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Final draft: Conventions and Guidelines for Perl Source Code

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From:
Dave Mitchell
Date:
August 6, 2001 05:22
Subject:
Final draft: Conventions and Guidelines for Perl Source Code
Message ID:
200108061221.NAA21091@gizmo.fdgroup.co.uk
Here's the final draft. Please consider it formally submitted; someone
(Dan? Ask? Adam?) needs to assign it a number, please.

Dave.


=head1 TITLE

Conventions and Guidelines for Perl Source Code

=head1 VERSION

=head2 CURRENT

   Maintainer: Dave Mitchell <davem@fdgroup.com>
   Class: Internals
   PDD Number: TBD 
   Version: 1
   Status: Developing
   Last Modified: 6 August 2001
   PDD Format: 1
   Language: English

=head2 HISTORY

Based on an earlier draft which covered only code comments.

=head1 CHANGES

None. First version

=head1 ABSTRACT

This document describes the various rules, guidelines and advice for
those wishing to contribute to the source code of Perl, in such areas
as code structure, naming conventions, comments etc.

=head1 DESCRIPTION

One of the criticisms of Perl 5 is that it's source code is
impenetrable to newcomers, due to such things as inconsistent or
obscure variable naming conventions, lack of comments in the source
code, and so on.  Hence this document.

We define three classes of conventions. Those that say I<must> are
mandatory, and code will not be accepted (apart from in exceptional
circumstances) unless it follows these rules. Those that say I<should>
are strong guidelines that should normally be be followed unless there
is a sensible reason to do otherwise.  Finally, where it says I<may>,
this is tentative suggestion to be used at your discretion.

Note this this particular PDD makes some recommendations that are
specific to the C programming language. This does not preclude Perl
being implemented in other languages, but in this case, additional PDDs
may need to be authored for the extra language-specific features.

=head1 IMPLEMENTATION

=head2 Coding style

The following I<must> apply:

=over 4

=item *

4-wide indents for code and 2-wide indents for nested CPP #directives.
All indentation must consist of spaces, no tabs (for ease of patching).

To ensure that tabs aren't inadvertently used for indentation, the following
boilerplate code must appear at the bottom of each source file. (This
rule may be rescinded if I'm ever threatened with a lynching....)

    /*
    * Local variables:
    * c-indentation-style: bsd
    * c-basic-offset: 4
    * indent-tabs-mode: nil
    * End
    *
    * vim:expandtab shiftwidth=4:
    */


=item *

Any other tabs are assumed to be on an 8-character boundary.

=item *

ANSI C function prototypes

=item *

"K&R" style for indenting control constructs: ie the closing C<}> should
line up with the opening C<if> etc.

=item *

When a conditional spans multiple lines, the opening brace must line up
with the "if" or "while", or be at the end-of-line otherwise.

=item *

Uncuddled elses: ie avoid  C<} else {>

=item *

No C++ style comments (C<//>): some C compilers may choke on them

=item *

Mark places that need to be revisited with XXX (and preferably your
initials too), and revisit often!

=item *

In function definitions, the name starts in column 0, with the
return type on the previous line

=item *

Single space after keywords that are followed by parens, eg
C<return (x+y)*2>, but no space between function name and following paren,
eg C<z = foo(x+y)*2>

=back

The following I<should> apply

=over 4

=item *

Do not exceed 79 columns

=item *

C<return foo;> rather than C<return (foo);>

=item *

C<if (!foo) ...> rather than C<if (foo == FALSE) ...> etc.

=item *

Avoid assignments in conditionals, but if they're unavoidable, use
Extra paren, e.g. C<if (a && (b = c)) ...>

=item *

Avoid double negatives, eg C<#ifndef NO_FEATURE_FOO>

=item *

binary operators should have a space on either side; parentheses should
not have space immediately after the opening paren nor immediately before
the closing paren, commas should have space after but not before, eg

        x = (a + b) * f(c, d / e)

=item *

Long lines should be split before an operator so that it is immediately
obvious when scanning the LHS of the code that this has happened, and two
extra levels of indent should be used.

=item *

and/or split on matching parens, with the content on separate line(s) and
with one extra indent:

    do_arbitrary_function(
	list_of_parameters_with_long_names, or_complex_subexpression(
	    of_more_params, or_expressions + 1
	)
    );



=back


=head2 Naming conventions

=over 4

=item Subsystems and APIs

Perl core will be split into a number of subsystems, each with an
associated API. For the purposes of naming files, data structures, etc,
each subsystem will be assigned a short nickname, eg pmc, gc, io.  All
code within the core will belong to a subsystem; miscellaneous code
with no obvious home will be placed in the special subsystem called
misc.

