oldlinux-files/gnu/standards.text

GNU Coding Standards				last updated 1 May 90

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 internally 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.

Other Contributors:

If someone else sends you a piece of code to add to the program you
are working on, we need legal papers to use it--the same sort of legal
papers we will need to get from you.  EACH significant contributor to
a program must sign some sort of legal papers in order for us to have
clear title to the program.  The main author alone is not enough.

So, before adding in any contributions from other people, tell us
so we can arrange to get the papers.  Then wait until we tell you
that we have received the signed papers, before you actually use the
contribution.

This applies both before you release the program and afterward.  If
you receive diffs to fix a bug, and they make significant change, we
need legal papers for it.

You don't need papers for changes of a few lines here or there, since
they are not significant for copyright purposes.  Also, you don't need
papers if all you get from the suggestion is some ideas, not actual code
which you use.  For example, if you write a different solution to the
problem, you don't need to get papers.

I know this is frustrating; it's frustrating for us as well.  But if
you don't wait, you are going out on a limb--for example, what if the
contributor's employer won't sign a disclaimer?  You might have to take
that code out again!

The very worst thing is if you forget to tell us about the other
contributor.  We could be very embarrassed in court some day as a
result.

Compatibility standards:

With certain exceptions, utility programs and libraries for GNU should
be upward compatible with those in Berkeley Unix, and upward
compatible with ANSI C if ANSI C specifies them, and upward compatible
with POSIX if POSIX specifies them.

When these standards conflict, it is useful to offer compatibility
modes for each of them.

ANSI C and POSIX prohibit many kinds of extensions.  Feel free to
make the extensions anyway, and include a -ansi or -compatible option
to turn them off.  However, if the extension has a significant chance
of breaking any real programs or scripts, then it is not really upward
compatible.  Try to redesign its interface.

When a feature is used only by users (not by programs or command
files), and it is done poorly in Unix, feel free 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 */
  ...
}

or, if you want to use ANSI C, format the definition like this:

static char *
concat (char *s1, char *s2)
{
  ...
}

In ANSI C, if the arguments don't fit nicely on one line,
split it like this:

int
lots_of_args (int an_integer, long a_long, short a_short,
              double a_double, float a_float)
...

For the body of the function, we prefer code formatted like this:

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

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

When you split an expression into multiple lines, split it
before an operator, not after one.  Here is the right way:

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

Try to avoid having two operators of different precedence at the same
level of indentation.  For example, don't write this:

      mode = (inmode[j] == VOIDmode
	      || GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
	      ? outmode[j] : inmode[j]);

Instead, use extra parentheses so that the indentation shows the nesting:

      mode = ((inmode[j] == VOIDmode
	       || (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
	      ? outmode[j] : inmode[j]);

Insert extra parentheses so that Emacs will indent the code properly.
For example, the following indentation looks nice if you do it by hand,
but Emacs would mess it up:

    v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
	+ rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;

But adding a set of parentheses solves the problem:

    v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
	 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);

Format do-while statements like this:

   do
     {
       a = foo (a);
     }
   while (a > 0);

Please use formfeed characters (^L) 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.  The formfeeds should appear alone on their lines,
just as they do in this file.

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.

Also explain the significance of the return value, if there is one.

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.  If a lower-case
identifer comes at the beginning of a sentence, don't capitalize it!
Changing the spelling makes it a different identifier.  If you don't
like starting a sentence with a lower case letter, write the sentence
differently (e.g. "The identifier lower-case is ...").

The comment on a function is much clearer if you use the argument
names to speak about the argument values.  The variable name itself
should be lower case, but write it in upper case when you are speaking
about the value rather than the variable itself.  Thus, "the inode
number NODE_NUM" rather than "an inode".

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 so long that the function
itself would be off the bottom of the screen.

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;

Every #endif should have a comment, except in the case of short conditionals
(just a few lines) that are not nested.  The comment should state the condition
of the conditional that is ending, *including its sense*.  #else should have
a comment describing the condition *and sense* of the code that follows.
For example:

    #ifdef foo
      ...
    #else /* not foo */
      ...
    #endif /* not foo */

but

    #ifndef foo
      ...
    #else /* foo */
      ...
    #endif /* foo */

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,
          bar;

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 ();
      else
	lose ();

always like this:

    if (foo)
      {
	if (bar)
	  win ();
	else
	  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 ("virtual memory exhausted");

instead, write this:

  foo = (char *) malloc (sizeof *foo);
  if (foo == 0)
    fatal ("virtual memory exhausted");

Don't make the program ugly to placate lint.  Please don't insert any
casts to void.  Zero without a cast is perfectly fine as a null
pointer constant.

