GNU coding styles and standards

coding programming

Reference standards

Don’t in any circumstances refer to Unix source code for or during your work on GNU! (Or to any other proprietary programs.)

If you have a vague recollection of the internals of a Unix program, this does not absolutely mean you can’t write an imitation of it, but do try to organize the imitation along different lines, because this is likely to make the details of the Unix version irrelevant and dissimilar to your results.

For example, Unix utilities were generally optimized to minimize memory use; if you go for speed instead, your program will be very different. You could keep the entire input file in core and scan it there instead of using stdio. Use a smarter algorithm discovered more recently than the Unix program. Eliminate use of temporary files. Do it in one pass instead of two (we did this in the assembler). Or, on the contrary, emphasize simplicity instead of speed. For some applications, the speed of today’s computers makes simpler algorithms adequate.

Or go for generality. For example, Unix programs often have static tables or fixed-size strings, which make for arbitrary limits; use dynamic allocation instead. Make sure your program handles nulls and other funny characters in the input files. Add a programming language for extensibility and write part of the program in that language.

Or turn some parts of the program into independently usable libraries. Or use a simple garbage collector instead of tracking precisely when to free memory, or use a new GNU facility such as obstacks.

Compatibility standards

Utility programs and libraries for GNU should be totally upward compatible with those in Berkeley Unix, with certain exceptions. When a feature is used only by users (not by programs or command files), and it is done poorly in Berkeley Unix, it is good to replace it completely with something totally different and better. (For example, vi is replaced with Emacs.) But it is nice to offer a compatible feature as well. (There is a free vi-clone, so we will offer it.)

Additional useful features not in Berkeley Unix are welcome. Additional programs with no counterpart in Unix may be useful, but our first priority is usually to duplicate what Unix already has.

Formatting standards:

It is important put the open-brace that starts the body of a C function in column zero, and avoid putting any other open-brace or open-parenthesis or open-bracket in column zero. Several tools look for open-braces in column zero to find the beginnings of C functions.
These tools will not work on code not formatted that way. It is also important for function definitions to start the name of the function in column zero. ‘ctags’ or ‘etags’ cannot recognize them otherwise. Thus, the proper format is this:

static char *
concat (s1, s2) /* Name starts in column zero here */char *s1, *s2;
{ /* Open brace in column zero here */  ...

Aside from this, I prefer code formatted like this:

if (x < foo (y, z))
  haha = bar[4] + 5;
  while (z)
    haha += foo (z, z);
  return ++x + bar ();

I find it easier to read a program when it has spaces before the open-parentheses and after the commas. Especially after the commas.

Please use form feed characters to divide the program into pages at logical places (but not within a function). It does not matter just how long the pages are, since they do not have to fit on a printed page.

When you split an expression into multiple lines, split it before an operator, not after one. For example:

if (foo_this_is_long && bar > win (x, y, z)
&& remaining_condition)

Commenting Standards

Every program should start with a comment saying briefly what it is for. Example: “fmt — filter for simple filling of text”. Please put a comment on each function saying what the function does, what sorts of arguments it gets, and what the possible values of arguments mean and are used for. It is not necessary to duplicate in words the meaning of the C argument declarations, if a C type is being used in its customary fashion. If there is anything nonstandard about its use (such as an argument of type `char *’ which is really the address of the second character of a string, not the first), or any possible values that would not work the way one would expect (such as, that strings containing newlines are not guaranteed to work), be sure to say so.

Please put two spaces after the end of a sentence in your comments, so that the Emacs sentence commands will work. Also, please write complete sentences and capitalize the first word. Avoid putting a case-sensitive identifier at the beginning of a sentence, because such identifiers look strange if lower case and yet are erroneous if capitalized.

The comment on a function is much clearer if you use the argument names to speak about the argument values. Thus, “the inode number
NODE_NUM” rather than “an inode”. Our convention is to put the argument name in all caps when speaking about its value in this way.

There is usually no purpose in restating the name of the function in the comment before it, because the reader can see that for himself. There might be an exception when the comment is very long. There should be a comment on each static variable as well, like this:

/* Nonzero means truncate lines in the display;
zero means continue them. */
int truncate_lines;

Syntactic Standards

Please explicitly declare all arguments to functions. Don’t omit them just because they are ints.

Declarations of external functions and functions to appear later in the source file should all go in one place near the beginning of the file (somewhere before the first function definition in the file), or else should go in a header file. Don’t put extern declarations inside functions.

Don’t declare multiple variables in one declaration that spans lines. Start a new declaration on each line, instead. For example, instead of this:

int foo,

write either this:

int foo, bar;

or this:

int foo;
int bar;

(If they are global variables, each should have a comment preceding it anyway.)

