# Copyright (C) 2001-2006, Parrot Foundation.
# $Id: c_functions.pod 49492 2010-10-10 14:40:36Z jkeenan $

=head1 NAME

docs/dev/c_functions.pod - C function decoration guidelines

=head1 Overview

Compilers have the ability to detect a wide class of potential errors in
code during the compilation phase, especially if certain metadata is
provided by the programmer to indicate details about specific operations.
This metadata is typically compiler-dependent, but using a system of
macros and our existing configuration system, we can instruct the
compiler to search for and prevent certain types of errors.

The net result is that errors (or potential errors) can be detected early
at compile-rime instead of later an runtime or during "make test".

Headerizer creates function declarations based on function
definitions. It scans the source files passed to it and extracts the
function declarations. Then it puts them into the appropriate .h file or,
in the case of static functions, back into the source file itself.

The headerizer also adds function attributes as specified by the
decorations on the source. It's important to properly-decorate functions
that are written, so that programs like headerizer can pass on important
metadata to the compiler.

Notice that not all of these decorations will have a real effect for all
compilers. In some cases, the various macros might be empty placeholders.
Also, where it says "compiler", it could also mean "lint or any
other static analysis tool like splint."

=head1 Function Parameter Decorators

=head2 What's a shim?

Think of "shim" as shorthand for "placeholder". It's 64% shorter.

GCC (and lint and splint and other analysis tools) likes to complain if
you pass an argument into a function and don't use it. If we know that we're
not going to use an argument, we can either remove the argument from
the function declaration, or mark it as unused.

Throwing the argument away is not always possible. Usually, it's because
the function is one that gets referred to by a function pointer, and
all functions of this group must have the same signature. Consider a
function with three args: Interp, Foo and Bar. Maybe a given function
doesn't use Foo, but we still have to accept Foo because all the other
functions like ours do. In this case we can use the C<UNUSED(Foo)> macro
in the body of the function to silence any compiler warnings. C<UNUSED>
lets the compiler know that we know the parameter isn't used, and that
we haven't just forgotten about it. C<UNUSED> is for cases where we don't
currently use a particular parameter, but we might in the future. If we
never will use it, mark it as a C<SHIM(Foo)> in the declaration. Here's an
example:

  void MyFunction(PARROT_INTERP, SHIM(int Foo), /* Never using Foo */
	  int Bar)
  {
      UNUSED(Bar);  /* We aren't using Bar YET */
	  ...
  }

If the interpreter structure in a function is a shim, there is a special
macro for that. 

=head2 Passing Interpreter Pointers

Most of the time, if you need an interpreter in your function,
define that argument as C<PARROT_INTERP>.  If your interpreter is
a shim, then use C<SHIM_INTERP>, not C<SHIM(PARROT_INTERP)>. 

=head2 What are input and output arguments?

Pointers are dangerous because they are so versatile. You can pass a pointer
to a function only to have that function modify the data that the pointer
is pointing to. In Parrot, we decorate all our pointer parameters with
keywords like C<ARGIN>, C<ARGOUT>, and C<ARGMOD> to specify whether the
pointer is an input only, an output only, or is modified.

Input pointers are pointers which are read, but the data they point to is
not changed. The data after the function call is the same as the data you
had before it. If you specify a parameter is an input parameter, and the
function tries to modify its contents anyway, you'll get a warning. Also,
if you pass in an uninitialized pointer, the compiler will throw a warning.

Output pointers are pointers that are passed into a function, its existing
contents are ignored, and new contents are created for it. It's called an
output because the data in the pointed-to structure are populated inside
the function and passed back out to the caller. If you have a pointer that
points to valid data and you pass it as an ARGOUT parameter, the compiler
will throw a warning. Unlike input arguments, you can typically pass an
uninitialized pointer as an ARGOUT parameter.

Modifiable, or "in-out" parameters are parameters that have both behaviors.
Some fields in it are read, some fields in it are changed.

Please note that these are only to be used on pointer types.  If you're
not absolutely sure that the argument is a pointer, don't use them.
(The "va_list" builtin type is sometimes a pointer, sometimes a struct,
depending on platform and compiler.)

