PDD 7: Conventions and Guidelines for Parrot Source Code

Abstract

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

One of the criticisms of Perl 5 is that its 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. We don't intend to make the same mistake when writing Parrot. Hence this document.

We define three classes of conventions:

"must"
"should"
"may"

Note that since Parrot is substantially implemented in C, these rules apply to C language source code unless otherwise specified.

Implementation

Language Standards and Portability

C code must generally depend on only those language and library features specified by the ISO C89 standard.

In addition, C code may assume that any pointer value can be coerced to an integral type (no smaller than typedef INTVAL in Parrot), then back to its original type, without loss.

Also C code may assume that there is a single NULL pointer representation and that it consists of a number, usually 4 or 8, of '\0' chars in memory.

C code that makes assumptions beyond these must depend on the configuration system, either to not compile an entire non-portable source where it will not work, or to provide an appropriate #ifdef macro.

Perl code must be written for Perl 5.8.0 and all later versions.

Perl code may use features not available in Perl 5.8.0 only if it is not vital to Parrot, and if it uses $^O and $] to degrade or fail gracefully when it is run where the features it depends on are not available.

Code Formatting

The following must apply:

Source line width is limited to 100 characters. Exceptions can be made for technical requirements, but not for style reasons. And please bear in mind that very long lines can be hard to read.
Indentation must consist only of spaces. (Tab characters just complicate things.)
C and Perl code must be indented four columns per nesting level.
Preprocessor #directives must be indented two columns per nesting level, with two exceptions: neither PARROT_IN_CORE nor the outermost _GUARD #ifdefs cause the level of indenting to increase.
Labels (including case labels) must be outdented two columns relative to the code they label.
Closing braces for control structures must line up vertically with the start of the control structures; e.g. } that closes an if must line up with the if.
Long lines, when split, must use at least one extra level of indentation on the continued line.
Cuddled elses are forbidden: i.e. avoid } else { .
C macro parameters must be parenthesized in macro bodies, to allow expressions passed as arguments; e.g.:

    #define POBJ_FLAG(n) ((UINTVAL)1 << (n))

The following should apply:

In function definitions, the function name must be on the left margin, with the return type on the previous line.
In function declarations (e.g. in header files), the function name must be on the same line as the return type.
Pointer types should be written with separation between the star and the base type, e.g. Interp *foo, but not Interp* foo.
To distinguish keywords from function calls visually, there should be at least one space between a C keyword and any subsequent open parenthesis, e.g. return (x+y)*2. There should be no space between a function name and the following open parenthesis, e.g. z = foo(x+y)*2
Use patterns of formatting to indicate patterns of semantics. Similar items should look similar, as the language permits. Note that some dimensions of similarity are incidental, not worth emphasizing; e.g. "these are all ints".
Binary operators (except . and ->) should have at least one space on either side; there should be no space between unary operators and their operands; parentheses should not have space immediately after the opening parenthesis nor immediately before the closing parenthesis; commas should have at least one space after, but not before; e.g.:

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

Use vertical alignment for clarity of parallelism. Compare this (bad):

     foo = 1 + 100;
     x = 100 + 1;
     whatever = 100 + 100;

... to this (good):

     foo      =   1 + 100;
     x        = 100 +   1;
     whatever = 100 + 100;

Do not routinely put single statements in statement blocks.

(Note that formatting consistency trumps this rule. For example, a long if/else if chain is easier to read if all (or none) of the conditional code is in blocks.)

Return values should not be parenthesized without need. It may be necessary to parenthesize a long return expression so that a smart editor will properly indent it.
When assigning inside a conditional, use extra parentheses, e.g. if (a && (b = c)) ... or if ((a = b)) ....
When splitting a long line at a binary operator (other than comma), the split should be before the operator, so that the continued line looks like one, e.g.:

    something_long_here = something_very_long + something_else_also_long
            - something_which_also_could_be_long;

When splitting a long line inside parentheses (or brackets), the continuation should be indented to the right of the innermost unclosed punctuation, e.g.:

    z = foo(bar + baz(something_very_long_here
                       * something_else_very_long),
            corge);

Code Structure

The following must apply:

C code must use C-style comments only, i.e. /* comment */. (Not all C compilers handle C++-style comments.)
Structure types must have tags.
Functions must have prototypes in scope at the point of use. Prototypes for extern functions must appear only in header files. If static functions are defined before use, their definitions serve as prototypes.
Parameters in function prototypes must be named. These names should match the parameters in the function definition.
Variable names must be included for all function parameters in the function declarations.
Header files must be wrapped with guard macros to prevent header redefinition. The guard macro must begin with PARROT_, followed by unique and descriptive text identifying the header file (usually the directory path and filename), and end with a _GUARD suffix. The matching #endif must have the guard macro name in a comment, to prevent confusion. For example, a file named parrot/foo.h might look like:

    #ifndef PARROT_FOO_H_GUARD
    #define PARROT_FOO_H_GUARD

    #include "parrot/config.h"
    #ifdef PARROT_HAS_FEATURE_FOO
    #  define FOO_TYPE bar
    typedef struct foo {
        ...
    } foo_t;
    #endif /* PARROT_HAS_FEATURE_FOO */

    #endif /* PARROT_FOO_H_GUARD */

The following should apply

Structure types should have typedefs with the same name as their tags, e.g.:

    typedef struct Foo {
        ...
    } Foo;

Avoid double negatives, e.g. #ifndef NO_FEATURE_FOO.
Do not compare directly against NULL, 0, or FALSE. Instead, write a boolean test, e.g. if (!foo) ....

