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Re-organized some sections and added a whole new section on avoiding the
preprocessor. Comments welcome.
This commit is contained in:
Mark Whitley
parent
9a71af54f5
commit
d58ff8731c
@ -16,6 +16,7 @@ right formatting rules to your file. Please _do_not_ run this on all the files
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in the directory, just your own.
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Declaration Order
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-----------------
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@ -31,15 +32,16 @@ Here is the order in which code should be laid out in a file:
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- function implementations
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Whitespace
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----------
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Whitespace and Formatting
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-------------------------
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This is everybody's favorite flame topic so let's get it out of the way right
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up front.
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Tabs vs. Spaces in Line Indentation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The preference in Busybox is to indent lines with tabs. Do not indent lines
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with spaces and do not indents lines using a mixture of tabs and spaces. (The
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@ -172,6 +174,7 @@ block. Example:
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}
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Variable and Function Names
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---------------------------
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@ -192,78 +195,195 @@ that can go through and convert files -- left as an exercise to the reader for
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now.
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Tip and Pointers
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----------------
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The following are simple coding guidelines that should be followed:
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Avoid The Preprocessor
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----------------------
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- When in doubt about the proper behavior of a Busybox program (output,
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formatting, options, etc.), model it after the equivalent GNU program.
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Doesn't matter how that program behaves on some other flavor of *NIX;
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doesn't matter what the POSIX standard says or doesn't say, just model
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Busybox programs after their GNU counterparts and nobody has to get hurt.
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At best, the preprocessor is a necessary evil, helping us account for platform
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and architecture differences. Using the preprocessor unnecessarily is just
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plain evil.
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- Don't use a '#define var 80' when you can use 'static const int var 80'
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instead. This makes the compiler do type checking for you (rather than
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relying on the more error-prone preprocessor) and it makes debugging
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programs much easier since the value of the variable can be easily
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displayed.
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- If a const variable is used in only one function, do not make it global to
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the file. Instead, declare it inside the function body.
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The Folly of #define
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~~~~~~~~~~~~~~~~~~~~
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- Inside applet files, all functions should be declared static so as to keep
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the global name space clean. The only exception to this rule is the
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"applet_main" function which must be declared extern.
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- If you write a function that performs a task that could be useful outside
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the immediate file, turn it into a general-purpose function with no ties to
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any applet and put it in the utility.c file instead.
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- Put all help/usage messages in usage.c. Put other strings in messages.c.
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Putting these strings into their own file is a calculated decision designed
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to confine spelling errors to a single place and aid internationalization
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efforts, if needed. (Side Note: we might want to use a single file instead
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of two, food for thought).
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- There's a right way and a wrong way to test for sting equivalence with
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strcmp:
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The wrong way:
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if (!strcmp(string, "foo")) {
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...
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The right way:
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if (strcmp(string, "foo") == 0){
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...
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The use of the "equals" (==) operator in the latter example makes it much
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more obvious that you are testing for equivalence. The former example with
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the "not" (!) operator makes it look like you are testing for an error. In
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a more perfect world, we would have a streq() function in the string
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library, but that ain't the world we're living in.
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- Do not use old-style function declarations that declare variable types
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between the parameter list and opening bracket. Example:
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Use 'const <type> var' for declaring constants.
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Don't do this:
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int foo(parm1, parm2)
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char parm1;
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float parm2;
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{
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....
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#define var 80
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Do this instead, when the variable is in a header file and will be used in
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several source files:
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const int var = 80;
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Or do this when the variable is used only in a single source file:
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static const int var = 80;
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Declaring variables as '[static] const' gives variables an actual type and
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makes the compiler do type checking for you; the preprocessor does _no_ type
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checking whatsoever, making it much more error prone. Declaring variables with
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'[static] const' also makes debugging programs much easier since the value of
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the variable can be easily queried and displayed.
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The Folly of Macros
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~~~~~~~~~~~~~~~~~~~
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Use 'static inline' instead of a macro.
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Don't do this:
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#define mini_func(param1, param2) (param1 << param2)
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Do this instead:
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int foo(char parm1, float parm2)
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static inline int mini_func(int param1, param2)
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{
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....
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return (param1 << param2);
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}
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- Please use brackets on all if and else statements, even if it is only one
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line. Example:
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Static inline functions are greatly preferred over macros. They provide type
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safety, have no length limitations, no formatting limitations, and under gcc
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they are as cheap as macros. Besides, really long macros with backslashes at
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the end of each line are ugly as sin.
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The Folly of #ifdef
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~~~~~~~~~~~~~~~~~~~
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Code cluttered with ifdefs is difficult to read and maintain. Don't do it.
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Instead, put your ifdefs in a header, and conditionally define 'static inline'
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functions, (or *maybe* macros), which are used in the code.
