
=encoding utf8

=for comment
Consistent formatting of this file is achieved with:
  perl ./Porting/podtidy pod/perlhacktips.pod

=head1 NAME

perlhacktips - Tips for Perl core C code hacking

=head1 DESCRIPTION

This document will help you learn the best way to go about hacking on
the Perl core C code.  It covers common problems, debugging, profiling,
and more.

If you haven't read L<perlhack> and L<perlhacktut> yet, you might want
to do that first.

=head1 COMMON PROBLEMS

Perl source now permits some specific C99 features which we know are
supported by all platforms, but mostly plays by ANSI C89 rules.  You
don't care about some particular platform having broken Perl?  I hear
there is still a strong demand for J2EE programmers.

=head2 Perl environment problems

=over 4

=item *

Not compiling with threading

Compiling with threading (-Duseithreads) completely rewrites the
function prototypes of Perl.  You better try your changes with that.
Related to this is the difference between "Perl_-less" and "Perl_-ly"
APIs, for example:

  Perl_sv_setiv(aTHX_ ...);
  sv_setiv(...);

The first one explicitly passes in the context, which is needed for
e.g. threaded builds.  The second one does that implicitly; do not get
them mixed.  If you are not passing in a aTHX_, you will need to do a
dTHX as the first thing in the function.

See L<perlguts/"How multiple interpreters and concurrency are
supported"> for further discussion about context.

=item *

Not compiling with -DDEBUGGING

The DEBUGGING define exposes more code to the compiler, therefore more
ways for things to go wrong.  You should try it.

=item *

Introducing (non-read-only) globals

Do not introduce any modifiable globals, truly global or file static.
They are bad form and complicate multithreading and other forms of
concurrency.  The right way is to introduce them as new interpreter
variables, see F<intrpvar.h> (at the very end for binary
compatibility).

Introducing read-only (const) globals is okay, as long as you verify
with e.g. C<nm libperl.a|egrep -v ' [TURtr] '> (if your C<nm> has
BSD-style output) that the data you added really is read-only.  (If it
is, it shouldn't show up in the output of that command.)

If you want to have static strings, make them constant:

  static const char etc[] = "...";

If you want to have arrays of constant strings, note carefully the
right combination of C<const>s:

    static const char * const yippee[] =
        {"hi", "ho", "silver"};

=item *

Not exporting your new function

Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any
function that is part of the public API (the shared Perl library) to be
explicitly marked as exported.  See the discussion about F<embed.pl> in
L<perlguts>.

=item *

Exporting your new function

The new shiny result of either genuine new functionality or your
arduous refactoring is now ready and correctly exported.  So what could
possibly go wrong?

Maybe simply that your function did not need to be exported in the
first place.  Perl has a long and not so glorious history of exporting
functions that it should not have.

If the function is used only inside one source code file, make it
static.  See the discussion about F<embed.pl> in L<perlguts>.

If the function is used across several files, but intended only for
Perl's internal use (and this should be the common case), do not export
it to the public API.  See the discussion about F<embed.pl> in
L<perlguts>.

=back

=head2 C99

Starting from 5.35.5 we now permit some C99 features in the core C
source. However, code in dual life extensions still needs to be C89
only, because it needs to compile against earlier version of Perl
running on older platforms.  Also note that our headers need to also be
valid as C++, because XS extensions written in C++ need to include
them, hence I<member structure initialisers> can't be used in headers.

C99 support is still far from complete on all platforms we currently
support. As a baseline we can only assume C89 semantics with the
specific C99 features described below, which we've verified work
everywhere.  It's fine to probe for additional C99 features and use
them where available, providing there is also a fallback for compilers
that don't support the feature.  For example, we use C11 thread local
storage when available, but fall back to POSIX thread specific APIs
otherwise.

Code can use (and rely on) the following C99 features being present

=over

=item *

mixed declarations and code

=item *

64 bit integer types

For consistency with the existing source code, use the typedefs C<I64>
and C<U64>, instead of using C<long long> and C<unsigned long long>
directly.

=item *

variadic macros

    void greet(char *file, unsigned int line, char *format, ...);
    #define logged_greet(...) greet(__FILE__, __LINE__, __VA_ARGS__);

Note that C<__VA_OPT__> is standardized as of C23 and C++20.  Before
that it was a gcc extension.

=item *

declarations in for loops

    for (const char *p = message; *p; ++p) {
        putchar(*p);
    }

=item *

member structure initialisers

But not in headers, as support was only added to C++ relatively
recently.

Hence this is fine in C and XS code, but not headers:

    struct message {
        char *action;
        char *target;
    };

    struct message mcguffin = {
        .target = "member structure initialisers",
        .action = "Built"
     };

You cannot use the similar syntax for compound literals, since we also
build perl using C++ compilers:

    /* this is fine */
    struct message m = {
        .target = "some target",
        .action = "some action"
    };
    /* this is not valid in C++ */
    m = (struct message){
        .target = "some target",
        .action = "some action"
    };

While structure designators are usable, the related array designators
are not, since they aren't supported by C++ at all.

=item *

flexible array members

This is standards conformant:

    struct greeting {
        unsigned int len;
        char message[];
    };

However, the source code already uses the "unwarranted chumminess with
the compiler" hack in many places:

    struct greeting {
        unsigned int len;
        char message[1];
    };

Strictly it B<is> undefined behaviour accessing beyond C<message[0]>,
but this has been a commonly used hack since K&R times, and using it
hasn't been a practical issue anywhere (in the perl source or any other
common C code). Hence it's unclear what we would gain from actively
changing to the C99 approach.

=item *

C<//> comments

All compilers we tested support their use. Not all humans we tested
support their use.

=item *

booleans

You can use C<bool>, C<true>, and C<false> as provided by C<< <stdbool.h> >>
(or natively in C++).

=back

Code explicitly should not use any other C99 features. For example

=over 4

=item *

variable length arrays

Not supported by B<any> MSVC, and this is not going to change.

Even "variable" length arrays where the variable is a constant
expression are syntax errors under MSVC.

=item *

C99 types in C<< <stdint.h> >>

Use C<PERL_INT_FAST8_T> etc as defined in F<handy.h>

=item *

C99 format strings in C<< <inttypes.h> >>

C<snprintf> in the VMS libc only added support for C<PRIdN> etc very
recently, meaning that there are live supported installations without
this, or formats such as C<%zu>.

(perl's C<sv_catpvf> etc use parser code in F<sv.c>, which
supports the C<z> modifier, along with perl-specific formats such as
C<SVf>.)

=back

If you want to use a C99 feature not listed above then you need to do
one of

=over 4

=item *

Probe for it in F<Configure>, set a variable in F<config.sh>, and add
fallback logic in the headers for platforms which don't have it.

=item *

Write test code and verify that it works on platforms we need to
support, before relying on it unconditionally.

=back

Likely you want to repeat the same plan as we used to get the current
C99 feature set. See the message at
L<https://markmail.org/thread/odr4fjrn72u2fkpz> for the C99 probes we
used before. Note that the two most "fussy" compilers appear to be MSVC
and the vendor compiler on VMS. To date all the *nix compilers have
been far more flexible in what they support.