=item Filenames

Filenames must be assumed to be case-insensitive, in the sense that
that you may not have two different files called Foo and foo. Normal
source-code filenames should be all lower-case; filenames with
upper-case letters in them are reserved for notice-me-first files such
as README, and for files which need some sort of pre-processing applied
to them or which do the preprocessing - eg a script F<foo.SH> might
read F<foo.TEMPLATE> and output F<foo.c>.

The characters making up filenames must be chosen from the ASCII set
A-Z,a-z,0-9 plus .-_

An underscore should be used to separate words rather than a hyphen
(-).  A file should not normally have more than a single '.' in it, and
this should be used to denote a suffix of some description. The
filename must still be unique if the main part is truncated to 8
characters and any suffix truncated to 3 characters. Ideally, filenames
should restricted to 8.3 in the first place, but this is not
essential.


Each subsystem I<foo> should supply the following files. This arrangement
is based on the assumption that each subsystem will - as far as is
practical - present an opaque interface to all other subsystems within the
core, as well as to extensions and embeddings.

=over 4

=item foo.h

This contains all the declarations needed for external users of that
API (and nothing more), ie it defines the API. It is permissible for
the API to include different or extra functionality when used by other
parts of the core, compared with its use in extensions and embeddings.
In this case, the extra stuff within the file is enabled by testing for
the macro PERL_IN_CORE.

=item foo_private.h

This contains declarations used internally by that subsystem, and which
must only be included within source files associated the subsystem.
This file defines the macro PERL_IN_FOO so that code knows when it is
being used within that subsystem. The file will also contain all the
'convenience' macros used to define shorter working names for functions
without the perl prefix (see below).

=item foo_globals.h

This file contains the declaration of a single structure containing the
private global variables used by the subsystem (see the section on
globals below for more details).

=item foo.sym

This file (format and contents TBD) contains information about global
symbols associated with the subsystem, and may be used by scripts to
auto-generate such stuff as the include files mentioned above, linker
map tables, documentation etc, based upon portability and extensibility
requirements.

=item foo_bar.[ch] etc

All other source files associated with the subsystem will have the prefix
foo_

=back

The top-level structure of the Perl source tarball should be as follows:

    /README, etc    a few top-level documents
    /doc/           Assorted miscellaneous documentation
    /pdd/           The current PDDs
    /perl/          The source code for Perl itself
    /perl/os/foo/   OS-specific source code for operating system foo
    /foo/           The source code for other families of binaries (eg /x2p/)
    /hints/         per-OS build hints files
    /scripts/       scripts needed during the building process
    /t/             scripts used by make test
    /lib/           perl modules ready for installation
    /ext/           perl modules that need compiling
    /pod/           src of the Perl man pages etc

plus others as it becomes necessary.

=item Names of code entities

Code entities such as variables, functions, macros etc (apart from strictly
local ones) should all follow these general guidelines.

=over 4

=item *

Multiple words or components should be separated with underscores rather
than using tricks such as capitalisation, eg C<new_foo_bar> rather than
C<NewFooBar> or (gasp) C<newfoobar>.

=item *

The names of entities should err on the side of verbosity, eg
C<create_foo_from_bar()> in preference to C<ct_foo_bar()>. Avoid cryptic
abbreviations wherever possible.

=item *

All entities should be prefixed with the name of the subsystem they appear
in, eg C<pmc_foo()>, C<struct io_bar>. They should be further prefixed
with the word 'perl' if they have external visibility or linkage,
namely, non-static functions, plus macros and typedefs etc which appear
in public header files. (Global variables are handled specially; see below.)
For example:

    perlpmc_foo()
    struct perlio_bar
    typedef struct perlio_bar Perlio_bar
    #define PERLPMC_readonly_TEST ...

In the specific case of the use of global variables and functions
within a subsystem, convenience macros will be defined (in
foo_private.h) that allow use of the shortened name in the case of
functions (ie C<pmc_foo()> instead of C<perlpmc_foo()>), and hide the
real representation in the case of global variables.


=item *

Variables and structure names should be all lower-case, eg C<pmc_foo>.