Here's a simple example of a function that uses these modifiers:

  void MyFunction(PARROT_INTERP, ARGIN(char *Foo),
	  ARGOUT(int *Bar), ARGMOD(float *Baz));

=head2 NOTNULL(x)

For function arguments and variables that must never have NULL
assigned to them, or passed into them.  For example, if we were
defining C<strlen()> in Parrot, we'd do it as
C<strlen(NOTNULL(const char *p))>. All the previous pointer decorations,
C<ARGIN>, C<ARGOUT> and C<ARGMOD> imply C<NOTNULL>. The compiler will
throw a warning if it detects a null value being passed to a C<NOTNULL>
parameter.

=head2 NULLOK(x)

For function arguments and variables where it's OK to pass in NULL.
For example, if we wrote C<free()> in Parrot, it would be
C<strlen(NULLOK(void *p))>. There are variants of C<ARGIN>, C<ARGOUT>,
and C<ARGMOD> that allow NULL values: C<ARGIN_NULLOK>, C<ARGOUT_NULLOK>,
and C<ARGMOD_NULLOK>. These have the same semantics as their
non-NULLOK counterparts, except the compiler will not throw errors if
a null value is passed.

=head1 Function Decorators

In addition to the C<SHIM>, C<ARGIN>, C<ARGOUT> and C<ARGMOD> parameters
and variants for parameters, there are a number of helpful modifiers that
can be applied directly to the function declaration itself.

=head2 PARROT_WARN_UNUSED_RESULT

Tells the compiler to warn if the function is called, but the result is
ignored. For instance, on a memory allocation function you would want to
keep track of the result so that you could free it later and not cause
a memory leak.

=head2 PARROT_IGNORABLE_RESULT

Tells the compiler that it's OK to ignore the function's return value.

=head2 PARROT_MALLOC

Functions marked with this are flagged as having received C<malloc>ed
memory. This lets the compiler do analysis on memory leaks.

=head2 PARROT_CONST_FUNCTION

The function is a deterministic one that will always return the
same value if given the same arguments, every time.  Examples include
functions like C<mod> or C<max>.  An anti-example is C<rand()> which
returns a different value every time. Some compilers can do optimizations
by replacing constant functions with lookup tables, if the results are
always going to be the same.

=head2 PARROT_PURE_FUNCTION

Less stringent than PARROT_CONST_FUNCTION, these functions only
operate on their arguments and the data they point to. These functions
have no other side effects to worry about, and clever compilers may find
ways to optimize these functions. Examples include C<strlen()> or
C<strchr()>.

=head2 PARROT_DOES_NOT_RETURN

For functions that can't return, like C<Parrot_exit()> or functions that
cause exceptions to be thrown. This helps the compiler's flow analysis
which can help detect unreachable code, or opportunities for optimization.

=head2 PARROT_CANNOT_RETURN_NULL

For functions that return a pointer, but the pointer is guaranteed to not
be NULL. The compiler can help to detect null pointer dereferences, and
this hint will simplify the process.

=head2 PARROT_CAN_RETURN_NULL

For functions that return a pointer that could be null. These return values
should be tested for null values before they are used or dereferenced.

=head2 PARROT_INLINE

For functions that could be inlined by the compiler for optimization. This
is more of a hint then a command, and many compilers might ignore it
entirely. Use this instead of the C<inline> keyword.

=head2 PARROT_EXPORT

For functions that are important API functions.

{{TODO: More detail is needed on this}}

=head1 Examples

    PARROT_EXPORT
    PARROT_WARN_UNUSED_RESULT
    INTVAL
    Parrot_str_find_index(PARROT_INTERP, NOTNULL(const STRING *s),
            NOTNULL(const STRING *s2), INTVAL start)


C<Parrot_str_find_index> is part of the Parrot API, and returns an INTVAL. The
interpreter is used somewhere in the function. String C<s> and C<s2>
cannot be NULL. If the calling function ignores the return value,
it's an error, because you'd never want to call C<Parrot_str_find_index()>
without wanting to know its value.

    PARROT_EXPORT
    PARROT_PURE_FUNCTION
    INTVAL
    parrot_hash_size(SHIM_INTERP, NOTNULL(const Hash *hash))
    {
        return hash->entries;
    }

This function is a pure function because it only looks at its parameters
or global memory. The interpreter doesn't get used, but needs to be
passed because all PARROT_EXPORT functions have interpreters passed, so is
flagged as a SHIM_INTERP.

We could put C<PARROT_WARN_UNUSED_RESULT> on this function, but since
all C<PARROT_PURE_FUNCTION>s and C<PARROT_CONST_FUNCTION>s get flagged
that way anyway, there's no need.