On *nix platforms, F<Configure> attempts to set compiler flags
appropriately. All vendor compilers that we tested defaulted to C99 (or
C11) support. However, older versions of gcc default to C89, or permit
I<most> C99 (with warnings), but forbid I<declarations in for loops>
unless C<-std=gnu99> is added. The alternative C<-std=c99> B<might>
seem better, but using it on some platforms can prevent C<< <unistd.h>
>> declaring some prototypes being declared, which breaks the build.
gcc's C<-ansi> flag implies C<-std=c89> so we can no longer set that,
hence the Configure option C<-gccansipedantic> now only adds
C<-pedantic>.

The Perl core source code files (the ones at the top level of the
source code distribution) are automatically compiled with as many as
possible of the C<-std=gnu99>, C<-pedantic>, and a selection of C<-W>
flags (see cflags.SH). Files in F<ext/> F<dist/> F<cpan/> etc are
compiled with the same flags as the installed perl would use to compile
XS extensions.

Basically, it's safe to assume that F<Configure> and F<cflags.SH> have
picked the best combination of flags for the version of gcc on the
platform, and attempting to add more flags related to enforcing a C
dialect will cause problems either locally, or on other systems that
the code is shipped to.

We believe that the C99 support in gcc 3.1 is good enough for us, but
we don't have a 19 year old gcc handy to check this :-) If you have
ancient vendor compilers that don't default to C99, the flags you might
want to try are

=over 4

=item AIX

C<-qlanglvl=stdc99>

=item HP/UX

C<-AC99>

=item Solaris

C<-xc99>

=back

=head2 Symbol Names and Namespace Pollution

=head3 Choosing legal symbol names

C reserves for its implementation any symbol whose name begins with an
underscore followed immediately by either an uppercase letter C<[A-Z]>
or another underscore.  C++ further reserves any symbol containing two
consecutive underscores, and further reserves in the global name space
any symbol beginning with an underscore, not just ones followed by a
capital.  We care about C++ because header files (F<*.h>) need to be
compilable by it, and some people do all their development using a C++
compiler.

The consequences of failing to do this are probably none.  Unless you
stumble on a name that the implementation uses, things will work.
Indeed, the perl core has more than a few instances of using
implementation-reserved symbols.  (These are gradually being changed.)
But your code might stop working any time that the implementation
decides to use a name you already had chosen, potentially many years
before.

It's best then to:

=over

=item B<Don't begin a file-level symbol name with an underscore>; (I<e.g.>, don't
use: C<_FOOBAR>)

It is fine to have a symbol in a function or block like C<_ref>,
beginning with an underscore followed by a lowercase letter.

=item B<Don't use two consecutive underscores in a symbol name>;
(I<e.g.>, don't use C<FOO__BAR>)

=back

POSIX also reserves many symbols.  See Section 2.2.2 in
L<https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html>.
Perl also has conflicts with that.

Perl reserves for its use any symbol beginning with C<Perl>, C<perl>,
or C<PL_>.  Any time you introduce a macro into a header file that
doesn't follow that convention, you are creating the possibility of a
namespace clash with an existing XS module, unless you restrict it by,
say,

 #ifdef PERL_CORE
 #  define my_symbol
 #endif

There are many symbols in header files that aren't of this form, and
which are accessible from XS namespace, intentionally or not, just
about anything in F<config.h>, for example.

Having to use one of these prefixes detracts from the readability of
the code, and hasn't been an actual issue for non-trivial names. Things
like perl defining its own C<MAX> macro have been problematic, but they
were quickly discovered, and a S<C<#ifdef PERL_CORE>> guard added.

So there's no rule imposed about using such symbols, just be aware of
the issues.

=head3 Choosing good symbol names

Ideally, a symbol name should correctly and precisely describe its
intended purpose.  But there is a tension between that and getting
names that are overly long and hence awkward to type and read.
Metaphors could be helpful (a poetic name), but those tend to be
culturally specific, and may not translate for someone whose native
language isn't English, or even comes from a different cultural
background.  Besides, the talent of writing poetry seems to be rare in
programmers.

Certain symbol names don't reflect their purpose, but are nonetheless
fine to use because of long-standing conventions.  These often
originated in the field of Mathematics, where C<i> and C<j> are
frequently used as subscripts, and C<n> as a population count.  Since
at least the 1950's, computer programs have used C<i>, I<etc.> as loop
variables.

Our guidance is to choose a name that reasonably describes the purpose,
and to comment its declaration more precisely.

One certainly shouldn't use misleading nor ambiguous names. C<last_foo>
could mean either the final C<foo> or the previous C<foo>, and so could
be confusing to the reader, or even to the writer coming back to the
code after a few months of working on something else. Sometimes the
programmer has a particular line of thought in mind, and it doesn't
occur to them that ambiguity is present.

There are probably still many off-by-1 bugs around because the name
L<perlapi/C<av_len>> doesn't correspond to what other I<-len>
constructs mean, such as L<perlapi/C<sv_len>>.  Awkward (and
controversial) synonyms were created to use instead that conveyed its
true meaning (L<perlapi/C<av_top_index>>).  Eventually, though, someone
had the better idea to create a new name to signify what most people
think C<-len> signifies.  So L<perlapi/C<av_count>> was born.  And we
wish it had been thought up much earlier.

=head2 Writing safer macros

Macros are used extensively in the Perl core for such things as hiding
internal details from the caller, so that it doesn't have to be
concerned about them.  For example, most lines of code don't need to
know if they are running on a threaded versus unthreaded perl.  That
detail is automatically mostly hidden.

It is often better to use an inline function instead of a macro.  They
are immune to name collisions with the caller, and don't magnify
problems when called with parameters that are expressions with side
effects.  There was a time when one might choose a macro over an inline
function because compiler support for inline functions was quite
limited.  Some only would actually only inline the first two or three
encountered in a compilation.  But those days are long gone, and inline
functions are fully supported in modern compilers.

Nevertheless, there are situations where a function won't do, and a
macro is required.  One example is when a parameter can be any of
several types.  A function has to be declared with a single explicit
parameter type, so a macro may be called for.

Or maybe the code involved is so trivial that a function would be just
complicating overkill, such as when the macro simply creates a mnemonic
name for some constant value.

If you do choose to use a non-trivial macro, be aware that there are
several avoidable pitfalls that can occur.  Keep in mind that a macro
is expanded within the lexical context of each place in the source it
is called.  If you have a token C<foo> in the macro and the source
happens also to have C<foo>, the meaning of the macro's C<foo> will
become that of the caller's.  Sometimes that is exactly the behavior
you want, but be aware that this tends to be confusing later on.  It
effectively turns C<foo> into a reserved word for any code that calls
the macro, and this fact is usually not documented nor considered.  It
is safer to pass C<foo> as a parameter, so that C<foo> remains freely
available to the caller and the macro interface is explicitly
specified.

Worse is when the equivalence between the two C<foo>'s is coincidental.
Suppose for example, that the macro declares a variable

 int foo

That works fine as long as the caller doesn't define the string C<foo>
in some way.  And it might not be until years later that someone comes
along with an instance where C<foo> is used.  For example a future
caller could do this:

 #define foo  bar

Then that declaration of C<foo> in the macro suddenly becomes

 int bar

That could mean that something completely different happens than
intended.  It is hard to debug; the macro and call may not even be in
the same file, so it would require some digging and gnashing of teeth
to figure out.