=item *

structure elements should be all lower-case, and the first component of
the name should incorporate the structure's name or an abbreviation of it.

=item *

Typedef names should be lower-case except for the first letter, eg
C<Foo_bar>. The exception to this is when the first component is a
short abbreviation, in which case the whole first component may be made
uppercase for readability purposes, eg C<IO_foo> rather than
C<Io_foo>.  Structures should generally be typedefed.

=item *

Macros should have their first component uppercase, and the majority
of the remaining components should be likewise. Where there is a family
of macros, the variable part can be indicated in lowercase, eg
C<PMC_foo_FLAG>, C<PMC_bar_FLAG>, ....

=item *

A macro which defines a flag bit should be suffixed with C<_FLAG>, eg
C<PMC_readonly_FLAG> (although you probably want to use an C<enum>
instead.)

=item *

A macro which tests a flag bit should be suffixed with C<_TEST>, eg
C<if (PMC_readonly_TEST(foo)) ...>

=item *

A macro which sets a flag bit should be suffixed with C<_SET>, eg
C<PMC_readonly_SET(foo);>

=item *

A macro which clears a flag bit should be suffixed with C<_CLEAR>, eg
C<PMC_readonly_CLEAR(foo);>

=item *

A macro defining a mask of flag bits should be suffixed with C<_MASK>,
eg C<foo &= ~PMC_STATUS_MASK> (but see notes on extensibility below).

=item *

Macros can be defined to cover common flag combinations, in which case they
should have C<_SETALL>, C<CLEARALL>, C<_TESTALL> or <_TESTANY> suffixes
as appropriate, to indicate aggregate bits, eg
C<PMC_valid_CLEARALL(foo)>

=item *

A macro defining an auto-configuration value should be prefixed with
C<HAS_>, eg C<HAS_BROKEN_FLOCK>, C<HAS_EBCDIC>.

=item *

A macro indicating the compilation 'location' should be prefixed with
C<IN_>, eg C<PERL_IN_CORE>, C<PERL_IN_PMC>, C<PERL_IN_X2P>. Individual
include file visitations should be marked with C<PERL_IN_FOO_H> for
file foo.h

=item *

A macro indicating major compilation switches should be prefixed with
C<USE_>, eg C<PERL_USE_STDIO>, C<USE_MULTIPLICITY>.

=item *

A macro that may declare stuff and thus needs to be at the start of a
block should be prefixed with C<DECL_>, eg C<DECL_SAVE_STACK>. Note
that macros which implicitly declare and then use variables are strongly
discouraged, unless it is essential for portability or extensibility.
The following are in decreasing preference style-wise, but increasing
preference extensibility-wise.

    { Stack sp = GETSTACK;  x = POPSTACK(sp) ... /* sp is an auto variable */
    { DECL_STACK(sp);  x = POPSTACK(sp); ... /* sp may or may not be auto */
    { DECL_STACK; x = POPSTACK; ... /* anybody's guess */


=back

=item Global Variables

Global variables must never be accessed directly outside the subsystem
in which they are used. Some other method, such as accessor functions,
must be provided by that subsystem's API. (For efficiency the 'accessor
functions' may occasionally actually be macros, but then the rule still
applies in spirit at least).

All global variables needed for the internal use of a particular
subsystem should all be declared within a single struct called
foo_globals for subsystem foo. This structure's declaration is placed
in the file foo_globals.h. Then somewhere a single compound structure
will be declared which has as members the individual structures from
each subsystem. Instances of this structure are then defined as a
one-off global variable, or as per-thread instances, or whatever is
required.

[Actually, three separate structures may be required, for global,
per-interpreter and per-thread variables.]

Within an individual subsystem, macros are defined for each global
variable of the form GLOBAL_foo (the name being deliberately clunky).
So we might for example have the following macros:

	/* perl_core.h or similar */

	#ifdef HAS_THREADS
	#  define GLOBALS_BASE (aTHX_->globals)
	#else
	#  define GLOBALS_BASE (Perl_globals)
	#endif

	/* pmc_private.h */

	#define GLOBAL_foo   GLOBALS_BASE.pmc.foo
	#define GLOBAL_bar   GLOBALS_BASE.pmc.bar
	... etc ...

=back


=head2 Code comments

The importance of good code documentation cannot be stressed enough.
To make your code understandable by others (and indeed by yourself when
you come to make changes a year later :-), the following conventions
apply to all source files.