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Don't do this:
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ret = my_func(bar, baz);
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if (!ret)
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return -1;
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#ifdef BB_FEATURE_FUNKY
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maybe_do_funky_stuff(bar, baz);
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#endif
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Do this instead:
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(in .h header file)
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#ifndef BB_FEATURE_FUNKY
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static inline void maybe_do_funky_stuff (int bar, int baz) {}
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#endif
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(in the .c source file)
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ret = my_func(bar, baz);
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if (!ret)
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return -1;
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maybe_do_funky_stuff(bar, baz);
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The great thing about this approach is that the compiler will optimize away
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the "no-op" case when the feature is turned off.
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Note also the use of the word 'maybe' in the function name to indicate
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conditional execution.
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Notes on Strings
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----------------
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Strings in C can get a little thorny. Here's some guidelines for dealing with
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strings in Busybox. (There is surely more that could be added to this
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section.)
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String Files
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~~~~~~~~~~~~
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Put all help/usage messages in usage.c. Put other strings in messages.c.
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Putting these strings into their own file is a calculated decision designed to
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confine spelling errors to a single place and aid internationalization
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efforts, if needed. (Side Note: we might want to use a single file - maybe
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called 'strings.c' - instead of two, food for thought).
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Testing String Equivalence
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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There's a right way and a wrong way to test for sting equivalence with
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strcmp():
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The wrong way:
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if (!strcmp(string, "foo")) {
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...
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The right way:
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if (strcmp(string, "foo") == 0){
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...
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The use of the "equals" (==) operator in the latter example makes it much more
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obvious that you are testing for equivalence. The former example with the
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"not" (!) operator makes it look like you are testing for an error. In a more
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perfect world, we would have a streq() function in the string library, but
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that ain't the world we're living in.
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Miscellaneous Coding Guidelines
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-------------------------------
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The following are important items that don't fit into any of the above
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sections.
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Model Busybox Applets After GNU Counterparts
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When in doubt about the proper behavior of a Busybox program (output,
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formatting, options, etc.), model it after the equivalent GNU program.
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Doesn't matter how that program behaves on some other flavor of *NIX; doesn't
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matter what the POSIX standard says or doesn't say, just model Busybox
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programs after their GNU counterparts and nobody has to get hurt.
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The only time we deviate from emulating the GNU behavior is when:
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- We are deliberately not supporting a feature (such as a command line
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switch)
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- Emulating the GNU behavior is prohibitively expensive (lots more code
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would be required, lots more memory would be used, etc.)
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- The differce is minor or cosmetic
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A note on the 'cosmetic' case: Output differences might be considered
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cosmetic, but if the output is significant enough to break other scripts that
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use the output, it should really be fixed.
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Scope
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~~~~~
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If a const variable is used only in a single source file, put it in the source
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file and not in a header file. Likewise, if a const variable is used in only
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one function, do not make it global to the file. Instead, declare it inside
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the function body. Bottom line: Make a concious effort to limit declarations
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to the smallest scope possible.
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Inside applet files, all functions should be declared static so as to keep the
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global name space clean. The only exception to this rule is the "applet_main"
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function which must be declared extern.
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If you write a function that performs a task that could be useful outside the
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immediate file, turn it into a general-purpose function with no ties to any
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applet and put it in the utility.c file instead.
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Brackets Are Your Friends
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~~~~~~~~~~~~~~~~~~~~~~~~~
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Please use brackets on all if and else statements, even if it is only one
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line. Example:
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Don't do this:
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@ -280,8 +400,8 @@ The following are simple coding guidelines that should be followed:
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stmt;
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}
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The "bracketless" approach is error prone because someday you might add a
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line like this:
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The "bracketless" approach is error prone because someday you might add a line
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like this:
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if (foo)
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stmt;
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@ -289,6 +409,32 @@ The following are simple coding guidelines that should be followed:
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else
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stmt;
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And the resulting behavior of your program would totally bewilder you.
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(Don't laugh, it happens to us all.) Remember folks, this is C, not
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Python.
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And the resulting behavior of your program would totally bewilder you. (Don't
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laugh, it happens to us all.) Remember folks, this is C, not Python.
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Function Declarations
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~~~~~~~~~~~~~~~~~~~~~
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Do not use old-style function declarations that declare variable types between
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the parameter list and opening bracket. Example:
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Don't do this:
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int foo(parm1, parm2)
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char parm1;
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float parm2;
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{
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....
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Do this instead:
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int foo(char parm1, float parm2)
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{
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....
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The only time you would ever need to use the old declaration syntax is to
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support ancient, antedeluvian compilers. To our good fortune, we have access
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to more modern compilers and the old declaration syntax is neither necessary
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nor desired.
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