Therefore, if a macro does use variables, their names should be such
that it is very unlikely that they would collide with any caller, now
or forever.  One way to do that, now being used in the perl source, is
to include the name of the macro itself as part of the name of each
variable in the macro.  Suppose the macro is named C<SvPV>  Then we
could have

 int foo_svpv_ = 0;

This is harder to read than plain C<foo>, but it is pretty much
guaranteed that a caller will never naively use C<foo_svpv_> (and run
into problems).  (The lowercasing makes it clearer that this is a
variable, but assumes that there won't be two elements whose names
differ only in the case of their letters.)  The trailing underscore
makes it even more unlikely to clash, as those, by convention, signify
a private variable name.  (See L</Choosing legal symbol names> for
restrictions on what names you can use.)

This kind of name collision doesn't happen with the macro's formal
parameters, so they don't need to have complicated names.  But there
are pitfalls when a parameter is an expression, or has some Perl
magic attached.  When calling a function, C will evaluate the parameter
once, and pass the result to the function.  But when calling a macro,
the parameter is copied as-is by the C preprocessor to each instance
inside the macro.  This means that when evaluating a parameter having
side effects, the function and macro results differ.  This is
particularly fraught when a parameter has overload magic, say it is a
tied variable that reads the next line in a file upon each evaluation.
Having it read multiple lines per call is probably not what the caller
intended.  If a macro refers to a potentially overloadable parameter
more than once, it should first make a copy and then use that copy the
rest of the time. There are macros in the perl core that violate this,
but are gradually being converted, usually by changing to use inline
functions instead.

Above we said "first make a copy".  In a macro, that is easier said
than done, because macros are normally expressions, and declarations
aren't allowed in expressions.  But the S<C<STMT_START> .. C<STMT_END>>
construct, described in L<perlapi|perlapi/STMT_START>, allows you to
have declarations in most contexts, as long as you don't need a return
value.  If you do need a value returned, you can make the interface
such that a pointer is passed to the construct, which then stores its
result there.  (Or you can use GCC brace groups.  But these require a
fallback if the code will ever get executed on a platform that lacks
this non-standard extension to C.  And that fallback would be another
code path, which can get out-of-sync with the brace group one, so doing
this isn't advisable.)  In situations where there's no other way, Perl
does furnish L<perlintern/C<PL_Sv>> and L<perlapi/C<PL_na>> to use
(with a slight performance penalty) for some such common cases.  But
beware that a call chain involving multiple macros using them will zap
the other's use.  These have been very difficult to debug.

For a concrete example of these pitfalls in action, see
L<https://perlmonks.org/?node_id=11144355>.

=head2 Portability problems

The following are common causes of compilation and/or execution
failures, not specific to Perl as such.  The C FAQ is good bedtime
reading.  Please test your changes with as many C compilers and
platforms as possible; we will, anyway, and it's nice to save oneself
from public embarrassment.

Also study L<perlport> carefully to avoid any bad assumptions about the
operating system, filesystems, character set, and so forth.

Do not assume an operating system indicates a certain compiler.

=over 4

=item *

Casting pointers to integers or casting integers to pointers

    void castaway(U8* p)
    {
      IV i = p;

or

    void castaway(U8* p)
    {
      IV i = (IV)p;

Both are bad, and broken, and unportable.  Use the PTR2IV() macro that
does it right.  (Likewise, there are PTR2UV(), PTR2NV(), INT2PTR(), and
NUM2PTR().)

=item *

Casting between function pointers and data pointers

Technically speaking casting between function pointers and data
pointers is unportable and undefined, but practically speaking it seems
to work, but you should use the FPTR2DPTR() and DPTR2FPTR() macros.
Sometimes you can also play games with unions.

=item *

Assuming C<sizeof(int) == sizeof(long)>

There are platforms where longs are 64 bits, and platforms where ints
are 64 bits, and while we are out to shock you, even platforms where
shorts are 64 bits.  This is all legal according to the C standard. (In
other words, C<long long> is not a portable way to specify 64 bits, and
C<long long> is not even guaranteed to be any wider than C<long>.)

Instead, use the definitions C<IV>, C<UV>, C<IVSIZE>, C<I32SIZE>, and
so forth. Avoid things like C<I32> because they are B<not> guaranteed
to be I<exactly> 32 bits (they are I<at least> 32 bits), nor are they
guaranteed to be C<int> or C<long>.  If you explicitly need 64-bit
variables, use C<I64> and C<U64>.

=item *

Assuming one can dereference any type of pointer for any type of data

  char *p = ...;
  long pony = *(long *)p;    /* BAD */

Many platforms, quite rightly so, will give you a core dump instead of
a pony if the p happens not to be correctly aligned.

=item *

Lvalue casts

  (int)*p = ...;    /* BAD */

Simply not portable.  Get your lvalue to be of the right type, or maybe
use temporary variables, or dirty tricks with unions.

=item *

Assume B<anything> about structs (especially the ones you don't
control, like the ones coming from the system headers)

=over 8

=item *

That a certain field exists in a struct

=item *

That no other fields exist besides the ones you know of

=item *

That a field is of certain signedness, sizeof, or type

=item *

That the fields are in a certain order

=over 8

=item *

While C guarantees the ordering specified in the struct definition,
between different platforms the definitions might differ

=back

=item *

That the C<sizeof(struct)> or the alignments are the same everywhere

=over 8

=item *

There might be padding bytes between the fields to align the fields -
the bytes can be anything

=item *

Structs are required to be aligned to the maximum alignment required by
the fields - which for native types is usually equivalent to
C<sizeof(the_field)>.

=back

=back

=item *

Assuming the character set is ASCIIish

Perl can compile and run under EBCDIC platforms.  See L<perlebcdic>.
This is transparent for the most part, but because the character sets
differ, you shouldn't use numeric (decimal, octal, nor hex) constants
to refer to characters.  You can safely say C<'A'>, but not C<0x41>.
You can safely say C<'\n'>, but not C<\012>.  However, you can use
macros defined in F<utf8.h> to specify any code point portably.
C<LATIN1_TO_NATIVE(0xDF)> is going to be the code point that means
LATIN SMALL LETTER SHARP S on whatever platform you are running on (on
ASCII platforms it compiles without adding any extra code, so there is
zero performance hit on those).  The acceptable inputs to
C<LATIN1_TO_NATIVE> are from C<0x00> through C<0xFF>.  If your input
isn't guaranteed to be in that range, use C<UNICODE_TO_NATIVE> instead.
C<NATIVE_TO_LATIN1> and C<NATIVE_TO_UNICODE> translate the opposite
direction.

If you need the string representation of a character that doesn't have
a mnemonic name in C, you should add it to the list in
F<regen/unicode_constants.pl>, and have Perl create C<#define>'s for
you, based on the current platform.

Note that the C<isI<FOO>> and C<toI<FOO>> macros in F<handy.h> work
properly on native code points and strings.

Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper
case alphabetic characters.  That is not true in EBCDIC.  Nor for 'a'
to 'z'.  But '0' - '9' is an unbroken range in both systems.  Don't
assume anything about other ranges.  (Note that special handling of
ranges in regular expression patterns and transliterations makes it
appear to Perl code that the aforementioned ranges are all unbroken.)