However, the sense of the expression being checked must be a boolean. Specifically, strcmp() and its brethren are three-state returns.

    if ( !strcmp(x,y) )     # BAD, checks for if x and y match, but looks
                            # like it is checking that they do NOT match.
    if ( strcmp(x,y) == 0 ) # GOOD, checks proper return value
    if ( STREQ(x,y) )       # GOOD, uses boolean wrapper macro

(Note: PMC * values should be checked for nullity with the PMC_IS_NULL macro, unfortunately leading to violations of the double-negative rule.)

Avoid dependency on "FIXME" and "TODO" labels: use the external bug tracking system. If a bug must be fixed soon, use "XXX" and put a ticket in the bug tracking system. This means that each "XXX" should have a Trac ticket number next to it.

Smart Editor Style Support

All developers using Emacs must ensure that their Emacs instances load the elisp source file editor/parrot.el before opening Parrot source files. See "README.pod" in editor for instructions.

All source files must end with an editor instruction coda:

C source files, and files largely consisting of C (e.g. yacc, lex, PMC, and opcode source files), must end with this coda:

   /*
    * Local variables:
    *   c-file-style: "parrot"
    * End:
    * vim: expandtab shiftwidth=4:
    */

Make source files must end with this coda:

    # Local Variables:
    #   mode: makefile
    # End:
    # vim: ft=make:

Perl source files must end with this coda:

    # Local Variables:
    #   mode: cperl
    #   cperl-indent-level: 4
    #   fill-column: 100
    # End:
    # vim: expandtab shiftwidth=4:

Exception: Files with __END__ or __DATA__ blocks do not require the coda. This is at least until there is some consensus as to how solve the issue of using editor hints in files with such blocks.

PIR source files should end with this coda:

    # Local Variables:
    #   mode: pir
    #   fill-column: 100
    # End:
    # vim: expandtab shiftwidth=4 ft=pir:

{{ XXX - Proper formatting and syntax coloring of C code under Emacs requires that Emacs know about typedefs. We should provide a simple script to update a list of typedefs, and parrot.el should read it or contain it. }}

Portability

Parrot 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, size of standard type, or anything else that varies from system to system.

Since most of Parrot's development uses GNU C, you might accidentally depend on a GNU feature without noticing. To avoid this, know what features of gcc are GNU extensions, and use them only when they're protected by #ifdefs.

Defensive Programming

Use Parrot data structures instead of C strings and arrays

C arrays, including strings, are very sharp tools without safety guards, and Parrot is a large program maintained by many people. Therefore:

Don't use a char * when a Parrot STRING would suffice. Don't use a C array when a Parrot array PMC would suffice. If you do use a char * or C array, check and recheck your code for even the slightest possibility of buffer overflow or memory leak.

Note that efficiency of some low-level operations may be a reason to break this rule. Be prepared to justify your choices to a jury of your peers.

Pass only `unsigned char` to `isxxx()` and `toxxx()`

Pass only values in the range of unsigned char (and the special value -1, a.k.a. EOF) to the isxxx() and toxxx() library functions. Passing signed characters to these functions is a very common error and leads to incorrect behavior at best and crashes at worst. And under most of the compilers Parrot targets, char is signed.

The `const` keyword on arguments

Use the const keyword as often as possible on pointers. It lets the compiler know when you intend to modify the contents of something. For example, take this definition:

    int strlen(const char *p);

The const qualifier tells the compiler that the argument will not be modified. The compiler can then tell you that this is an uninitialized variable:

    char *p;
    int n = strlen(p);

Without the const, the compiler has to assume that strlen() is actually initializing the contents of p.

The `const` keyword on variables

If you're declaring a temporary pointer, declare it const, with the const to the right of the *, to indicate that the pointer should not be modified.

    Wango * const w = get_current_wango();
    w->min  = 0;
    w->max  = 14;
    w->name = "Ted";

This prevents you from modifying w inadvertently.

    new_wango = w++; /* Error */

If you're not going to modify the target of the pointer, put a const to the left of the type, as in:

    const Wango * const w = get_current_wango();
    if (n < wango->min || n > wango->max) {
        /* do something */
    }

Localizing variables

Declare variables in the innermost scope possible.

    if (foo) {
        int i;
        for (i = 0; i < n; i++)
            do_something(i);
    }

Don't reuse unrelated variables. Localize as much as possible, even if the variables happen to have the same names.

    if (foo) {
        int i;
        for (i = 0; i < n; i++)
            do_something(i);
    }
    else {
        int i;
        for (i = 14; i > 0; i--)
            do_something_else(i * i);
    }

You could hoist the int i; outside the test, but then you'd have an i that's visible after it's used, which is confusing at best.

Naming Conventions

Filenames
Names of code entities
Global Variables

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.

Developer files
Per-section comments
Per-entity comments
Optimizations
General comments
Copyright notice

Extensibility

Over the lifetime of Parrot, the source code will undergo many major changes never envisaged by its original authors. 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 source 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: c.f. 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.

Performance

We want Parrot 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 optimization 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.

If you do put an optimization 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 optimization 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.

Exemptions

Not all files can strictly fall under these guidelines as they are automatically generated by other tools, or are external files included in the Parrot repository for convenience. Such files include the C header and source files automatically generated by (f)lex and yacc/bison, and some of the Perl modules under the lib/ directory.

To exempt a file (or directory of files) from checking by the coding standards tests, one must edit the appropriate exemption list within lib/Parrot/Distribution.pm (in either of the methods is_c_exemption() or is_perl_exemption()). One can use wildcards in the list to exempt, for example, all files under a given directory.

References

None.