=over 4

=item Developer files

For each source file (eg a F<foo.c> F<foo.h> pair), there should be an
accompanying developer file called F<foo.dev>. This text file contains
documentation on all the implementation decisions associated with the
source file. (Note that this is in contrast to PDDs, which describe
design decisions). This is the place for mini-essays on how to avoid
overflows in unsigned arithmetic, or on the pros and cons of differing
hash algorithms, and why the current one was chosen, and how it works.
In principle, someone coming to a particular source file for the first
time should be able to read the F<.dev> file and gain an immediate
overview of what the source file is for, the algorithms it implements,
etc.

Currently no particular format or structure is imposed on the developer
file, but it should have as a minimum the following sections:

=over 4

=item Overview

Explain the purpose of the source file.

=item Data structures and algorithms

Explain how it all works.

=item History

Record major changes to the file, eg "we moved from a linked list to a
hash table implementation for storing Foos, as it was found to be much
faster".

=item Notes

Anything that may be of interest to your successors, eg benchmarks of
differing hash algorithms, essays on how to do integer arithmetic.

=item References

Links to pages and books that may contain useful info relevant to the
stuff going on in the code - eg the book you stole the hash function
from.

=back

=item Top-of-file comments

In addition to the copyright message and optional quote, each source
file must have a short comment at the top explaining the basic purpose
of the file, eg

	/* pp_hot.c - like pp.c, this file contains functions that operate
	 * on the contents of the stack (pp == 'push & pop'), but in this
	 * case, frequently used ('hot') functions have been moved here
	 * from pp.c to (hopefully) improve CPU cache hit rates.
	 */

=item Per-section comments

If there is a collection of functions, structures or whatever which are
grouped together and have a common theme or purpose, there should be a
general comment at the start of the section briefly explaining their
overall purpose. (Detailed essays should be left to the developer
file). If there is really only one section, then the top-of-file
comment already satisfies this requirement.

	/* This section deals with 'arenas', which are chunks of PMCs of
	 * a particular type that are allocated in one go. Individual
	 * requests can then be made to grab or release individual PMCs.
	 * For each type foo, there is a pointer called GLOBAL_arena_foo
	 * which blah blah....
	 */

=item Per-entity comments

Every non-local named entity, be it a function, variable, structure,
macro or whatever, must have an accompanying comment explaining it's
purpose.  This comment must be in the special format described below,
in order to allow automatic extraction by tools - for example, to
generate per API man pages, B<perldoc -f> style utilities and so on.

Often the comment need only be a single line explaining its purpose,
but sometimes more explanation may be needed. For example, "return an
Integer Foo to its allocation pool" may be enough to demystify the
function C<del_I_foo()>

Each comment should be of the form

    /*=for api apiname entityname[,entityname..] flags ....(TBC)....
    comments....
    */

where I<apiname> is the API the entity belongs to, eg I<pmc>, and entity
name is the actual name of the function or macro or whatever. Where
there is a whole family of entities that have the same properties and
can be collectively described with a single comment, a list of
entity names can be provided.

TBC ...


=item Optimisations

Whenever code has deliberately been written in an odd way for
performance reasons, you should point this out - if nothing else, to
avoid some poor shmuck trying subsequently to replace it with something
'cleaner'.

    /* The loop is partially unrolled here as it makes it a lot faster.
     * See the .dev file for the full details
     */

=item General comments

While there is no need to go mad commenting every line of code, it is
immensely helpful to to provide a "running commentary" every 10 or so
lines say; if nothing else, this makes it easy to quickly locate a
specific chunk of code. Such comments are particularly useful at the top
of each major branch, eg

    if (FOO_bar_BAZ(**p+*q) <= (r-s[FOZ & FAZ_MASK]) || FLOP_2(z99)) {
	/* we're in foo mode: clean up lexicals */
	... (20 lines of gibberish) ...
    }
    else if (...) {
	/* we're in bar mode: clean up globals */
	... (20 more lines of gibberish) ...
    }
    else {
	/* we're in baz mode: self-destruct */
	....
    }

=back

=head2 Extensibility

If Perl 5 is anything to go by, the lifetime of Perl 6 will be at least
seven years. During this period, the source code will undergo many
major changes never envisaged by its original authors - cf threads,
unicode in perl 5. To this end, Your code should balance out the
assumptions that make things possible, fast or small, with the
assumptions that make it difficult to change things in future. This is
especially important for parts of the code which are exposed through
APIs - the requirements of src or binary compatibility for such things as
extensions can make it very hard to change things later on.