Many of the comments in the existing code ignore the possibility of
EBCDIC, and may be wrong therefore, even if the code works.  This is
actually a tribute to the successful transparent insertion of being
able to handle EBCDIC without having to change pre-existing code.

UTF-8 and UTF-EBCDIC are two different encodings used to represent
Unicode code points as sequences of bytes.  Macros  with the same names
(but different definitions) in F<utf8.h> and F<utfebcdic.h> are used to
allow the calling code to think that there is only one such encoding.
This is almost always referred to as C<utf8>, but it means the EBCDIC
version as well.  Again, comments in the code may well be wrong even if
the code itself is right.  For example, the concept of UTF-8
C<invariant characters> differs between ASCII and EBCDIC.  On ASCII
platforms, only characters that do not have the high-order bit set
(i.e.  whose ordinals are strict ASCII, 0 - 127) are invariant, and the
documentation and comments in the code may assume that, often referring
to something like, say, C<hibit>.  The situation differs and is not so
simple on EBCDIC machines, but as long as the code itself uses the
C<NATIVE_IS_INVARIANT()> macro appropriately, it works, even if the
comments are wrong.

As noted in L<perlhack/TESTING>, when writing test scripts, the file
F<t/charset_tools.pl> contains some helpful functions for writing tests
valid on both ASCII and EBCDIC platforms.  Sometimes, though, a test
can't use a function and it's inconvenient to have different test
versions depending on the platform.  There are 20 code points that are
the same in all 4 character sets currently recognized by Perl (the 3
EBCDIC code pages plus ISO 8859-1 (ASCII/Latin1)).  These can be used
in such tests, though there is a small possibility that Perl will
become available in yet another character set, breaking your test.  All
but one of these code points are C0 control characters.  The most
significant controls that are the same are C<\0>, C<\r>, and C<\N{VT}>
(also specifiable as C<\cK>, C<\x0B>, C<\N{U+0B}>, or C<\013>).  The
single non-control is U+00B6 PILCROW SIGN.  The controls that are the
same have the same bit pattern in all 4 character sets, regardless of
the UTF8ness of the string containing them.  The bit pattern for U+B6
is the same in all 4 for non-UTF8 strings, but differs in each when its
containing string is UTF-8 encoded.  The only other code points that
have some sort of sameness across all 4 character sets are the pair
0xDC and 0xFC. Together these represent upper- and lowercase LATIN
LETTER U WITH DIAERESIS, but which is upper and which is lower may be
reversed: 0xDC is the capital in Latin1 and 0xFC is the small letter,
while 0xFC is the capital in EBCDIC and 0xDC is the small one.  This
factoid may be exploited in writing case insensitive tests that are the
same across all 4 character sets.

=item *

Assuming the character set is just ASCII

ASCII is a 7 bit encoding, but bytes have 8 bits in them.  The 128
extra characters have different meanings depending on the locale.
Absent a locale, currently these extra characters are generally
considered to be unassigned, and this has presented some problems. This
has being changed starting in 5.12 so that these characters can be
considered to be Latin-1 (ISO-8859-1).

=item *

Mixing #define and #ifdef

  #define BURGLE(x) ... \
  #ifdef BURGLE_OLD_STYLE        /* BAD */
  ... do it the old way ... \
  #else
  ... do it the new way ... \
  #endif

You cannot portably "stack" cpp directives.  For example in the above
you need two separate BURGLE() #defines, one for each #ifdef branch.

=item *

Adding non-comment stuff after #endif or #else

  #ifdef SNOSH
  ...
  #else !SNOSH    /* BAD */
  ...
  #endif SNOSH    /* BAD */

The #endif and #else cannot portably have anything non-comment after
them.  If you want to document what is going (which is a good idea
especially if the branches are long), use (C) comments:

  #ifdef SNOSH
  ...
  #else /* !SNOSH */
  ...
  #endif /* SNOSH */

The gcc option C<-Wendif-labels> warns about the bad variant (by
default on starting from Perl 5.9.4).

=item *

Having a comma after the last element of an enum list

  enum color {
    CERULEAN,
    CHARTREUSE,
    CINNABAR,     /* BAD */
  };

is not portable.  Leave out the last comma.

Also note that whether enums are implicitly morphable to ints varies
between compilers, you might need to (int).

=item *

Mixing signed char pointers with unsigned char pointers

  int foo(char *s) { ... }
  ...
  unsigned char *t = ...; /* Or U8* t = ... */
  foo(t);   /* BAD */

While this is legal practice, it is certainly dubious, and downright
fatal in at least one platform: for example VMS cc considers this a
fatal error.  One cause for people often making this mistake is that a
"naked char" and therefore dereferencing a "naked char pointer" have an
undefined signedness: it depends on the compiler and the flags of the
compiler and the underlying platform whether the result is signed or
unsigned.  For this very same reason using a 'char' as an array index
is bad.

=item *

Macros that have string constants and their arguments as substrings of
the string constants

  #define FOO(n) printf("number = %d\n", n)    /* BAD */
  FOO(10);

Pre-ANSI semantics for that was equivalent to

  printf("10umber = %d\10");

which is probably not what you were expecting.  Unfortunately at least
one reasonably common and modern C compiler does "real backward
compatibility" here, in AIX that is what still happens even though the
rest of the AIX compiler is very happily C89.

=item *

Using printf formats for non-basic C types

   IV i = ...;
   printf("i = %d\n", i);    /* BAD */

While this might by accident work in some platform (where IV happens to
be an C<int>), in general it cannot.  IV might be something larger.
Even worse the situation is with more specific types (defined by Perl's
configuration step in F<config.h>):

   Uid_t who = ...;
   printf("who = %d\n", who);    /* BAD */

The problem here is that Uid_t might be not only not C<int>-wide but it
might also be unsigned, in which case large uids would be printed as
negative values.

There is no simple solution to this because of printf()'s limited
intelligence, but for many types the right format is available as with
either 'f' or '_f' suffix, for example:

   IVdf /* IV in decimal */
   UVxf /* UV in hexadecimal */
   U32of /* A U32 in octal */

   printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */

   Uid_t_f /* Uid_t in decimal */

   printf("who = %"Uid_t_f"\n", who);

Or you can try casting to a "wide enough" type:

   printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);

See L<perlguts/Formatted Printing of Size_t and SSize_t> for how to
print those.

Also remember that the C<%p> format really does require a void pointer:

   U8* p = ...;
   printf("p = %p\n", (void*)p);

The gcc option C<-Wformat> scans for such problems.

=item *

Blindly passing va_list

Not all platforms support passing va_list to further varargs (stdarg)
functions.  The right thing to do is to copy the va_list using the
Perl_va_copy() if the NEED_VA_COPY is defined.

=for apidoc_section $genconfig
=for apidoc Amnh||NEED_VA_COPY

=item *

Using gcc statement expressions

   val = ({...;...;...});    /* BAD */

While a nice extension, it's not portable.  Historically, Perl used
them in macros if available to gain some extra speed (essentially as a
funky form of inlining), but we now support (or emulate) C99 C<static
inline> functions, so use them instead. Declare functions as
C<PERL_STATIC_INLINE> to transparently fall back to emulation where
needed.