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;

When you want to define names with constant integer values, use `enum'
rather than `#define'.  GDB knows about enumeration constants.

Use file names of 14 characters or less, to avoid creating gratuitous
problems on System V.

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 system
that rounds block sizes to a power of 2, `realloc' may get a different
block if you ask for less space.

In Unix, `realloc' can destroy the storage block if it returns zero.
GNU `realloc' does not have this bug: if it fails, the original block
is unchanged.  Feel free to assume the bug is fixed.  If you wish to
run your program on Unix, and wish to avoid lossage in this case, you
can use the GNU `malloc'.

You must expect `free' to alter the contents of the block that was
freed.  Anything you want to fetch from the block, you must fetch
before calling `free'.

Use `getopt' to decode arguments, unless the argument syntax makes this
unreasonable.

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.

In error checks that detect "impossible" conditions, just abort.
There is usually no point in printing any message.  These checks
indicate the existence of bugs.  Whoever wants to fix the bugs will
have to read the source code and run a debugger.  So explain the
problem with comments in the source.  The relevant data will be in
variables, which are easy to examine with the debugger, so there is no
point moving them elsewhere.

Library standards:

Try to make library functions reentrant.  If they need to do dynamic
storage allocation, at least try to avoid any nonreentrancy aside from
that of malloc itself.

Here are certain name conventions for libraries, to avoid name
conflicts.

Choose a name prefix for the library, more than two characters long.
All external function and variable names should start with this
prefix.  In addition, there should only be one of these in any given
library member.  This usually means putting each one in a separate
source file.

An exception can be made when two external symbols are always used
together, so that no reasonable program could use one without the
other; then they can both go in the same file.

External symbols that are not documented entry points for the user
should have names beginning with `_'.  They should also contain
the chosen name prefix for the library, to prevent collisions with
other libraries.  These can go in the same files with user entry 
points if you like.

Static functions and variables can be used as you like and need not
fit any naming convention.

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 (e.g., directories) when there is a higher-level alternative
(readdir).

You can freely assume any reasonably standard facilities in the C
language, libraries or kernel, because we will find it necessary to
support these facilities in the full GNU system, whether or not we
have already done so.  The fact that there may exist kernels or C
compilers that lack these 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.  It's
unlikely that 16-bit machines will ever 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 all pointers have the same format, regardless
of the type they point to, and that this is really an integer.
There are some weird machines where this isn't true, but they aren't
important; don't waste time catering to them.  Besides, eventually
we will put function prototypes into all GNU programs, and that will
probably make your program work even on weird machines.

Since some important machines (including the 68000) are big-endian,
it is important not to assume that the addres of an int object
is also the address of its least-significant byte.  Thus, don't
make the following mistake:

                int c;
                ...
                while ((c = getchar()) != EOF)
                        write(file_descriptor, &c, 1);

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.

Utility Interface Standards:

Please don't make the behavior of a utility depend on the name used
to invoke it.  It is useful sometimes to make a link to a utility
with a different name, and that should not change what it does.

Instead, use a run time option or a compilation switch or both
to select among the alternate behaviors.

It is a good idea to follow the Posix guidelines for the command-line
options of a program.  The easiest way to do this is to use getopt to
parse them.  Note that the GNU version of getopt will normally permit
options anywhere among the arguments unless the special argument `--' 
is used.  This is not what Posix specifies; it is a GNU extension.

Please define long-named options that are equivalent to the
single-letter Unix-style options.  We hope to make GNU more user
friendly this way.  This is easy to do with the GNU version of
getopt.

It is usually a good idea for file names given as ordinary arguments
to be input files only; any output files would be specified using
options (preferably -o).  Even if you allow an output file name as an
ordinary argument for compatibility, try to provide a suitable option
as well.  This will lead to more consistency among GNU utilities, so
that there are fewer idiosyncracies for users to remember.

Documentation Standards:

Please use Texinfo for documenting GNU programs.  See the Texinfo
manual, either the hardcopy or the version in the GNU Emacs Info
sub-system (C-h i).

See existing GNU texinfo files (e.g. those under the man/ directory in
the GNU Emacs Distribution) for examples.

Write well.  Document all -switches and their interactions.  Document
all commands.  Give examples of their use.  Tell people how and when
to use the features for what they typically want to accomplish, not
just what each feature does.