Compiler warnings are meant to help you find bugs. You should turn on as many as possible. With GCC, the switch is "-Wall". Try and fix as many warnings as possible.

### 4.7.2. Provide a default case for all switch statements

Explanation:

What you think is guaranteed is never really guaranteed. The value that you don't think you need to check is the one that someday will be passed. So, to protect yourself from the unknown, always have a default step in a switch statement.

Example:

    switch (hash_string(cmd))
    {
       case hash_actions_file:
          ... code ...
          break;

       case hash_confdir:
          ... code ...
          break;

       default:
          log_error( ... );
          ... anomaly code goes here ...
          continue; / break; / exit( 1 ); / etc ...

    } /* end switch (hash_string(cmd)) */

Note: If you already have a default condition, you are obviously exempt from this point. Of note, most of the WIN32 code calls `DefWindowProc' after the switch statement. This API call *should* be included in a default statement.

Another Note: This is not so much a readability issue as a robust programming issue. The "anomaly code goes here" may be no more than a print to the STDERR stream (as in load_config). Or it may really be an abort condition.

Status: Programmer discretion is advised.

### 4.7.3. Try to avoid falling through cases in a switch statement.

Explanation:

In general, you will want to have a 'break' statement within each 'case' of a switch statement. This allows for the code to be more readable and understandable, and furthermore can prevent unwanted surprises if someone else later gets creative and moves the code around.

The language allows you to plan the fall through from one case statement to another simply by omitting the break statement within the case statement. This feature does have benefits, but should only be used in rare cases. In general, use a break statement for each case statement.

If you choose to allow fall through, you should comment both the fact of the fall through and reason why you felt it was necessary.

### 4.7.4. Don't mix size_t and other types

Explanation:

The type of size_t varies across platforms. Do not make assumptions about whether it is signed or unsigned, or about how long it is. Do not compare a size_t against another variable of a different type (or even against a constant) without casting one of the values.

### 4.7.5. Declare each variable and struct on its own line.

Explanation:

It can be tempting to declare a series of variables all on one line. Don't.

Example:

    long a = 0;
    long b = 0;
    long c = 0;

Instead of:

long a, b, c;

Explanation: - there is more room for comments on the individual variables - easier to add new variables without messing up the original ones - when searching on a variable to find its type, there is less clutter to "visually" eliminate

Exceptions: when you want to declare a bunch of loop variables or other trivial variables; feel free to declare them on one line. You should, although, provide a good comment on their functions.

Status: developer-discretion.

### 4.7.6. Use malloc/zalloc sparingly

Explanation:

Create a local struct (on the stack) if the variable will live and die within the context of one function call.

Only "malloc" a struct (on the heap) if the variable's life will extend beyond the context of one function call.

Example:

    If a function creates a struct and stores a pointer to it in a
    list, then it should definitely be allocated via `malloc'.

### 4.7.7. The Programmer Who Uses 'malloc' is Responsible for Ensuring 'free'

Explanation:

If you have to "malloc" an instance, you are responsible for insuring that the instance is `free'd, even if the deallocation event falls within some other programmer's code. You are also responsible for ensuring that deletion is timely (i.e. not too soon, not too late). This is known as "low-coupling" and is a "good thing (tm)". You may need to offer a free/unload/destructor type function to accommodate this.

Example:

    int load_re_filterfile(struct client_state *csp) { ... }
    static void unload_re_filterfile(void *f) { ... }

Exceptions:

The developer cannot be expected to provide `free'ing functions for C run-time library functions ... such as `strdup'.

Status: developer-discretion. The "main" use of this standard is for allocating and freeing data structures (complex or nested).

### 4.7.8. Add loaders to the `file_list' structure and in order

Explanation:

I have ordered all of the "blocker" file code to be in alpha order. It is easier to add/read new blockers when you expect a certain order.

Note: It may appear that the alpha order is broken in places by POPUP tests coming before PCRS tests. But since POPUPs can also be referred to as KILLPOPUPs, it is clear that it should come first.