=item *

Binding together several statements in a macro

Use the macros C<STMT_START> and C<STMT_END>.

   STMT_START {
      ...
   } STMT_END

But there can be subtle (but avoidable if you do it right) bugs
introduced with these; see L<perlapi/C<STMT_START>> for best practices
for their use.

=item *

Testing for operating systems or versions when you should be testing
for features

  #ifdef __FOONIX__    /* BAD */
  foo = quux();
  #endif

Unless you know with 100% certainty that quux() is only ever available
for the "Foonix" operating system B<and> that is available B<and>
correctly working for B<all> past, present, B<and> future versions of
"Foonix", the above is very wrong.  This is more correct (though still
not perfect, because the below is a compile-time check):

  #ifdef HAS_QUUX
  foo = quux();
  #endif

How does the HAS_QUUX become defined where it needs to be?  Well, if
Foonix happens to be Unixy enough to be able to run the Configure
script, and Configure has been taught about detecting and testing
quux(), the HAS_QUUX will be correctly defined.  In other platforms,
the corresponding configuration step will hopefully do the same.

In a pinch, if you cannot wait for Configure to be educated, or if you
have a good hunch of where quux() might be available, you can
temporarily try the following:

  #if (defined(__FOONIX__) || defined(__BARNIX__))
  # define HAS_QUUX
  #endif

  ...

  #ifdef HAS_QUUX
  foo = quux();
  #endif

But in any case, try to keep the features and operating systems
separate.

A good resource on the predefined macros for various operating systems,
compilers, and so forth is
L<https://sourceforge.net/p/predef/wiki/Home/>.

=item *

Assuming the contents of static memory pointed to by the return values
of Perl wrappers for C library functions doesn't change.  Many C
library functions return pointers to static storage that can be
overwritten by subsequent calls to the same or related functions.  If
you handle those returns before one of those functions that share the
storage gets called, this is fine, but in embedded perls, or when using
threads, such a function may get called before you get a chance to
handle it.

L<perlclib/Dealing with embedded perls and threads> contains a list of
problematic functions with good advice as to how to cope with them.

=back

=head2 Problematic System Interfaces

There are lots of issues with using various C library functions,
including security ones.  You should read L<perlclib> which covers
things in detail.

Remember that Perl strings are NOT the same as C strings:  They may
contain C<NUL> characters, whereas a C string is terminated by the first
C<NUL>. That is why Perl API functions that deal with strings generally
take a pointer to the first byte and either a length or a pointer to the
byte just beyond the final one.

And this is the reason that many of the C library string handling
functions should not be used.  They don't cope with the full generality
of Perl strings.  It may be that your test cases don't have embedded
C<NUL>s, and so the tests pass, whereas there may well eventually arise
real-world cases where they fail.  A lesson here is to include C<NUL>s
in your tests.  Now it's fairly rare in most real world cases to get
C<NUL>s, so your code may seem to work, until one day a C<NUL> comes
along.

Here's an example.  It used to be a common paradigm, for decades, in
the perl core to use S<C<strchr("list", c)>> to see if the character
C<c> is any of the ones given in C<"list">, a double-quote-enclosed
string of the set of characters that we are seeing if C<c> is one of.
As long as C<c> isn't a C<NUL>, it works.  But when C<c> is a C<NUL>,
C<strchr> returns a pointer to the terminating C<NUL> in C<"list">.
This likely will result in a segfault or a security issue when the
caller uses that end pointer as the starting point to read from.

A solution to this and many similar issues is to use the C<mem>I<-foo>
C library functions instead.  In this case C<memchr> can be used to see
if C<c> is in C<"list"> and works even if C<c> is C<NUL>.  These
functions need an additional parameter to give the string length. In
the case of literal string parameters, perl has defined macros that
calculate the length for you.  See L<perlapi/String Handling>.

=head1 DEBUGGING

You can compile a special debugging version of Perl, which allows you
to use the C<-D> option of Perl to tell more about what Perl is doing.
But sometimes there is no alternative than to dive in with a debugger,
either to see the stack trace of a core dump (very useful in a bug
report), or trying to figure out what went wrong before the core dump
happened, or how did we end up having wrong or unexpected results.

=head2 Poking at Perl

To really poke around with Perl, you'll probably want to build Perl for
debugging, like this:

    ./Configure -d -DDEBUGGING
    make

C<-DDEBUGGING> turns on the C compiler's C<-g> flag to have it produce
debugging information which will allow us to step through a running
program, and to see in which C function we are at (without the
debugging information we might see only the numerical addresses of the
functions, which is not very helpful). It will also turn on the
C<DEBUGGING> compilation symbol which enables all the internal
debugging code in Perl. There are a whole bunch of things you can debug
with this: L<perlrun|perlrun/-Dletters> lists them all, and the best
way to find out about them is to play about with them.  The most useful
options are probably

    l  Context (loop) stack processing
    s  Stack snapshots (with v, displays all stacks)
    t  Trace execution
    o  Method and overloading resolution
    c  String/numeric conversions

For example

    $ perl -Dst -e '$x + 1'
    ....
    (-e:1)	gvsv(main::x)
        =>  UNDEF
    (-e:1)	const(IV(1))
        =>  UNDEF  IV(1)
    (-e:1)	add
        =>  NV(1)


Some of the functionality of the debugging code can be achieved with a
non-debugging perl by using XS modules:

    -Dr => use re 'debug'
    -Dx => use O 'Debug'

=head2 Using a source-level debugger

If the debugging output of C<-D> doesn't help you, it's time to step
through perl's execution with a source-level debugger.

=over 3

=item *

We'll use C<gdb> for our examples here; the principles will apply to
any debugger (many vendors call their debugger C<dbx>), but check the
manual of the one you're using.

=back

To fire up the debugger, type

    gdb ./perl

Or if you have a core dump:

    gdb ./perl core

You'll want to do that in your Perl source tree so the debugger can
read the source code.  You should see the copyright message, followed
by the prompt.

    (gdb)

C<help> will get you into the documentation, but here are the most
useful commands:

=over 3

=item * run [args]

Run the program with the given arguments.

=item * break function_name

=item * break source.c:xxx

Tells the debugger that we'll want to pause execution when we reach
either the named function (but see L<perlguts/Internal Functions>!) or
the given line in the named source file.

=item * step

Steps through the program a line at a time.

=item * next

Steps through the program a line at a time, without descending into
functions.

=item * continue

Run until the next breakpoint.

=item * finish

Run until the end of the current function, then stop again.

=item * 'enter'

Just pressing Enter will do the most recent operation again - it's a
blessing when stepping through miles of source code.

=item * ptype

Prints the C definition of the argument given.