For example, if you define suitable macros to set/test flags in a
struct, then you can later add a second word of flags to the struct
without breaking source compatibility. (Although you might still break
binary compatibility if you're not careful.) Of the following two
methods of setting a common combination of flags, the second doesn't
assume that all the flags are contained within a single field:

    foo->flags |= (FOO_int_FLAG | FOO_num_FLAG | FOO_str_FLAG);
    FOO_valid_value_SETALL(foo);

Similarly, avoid using a char* (or {char*,length}) if it is feasible to
later use a PMC* at the same point: cf UTF-8 hash keys in Perl 5.

Of course, private code hidden behind an API can play more fast and
loose than code which gets exposed.


=head2 Portability

Related to extensibility is portability. Perl runs on many, many
platforms, and will no doubt be ported to ever more bizarre and obscure
ones over time.  You should never assume an operating system, processor
architecture, endian-ness, word size, or whatever. In particular, don't
fall into the any of the following common traps:

Internal data types and their utility functions (especially for strings)
should be used over a bare char * whenever possible. Ideally there should
be no char * in the source anywhere, and no use of C's standard string
library.

Dont assume GNU C, and don't use any GNU extensions unless protected by
#ifdefs for non-GNU-C builds.


TBC ... Any contributions welcome !!!


=head2 Performance

We want Perl to be fast. Very fast. But we also want it to be portable
and extensible. Based on the 90/10 principle, (or 80/20, or 95/5,
depending on who you speak to), most performance is gained or lost in a
few small but critical areas of code. Concentrate your optimisation
efforts there.

Note that the most overwhelmingly important factor in performance is in
choosing the correct algorithms and data structures in the first place.
Any subsequent tweaking of code is secondary to this. Also, any
tweaking that is done should as far as possible be platform
independent, or at least likely to cause speed-ups in a wide variety of
environments, and do no harm elsewhere. Only in exceptional
circumstances should assembly ever even be considered, and then only if
generic fallback code is made available that can still be used by all
other non-optimised platforms.

Probably the dominant factor (circa 2001) that effects processor
performance is the cache. Processor clock rates have increased far in
excess of of main memory access rates, and the only way for the
processor to proceed without stalling is for most of the data items it
needs to be found to hand in the cache. It is reckoned that even a 2%
cache miss rate can cause a slowdown in the region of 50%. It is for
this reason that algorithms and data structures must be designed to be
'cache-friendly'.

A typical cache may have a block size of anywhere between 4 and 256
bytes.  When a program attempts to read a word from memory and the word
is already in the cache, then processing continues unaffected.
Otherwise, the processor is typically stalled while a whole contiguous
chunk of main memory is read in and stored in a cache block. Thus,
after incurring the initial time penalty, you then get all the memory
adjacent to the initially read data item for free.  Algorithms that make
use of this fact can experience quite dramatic speedups.  For example,
the following pathological code ran four times faster on my machine by
simply swapping C<i> and C<j>.

    int a[1000][1000];
    
    ... (a gets populated) ...
    
    int i,j,k;
    for (i=0; i<1000; i++) {
	for (j=0; j<1000; j++) {
	    k += a[j][i];
	}
    }

This all boils down to: keep things near to each other that get
accessed at around the same time. (This is why the important
optimisations occur in data structure and algorithm design rather than
in the detail of the code.) This rule applies both to the layout of
different objects relative to each other, and to the relative
positioning of individual fields within a single structure.

If you do put an optimisation in, time it on as many architectures as
you can, and be suspicious of it if it slows down on any of them! Perhaps
it will be slow on other architectures too (current and future).
Perhaps it wasn't so clever after all? If the optimisation is platform
specific, you should probably put it in a platform-specific function in
a platform-specific file, rather than cluttering the main source with
zillions of #ifdefs.

And remember to document it.

Loosely speaking, Perl tends to optimise for speed rather than space,
So you may want to code for speed first, then tweak to reclaim some
space while not affecting performance.

=head1 REFERENCES


The section on coding style is based on Perl5's F<Porting/patching.pod>
by Daniel Grisinger. The section on naming conventions grew from some
suggestions by Paolo Molaro <lupus@lettere.unipd.it>. Other snippets
came from various P5Pers. The rest of it is probably my fault.



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