When you have an if-else statement nested in another if statement, always put braces around the if-else. Thus, never write like this:

if (foo)
if (bar)
win ();
lose ();

always like this:

if (foo)
if (bar)
win ();
lose ();

Don’t declare both a structure tag and variables or typedefs in the same declaration. Instead, declare the structure tag separately and then use it to declare the variables or typedefs. Try to avoid assignments inside if-conditions. For example, don’t write this:

if ((foo = (char *) malloc (sizeof *foo)) == 0)
fatal ("out of memory");

instead, write this:

foo = (char *) malloc (sizeof *foo);
if (foo == 0)
fatal ("out of memory");

Naming Standards:

Please use underscores to separate words in a name, so that the Emacs word commands can be useful within them. Stick to lower case; reserve upper case for macros and enum constants, and for name-prefixes that follow a uniform convention.

For example, use names like `ignore_space_change_flag’; don’t use names like `iCantReadThis’.

Variables that indicate whether command-line options have been specified should be named after the meaning of the option, not after the option-letter. A comment should state both the exact meaning of the option and its letter. For example,

/* Ignore changes in horizontal whitespace (-b). */int ignore_space_change_flag;

Semantic Standards:

Avoid arbitrary limits on the length or number of ANY data structure, including filenames, lines, files, and symbols, by allocating all data structures dynamically. In most Unix utilities, “long lines are silently truncated”. This is not acceptable in a GNU utility. Utilities reading files should not drop null characters, or any other nonprinting characters including those with codes above 0177, except perhaps utilities specifically intended for interface to printers.

Check every system call for an error return, unless you know you wish to ignore errors. Include the system error text (from perror or equivalent) in *every* error message resulting from a failing system call, as well as the name of the file if any and the name of the utility. Just “cannot open foo.c” or “stat failed” is not sufficient.

Check every call to `malloc’ or `realloc’ to see if it returned zero. Check `realloc’ even if you are making the block smaller; in a power-of-2
system, this can require allocating new space!

In Unix, `realloc’ can destroy the storage block if it returns zero. GNU `realloc’ does not have this bug, so your utilities can assume that `realloc’ works conveniently (does not destroy the original block when it returns zero.) If you wish to run them on Unix, and wish to avoid lossage in this case, you can use the GNU `malloc’.

Do not assume anything about the contents of a block of memory after it has been passed to `free’.

When static storage is to be written in during program execution, use explicit C code to initialize it. Reserve C initialized declarations for data that will not be changed.

Try to avoid low-level interfaces to obscure Unix data structures (such as file directories, utmp, or the layout of kernel memory), since these are less likely to work compatibly. If you need to find all the files in a directory, use `readdir’ or some other high-level interface. These will be supported compatibly by GNU. GNU signal handling will probably be like that in BSD, rather than that in system V, because BSD’s is more powerful and easier to use.

Portability standards

Much of what is called “portability” in the Unix world refers to porting to different Unix versions. This is not relevant to GNU software, because its purpose is to run on top of one and only one kernel, the GNU kernel, compiled with one and only one C compiler, the GNU C compiler. The amount and kinds of variation among GNU systems on different cpu’s will be like the variation among Berkeley 4.3 systems on different cpu’s.

It is difficult to be sure exactly what facilities the GNU kernel will provide, since it isn’t finished yet. Therefore, assume you can use anything in 4.3; just avoid using the format of semi-internal data bases (utmp, directories, kmem) when there is a higher-level alternative.

You can freely assume any reasonably standard facilities in the C language, libraries or kernel, because any such facility is part of GNU’s job to support. The fact that there may exist kernels or C compilers that lack those facilities is irrelevant as long as the GNU kernel and C compiler support them.

It remains necessary to worry about differences among cpu types, such as the difference in byte ordering and alignment restrictions. However, I don’t expect 16-bit machines ever to be supported by GNU, so there is no point in spending any time to consider the possibility that an int will be less than 32 bits.

You can assume that it is reasonable to use a meg of memory. Don’t strain to reduce memory usage unless it can get to that level. If your program creates complicated data structures, just make them in core and give a fatal error if malloc returns zero.

If a program works by lines and could be applied to arbitrary user- supplied input files, it should keep only a line in memory, because this is not very hard and users will want to be able to operate on input files that are bigger than will fit in core all at once.

M. Saqib: Saqib is Master-level Senior Software Engineer with over 14 years of experience in designing and developing large-scale software and web applications. He has more than eight years experience of leading software development teams. Saqib provides consultancy to develop software systems and web services for Fortune 500 companies. He has hands-on experience in C/C++ Java, JavaScript, PHP and .NET Technologies. Saqib owns and write contents on since 2004.
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