  (gdb) ptype PL_op
  type = struct op {
      OP *op_next;
      OP *op_sibparent;
      OP *(*op_ppaddr)(void);
      PADOFFSET op_targ;
      unsigned int op_type : 9;
      unsigned int op_opt : 1;
      unsigned int op_slabbed : 1;
      unsigned int op_savefree : 1;
      unsigned int op_static : 1;
      unsigned int op_folded : 1;
      unsigned int op_spare : 2;
      U8 op_flags;
      U8 op_private;
  } *

=item * print

Execute the given C code and print its results.  B<WARNING>: Perl makes
heavy use of macros, and F<gdb> does not necessarily support macros
(see later L</"gdb macro support">).  You'll have to substitute them
yourself, or to invoke cpp on the source code files (see L</"The .i
Targets">) So, for instance, you can't say

    print SvPV_nolen(sv)

but you have to say

    print Perl_sv_2pv_nolen(sv)

=back

You may find it helpful to have a "macro dictionary", which you can
produce by saying C<cpp -dM perl.c | sort>.  Even then, F<cpp> won't
recursively apply those macros for you.

=head2 gdb macro support

Recent versions of F<gdb> have fairly good macro support, but in order
to use it you'll need to compile perl with macro definitions included
in the debugging information.  Using F<gcc> version 3.1, this means
configuring with C<-Doptimize=-g3>.  Other compilers might use a
different switch (if they support debugging macros at all).

=head2 Dumping Perl Data Structures

One way to get around this macro hell is to use the dumping functions
in F<dump.c>; these work a little like an internal
L<Devel::Peek|Devel::Peek>, but they also cover OPs and other
structures that you can't get at from Perl.  Let's take an example.
We'll use the C<$x = $y + $z> we used before, but give it a bit of
context: C<$y = "6XXXX"; $z = 2.3;>.  Where's a good place to stop and
poke around?

What about C<pp_add>, the function we examined earlier to implement the
C<+> operator:

    (gdb) break Perl_pp_add
    Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.

Notice we use C<Perl_pp_add> and not C<pp_add> - see
L<perlguts/Internal Functions>.  With the breakpoint in place, we can
run our program:

    (gdb) run -e '$y = "6XXXX"; $z = 2.3; $x = $y + $z'

Lots of junk will go past as gdb reads in the relevant source files and
libraries, and then:

    Breakpoint 1, Perl_pp_add () at pp_hot.c:309
    1396    dSP; dATARGET; bool useleft; SV *svl, *svr;
    (gdb) step
    311           dPOPTOPnnrl_ul;
    (gdb)

We looked at this bit of code before, and we said that
C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and
C<right> - let's slightly expand it:

 #define dPOPTOPnnrl_ul  NV right = POPn; \
                         SV *leftsv = TOPs; \
                         NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0

C<POPn> takes the SV from the top of the stack and obtains its NV
either directly (if C<SvNOK> is set) or by calling the C<sv_2nv>
function.  C<TOPs> takes the next SV from the top of the stack - yes,
C<POPn> uses C<TOPs> - but doesn't remove it.  We then use C<SvNV> to
get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses
C<SvNV>.

Since we don't have an NV for C<$y>, we'll have to use C<sv_2nv> to
convert it.  If we step again, we'll find ourselves there:

    (gdb) step
    Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
    1669        if (!sv)
    (gdb)

We can now use C<Perl_sv_dump> to investigate the SV:

    (gdb) print Perl_sv_dump(sv)
    SV = PV(0xa057cc0) at 0xa0675d0
    REFCNT = 1
    FLAGS = (POK,pPOK)
    PV = 0xa06a510 "6XXXX"\0
    CUR = 5
    LEN = 6
    $1 = void

We know we're going to get C<6> from this, so let's finish the
subroutine:

    (gdb) finish
    Run till exit from #0  Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
    0x462669 in Perl_pp_add () at pp_hot.c:311
    311           dPOPTOPnnrl_ul;

We can also dump out this op: the current op is always stored in
C<PL_op>, and we can dump it with C<Perl_op_dump>.  This'll give us
similar output to CPAN module L<B::Debug>.

=for apidoc_section $debugging
=for apidoc Amnh||PL_op

    (gdb) print Perl_op_dump(PL_op)
    {
    13  TYPE = add  ===> 14
        TARG = 1
        FLAGS = (SCALAR,KIDS)
        {
            TYPE = null  ===> (12)
              (was rv2sv)
            FLAGS = (SCALAR,KIDS)
            {
    11          TYPE = gvsv  ===> 12
                FLAGS = (SCALAR)
                GV = main::b
            }
        }

# finish this later #

=head2 Using gdb to look at specific parts of a program

With the example above, you knew to look for C<Perl_pp_add>, but what
if there were multiple calls to it all over the place, or you didn't
know what the op was you were looking for?

One way to do this is to inject a rare call somewhere near what you're
looking for.  For example, you could add C<study> before your method:

    study;

And in gdb do:

    (gdb) break Perl_pp_study

And then step until you hit what you're looking for.  This works well
in a loop if you want to only break at certain iterations:

    for my $i (1..100) {
        study if $i == 50;
    }

=head2 Using gdb to look at what the parser/lexer are doing

If you want to see what perl is doing when parsing/lexing your code,
you can use C<BEGIN {}>:

    print "Before\n";
    BEGIN { study; }
    print "After\n";

And in gdb:

    (gdb) break Perl_pp_study

If you want to see what the parser/lexer is doing inside of C<if>
blocks and the like you need to be a little trickier:

    if ($x && $y && do { BEGIN { study } 1 } && $z) { ... }

=head1 SOURCE CODE STATIC ANALYSIS

Various tools exist for analysing C source code B<statically>, as
opposed to B<dynamically>, that is, without executing the code.  It is
possible to detect resource leaks, undefined behaviour, type
mismatches, portability problems, code paths that would cause illegal
memory accesses, and other similar problems by just parsing the C code
and looking at the resulting graph, what does it tell about the
execution and data flows.  As a matter of fact, this is exactly how C
compilers know to give warnings about dubious code.

=head2 lint

The good old C code quality inspector, C<lint>, is available in several
platforms, but please be aware that there are several different
implementations of it by different vendors, which means that the flags
are not identical across different platforms.

There is a C<lint> target in Makefile, but you may have to diddle with
the flags (see above).

=head2 Coverity

Coverity (L<https://www.coverity.com/>) is a product similar to lint and
as a testbed for their product they periodically check several open
source projects, and they give out accounts to open source developers
to the defect databases.

There is Coverity setup for the perl5 project:
L<https://scan.coverity.com/projects/perl5>

=head2 HP-UX cadvise (Code Advisor)

HP has a C/C++ static analyzer product for HP-UX caller Code Advisor.
(Link not given here because the URL is horribly long and seems
horribly unstable; use the search engine of your choice to find it.)
The use of the C<cadvise_cc> recipe with C<Configure ...
-Dcc=./cadvise_cc> (see cadvise "User Guide") is recommended; as is the
use of C<+wall>.

=head2 cpd (cut-and-paste detector)

The cpd tool detects cut-and-paste coding.  If one instance of the
cut-and-pasted code changes, all the other spots should probably be
changed, too.  Therefore such code should probably be turned into a
subroutine or a macro.

cpd (L<https://docs.pmd-code.org/latest/pmd_userdocs_cpd.html>) is part
of the pmd project (L<https://pmd.github.io/>).  pmd was originally
written for static analysis of Java code, but later the cpd part of it
was extended to parse also C and C++.

Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
pmd-X.Y.jar from it, and then run that on source code thusly:

  java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \
   --minimum-tokens 100 --files /some/where/src --language c > cpd.txt

You may run into memory limits, in which case you should use the -Xmx
option:

  java -Xmx512M ...