### 4.7.9. "Uncertain" new code and/or changes to existing code, use XXX

Explanation:

If you have enough confidence in new code or confidence in your changes, but are not *quite* sure of the repercussions, add this:

/* XXX: this code has a logic error on platform XYZ, * attempting to fix */ #ifdef PLATFORM ...changed code here... #endif

or:

/* XXX: I think the original author really meant this... */ ...changed code here...

or:

/* XXX: new code that *may* break something else... */ ...new code here...

Note: If you make it clear that this may or may not be a "good thing (tm)", it will be easier to identify and include in the project (or conversely exclude from the project).

## 4.8. Addendum: Template for files and function comment blocks:

Example for file comments:

    /*********************************************************************
     *
     * File        :  $Source
     *
     * Purpose     :  (Fill me in with a good description!)
     *
     * Copyright   :  Written by and Copyright (C) 2001-2009
     *                the Privoxy team. https://www.privoxy.org/
     *
     *                This program is free software; you can redistribute it
     *                and/or modify it under the terms of the GNU General
     *                Public License as published by the Free Software
     *                Foundation; either version 2 of the License, or (at
     *                your option) any later version.
     *
     *                This program is distributed in the hope that it will
     *                be useful, but WITHOUT ANY WARRANTY; without even the
     *                implied warranty of MERCHANTABILITY or FITNESS FOR A
     *                PARTICULAR PURPOSE.  See the GNU General Public
     *                License for more details.
     *
     *                The GNU General Public License should be included with
     *                this file.  If not, you can view it at
     *                http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
     *                or write to the Free Software Foundation, Inc.,
     *                51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 ,
     *                USA
     *
     *********************************************************************/

    #include "config.h"

       ...necessary include files for us to do our work...

    const char FILENAME_h_rcs[] = FILENAME_H_VERSION;

Note: This declares the rcs variables that should be added to the "show-version" page. If this is a brand new creation by you, you are free to change the "Copyright" section to represent the rights you wish to maintain.

Note: The formfeed character that is present right after the comment flower box is handy for (X|GNU)Emacs users to skip the verbiage and get to the heart of the code (via `forward-page' and `backward-page'). Please include it if you can.

Example for file header comments:

    #ifndef _FILENAME_H
    #define _FILENAME_H
    /*********************************************************************
     *
     * File        :  $Source
     *
     * Purpose     :  (Fill me in with a good description!)
     *
     * Copyright   :  Written by and Copyright (C) 2001-2009
     *                the Privoxy team. https://www.privoxy.org/
     *
     *                This program is free software; you can redistribute it
     *                and/or modify it under the terms of the GNU General
     *                Public License as published by the Free Software
     *                Foundation; either version 2 of the License, or (at
     *                your option) any later version.
     *
     *                This program is distributed in the hope that it will
     *                be useful, but WITHOUT ANY WARRANTY; without even the
     *                implied warranty of MERCHANTABILITY or FITNESS FOR A
     *                PARTICULAR PURPOSE.  See the GNU General Public
     *                License for more details.
     *
     *                The GNU General Public License should be included with
     *                this file.  If not, you can view it at
     *                http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
     *                or write to the Free Software Foundation, Inc.,
     *                51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 ,
     *                USA
     *
     *********************************************************************/

    #include "project.h"

    #ifdef __cplusplus
    extern "C" {
    #endif

       ... function headers here ...

    /* Revision control strings from this header and associated .c file */
    extern const char FILENAME_rcs[];
    extern const char FILENAME_h_rcs[];

    #ifdef __cplusplus
    } /* extern "C" */
    #endif

    #endif /* ndef _FILENAME_H */

    /*
      Local Variables:
      tab-width: 3
      end:
    */

Example for function comments:

    /*********************************************************************
     *
     * Function    :  FUNCTION_NAME
     *
     * Description :  (Fill me in with a good description!)
     *
     * parameters  :
     *          1  :  param1 = pointer to an important thing
     *          2  :  x      = pointer to something else
     *
     * Returns     :  0 => Ok, everything else is an error.
     *
     *********************************************************************/
    int FUNCTION_NAME(void *param1, const char *x)
    {
       ...
       return 0;

    }

Note: If we all follow this practice, we should be able to parse our code to create a "self-documenting" web page.

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