=head2 gcc warnings

Though much can be written about the inconsistency and coverage
problems of gcc warnings (like C<-Wall> not meaning "all the warnings",
or some common portability problems not being covered by C<-Wall>, or
C<-ansi> and C<-pedantic> both being a poorly defined collection of
warnings, and so forth), gcc is still a useful tool in keeping our
coding nose clean.

The C<-Wall> is by default on.

It would be nice for C<-pedantic>) to be on always, but unfortunately
it is not safe on all platforms - for example fatal conflicts with the
system headers (Solaris being a prime example).  If Configure
C<-Dgccansipedantic> is used, the C<cflags> frontend selects
C<-pedantic> for the platforms where it is known to be safe.

The following extra flags are added:

=over 4

=item *

C<-Wendif-labels>

=item *

C<-Wextra>

=item *

C<-Wc++-compat>

=item *

C<-Wwrite-strings>

=item *

C<-Werror=pointer-arith>

=item *

C<-Werror=vla>

=back

The following flags would be nice to have but they would first need
their own Augean stablemaster:

=over 4

=item *

C<-Wshadow>

=item *

C<-Wstrict-prototypes>

=back

The C<-Wtraditional> is another example of the annoying tendency of gcc
to bundle a lot of warnings under one switch (it would be impossible to
deploy in practice because it would complain a lot) but it does contain
some warnings that would be beneficial to have available on their own,
such as the warning about string constants inside macros containing the
macro arguments: this behaved differently pre-ANSI than it does in
ANSI, and some C compilers are still in transition, AIX being an
example.

=head2 Warnings of other C compilers

Other C compilers (yes, there B<are> other C compilers than gcc) often
have their "strict ANSI" or "strict ANSI with some portability
extensions" modes on, like for example the Sun Workshop has its C<-Xa>
mode on (though implicitly), or the DEC (these days, HP...) has its
C<-std1> mode on.

=head1 MEMORY DEBUGGERS

B<NOTE 1>: Running under older memory debuggers such as Purify,
valgrind or Third Degree greatly slows down the execution: seconds
become minutes, minutes become hours.  For example as of Perl 5.8.1,
the F<ext/Encode/t/Unicode.t> test takes extraordinarily long to
complete under e.g. Purify, Third Degree, and valgrind.  Under valgrind
it takes more than six hours, even on a snappy computer.  Said test
must be doing something that is quite unfriendly for memory debuggers.
If you don't feel like waiting, you can simply kill the perl process.
Roughly valgrind slows down execution by factor 10, AddressSanitizer by
factor 2.

B<NOTE 2>: To minimize the number of memory leak false alarms (see
L</PERL_DESTRUCT_LEVEL> for more information), you have to set the
environment variable C<PERL_DESTRUCT_LEVEL> to 2.  For example, like
this:

    env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...

B<NOTE 3>: There are known memory leaks when there are compile-time
errors within C<eval> or C<require>; seeing C<S_doeval> in the call
stack is a good sign of these.  Fixing these leaks is non-trivial,
unfortunately, but they must be fixed eventually.

B<NOTE 4>: L<DynaLoader> will not clean up after itself completely
unless Perl is built with the Configure option
C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>.

=head2 valgrind

The valgrind tool can be used to find out both memory leaks and illegal
heap memory accesses.  As of version 3.3.0, Valgrind only supports
Linux on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64.
The special "test.valgrind" target can be used to run the tests under
valgrind.  Found errors and memory leaks are logged in files named
F<testfile.valgrind> and by default output is displayed inline.

Example usage:

    make test.valgrind

Since valgrind adds significant overhead, tests will take much longer
to run.  The valgrind tests support being run in parallel to help with
this:

    TEST_JOBS=9 make test.valgrind

Note that the above two invocations will be very verbose as reachable
memory and leak-checking is enabled by default.  If you want to just
see pure errors, try:

    VG_OPTS='-q --leak-check=no --show-reachable=no' TEST_JOBS=9 \
        make test.valgrind

Valgrind also provides a cachegrind tool, invoked on perl as:

    VG_OPTS=--tool=cachegrind make test.valgrind

As system libraries (most notably glibc) are also triggering errors,
valgrind allows to suppress such errors using suppression files.  The
default suppression file that comes with valgrind already catches a lot
of them.  Some additional suppressions are defined in F<t/perl.supp>.

To get valgrind and for more information see L<https://valgrind.org/>.

=head2 AddressSanitizer

AddressSanitizer ("ASan") consists of a compiler instrumentation module
and a run-time C<malloc> library. ASan is available for a variety of
architectures, operating systems, and compilers (see project link
below). It checks for unsafe memory usage, such as use after free and
buffer overflow conditions, and is fast enough that you can easily
compile your debugging or optimized perl with it. Modern versions of
ASan check for memory leaks by default on most platforms, otherwise
(e.g. x86_64 OS X) this feature can be enabled via
C<ASAN_OPTIONS=detect_leaks=1>.


To build perl with AddressSanitizer, your Configure invocation should
look like:

    sh Configure -des -Dcc=clang \
       -Accflags=-fsanitize=address -Aldflags=-fsanitize=address \
       -Alddlflags=-shared\ -fsanitize=address \
       -fsanitize-blacklist=`pwd`/asan_ignore

where these arguments mean:

=over 4

=item * -Dcc=clang

This should be replaced by the full path to your clang executable if it
is not in your path.

=item * -Accflags=-fsanitize=address

Compile perl and extensions sources with AddressSanitizer.

=item * -Aldflags=-fsanitize=address

Link the perl executable with AddressSanitizer.

=item * -Alddlflags=-shared\ -fsanitize=address

Link dynamic extensions with AddressSanitizer.  You must manually
specify C<-shared> because using C<-Alddlflags=-shared> will prevent
Configure from setting a default value for C<lddlflags>, which usually
contains C<-shared> (at least on Linux).

=item * -fsanitize-blacklist=`pwd`/asan_ignore

AddressSanitizer will ignore functions listed in the C<asan_ignore>
file.  (This file should contain a short explanation of why each of the
functions is listed.)

=back

See also L<https://github.com/google/sanitizers/wiki/AddressSanitizer>.

=head2 Dr Memory

Dr. Memory is a tool similar to valgrind which is usable on Windows
and Linux.

It supports heap checking like C<memcheck> from valgrind.  There are
also other tools included.

See L<https://drmemory.org/>.


=head1 PROFILING

Depending on your platform there are various ways of profiling Perl.

There are two commonly used techniques of profiling executables:
I<statistical time-sampling> and I<basic-block counting>.

The first method takes periodically samples of the CPU program counter,
and since the program counter can be correlated with the code generated
for functions, we get a statistical view of in which functions the
program is spending its time.  The caveats are that very small/fast
functions have lower probability of showing up in the profile, and that
periodically interrupting the program (this is usually done rather
frequently, in the scale of milliseconds) imposes an additional
overhead that may skew the results.  The first problem can be
alleviated by running the code for longer (in general this is a good
idea for profiling), the second problem is usually kept in guard by the
profiling tools themselves.

The second method divides up the generated code into I<basic blocks>.
Basic blocks are sections of code that are entered only in the
beginning and exited only at the end.  For example, a conditional jump
starts a basic block.  Basic block profiling usually works by
I<instrumenting> the code by adding I<enter basic block #nnnn>
book-keeping code to the generated code.  During the execution of the
code the basic block counters are then updated appropriately.  The
caveat is that the added extra code can skew the results: again, the
profiling tools usually try to factor their own effects out of the
results.

=head2 Gprof Profiling

I<gprof> is a profiling tool available in many Unix platforms which
uses I<statistical time-sampling>.  You can build a profiled version of
F<perl> by compiling using gcc with the flag C<-pg>.  Either edit
F<config.sh> or re-run F<Configure>.  Running the profiled version of
Perl will create an output file called F<gmon.out> which contains the
profiling data collected during the execution.

quick hint:

    $ sh Configure -des -Dusedevel -Accflags='-pg' \
        -Aldflags='-pg' -Alddlflags='-pg -shared' \
        && make perl
    $ ./perl ... # creates gmon.out in current directory
    $ gprof ./perl > out
    $ less out

(you probably need to add C<-shared> to the <-Alddlflags> line until RT
#118199 is resolved)

The F<gprof> tool can then display the collected data in various ways.
Usually F<gprof> understands the following options:

=over 4

=item * -a

Suppress statically defined functions from the profile.

=item * -b

Suppress the verbose descriptions in the profile.

=item * -e routine

Exclude the given routine and its descendants from the profile.

=item * -f routine

Display only the given routine and its descendants in the profile.

=item * -s

Generate a summary file called F<gmon.sum> which then may be given to
subsequent gprof runs to accumulate data over several runs.

=item * -z

Display routines that have zero usage.

=back

For more detailed explanation of the available commands and output
formats, see your own local documentation of F<gprof>.

=head2 GCC gcov Profiling

I<basic block profiling> is officially available in gcc 3.0 and later.
You can build a profiled version of F<perl> by compiling using gcc with
the flags C<-fprofile-arcs -ftest-coverage>.  Either edit F<config.sh>
or re-run F<Configure>.

quick hint:

    $ sh Configure -des -Dusedevel -Doptimize='-g' \
        -Accflags='-fprofile-arcs -ftest-coverage' \
        -Aldflags='-fprofile-arcs -ftest-coverage' \
        -Alddlflags='-fprofile-arcs -ftest-coverage -shared' \
        && make perl
    $ rm -f regexec.c.gcov regexec.gcda
    $ ./perl ...
    $ gcov regexec.c
    $ less regexec.c.gcov

(you probably need to add C<-shared> to the <-Alddlflags> line until RT
#118199 is resolved)

Running the profiled version of Perl will cause profile output to be
generated.  For each source file an accompanying F<.gcda> file will be
created.

To display the results you use the I<gcov> utility (which should be
installed if you have gcc 3.0 or newer installed).  F<gcov> is run on
source code files, like this

    gcov sv.c

which will cause F<sv.c.gcov> to be created.  The F<.gcov> files
contain the source code annotated with relative frequencies of
execution indicated by "#" markers.  If you want to generate F<.gcov>
files for all profiled object files, you can run something like this:

    for file in `find . -name \*.gcno`
    do sh -c "cd `dirname $file` && gcov `basename $file .gcno`"
    done

Useful options of F<gcov> include C<-b> which will summarise the basic
block, branch, and function call coverage, and C<-c> which instead of
relative frequencies will use the actual counts.  For more information
on the use of F<gcov> and basic block profiling with gcc, see the
latest GNU CC manual.  As of gcc 4.8, this is at
L<https://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>.

=head2 callgrind profiling

callgrind is a valgrind tool for profiling source code. Paired with
kcachegrind (a Qt based UI), it gives you an overview of where code is
taking up time, as well as the ability to examine callers, call trees,
and more. One of its benefits is you can use it on perl and XS modules
that have not been compiled with debugging symbols.

If perl is compiled with debugging symbols (C<-g>), you can view the
annotated source and click around, much like L<Devel::NYTProf>'s HTML
output.

For basic usage:

    valgrind --tool=callgrind ./perl ...

By default it will write output to F<callgrind.out.PID>, but you can
change that with C<--callgrind-out-file=...>

To view the data, do:

    kcachegrind callgrind.out.PID

If you'd prefer to view the data in a terminal, you can use
F<callgrind_annotate>.  In its basic form:

    callgrind_annotate callgrind.out.PID | less

Some useful options are:

=over 4

=item * --threshold

Percentage of counts (of primary sort event) we are interested in. The
default is 99%, 100% might show things that seem to be missing.

=item * --auto

Annotate all source files containing functions that helped reach the
event count threshold.

=back

=head2 C<profiler> profiling (Cygwin)

Cygwin allows for C<gprof> profiling and C<gcov> coverage testing, but
this only profiles the main executable.

You can use the C<profiler> tool to perform sample based profiling, it
requires no special preparation of the executables beyond debugging
symbols.

This produces sampling data which can be processed with C<gprof>.

There is L<limited
documentation|https://www.cygwin.com/cygwin-ug-net/profiler.html> on
the Cygwin web site.

=head2 Visual Studio Profiling

You can use the Visual Studio profiler to profile perl if you've built
perl with MSVC, even though we build perl at the command-line.  You
will need to build perl with C<CFG=Debug> or C<CFG=DebugSymbols>.

The Visual Studio profiler is a sampling profiler.

See L<the Visual Studio
documentation|https://github.com/MicrosoftDocs/visualstudio-docs/blob/main/docs/profiling/beginners-guide-to-performance-profiling.md>
to get started.

=head1 MISCELLANEOUS TRICKS

=head2 PERL_DESTRUCT_LEVEL

If you want to run any of the tests yourself manually using e.g.
valgrind, please note that by default perl B<does not> explicitly clean
up all the memory it has allocated (such as global memory arenas) but
instead lets the C<exit()> of the whole program "take care" of such
allocations, also known as "global destruction of objects".

There is a way to tell perl to do complete cleanup: set the environment
variable C<PERL_DESTRUCT_LEVEL> to a non-zero value.  The F<t/TEST>
wrapper does set this to 2, and this is what you need to do too, if you
don't want to see the "global leaks": For example, for running under
valgrind

    env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib t/foo/bar.t

(Note: the mod_perl Apache module uses this environment variable for
its own purposes and extends its semantics.  Refer to L<the mod_perl
documentation|https://perl.apache.org/docs/> for more information.
Also, spawned threads do the equivalent of setting this variable to the
value 1.)

If, at the end of a run, you get the message I<N scalars leaked>, you
can recompile with C<-DDEBUG_LEAKING_SCALARS> (C<Configure
-Accflags=-DDEBUG_LEAKING_SCALARS>), which will cause the addresses of
all those leaked SVs to be dumped along with details as to where each
SV was originally allocated.  This information is also displayed by
L<Devel::Peek>.  Note that the extra details recorded with each SV
increase memory usage, so it shouldn't be used in production
environments.  It also converts C<new_SV()> from a macro into a real
function, so you can use your favourite debugger to discover where
those pesky SVs were allocated.

If you see that you're leaking memory at runtime, but neither valgrind
nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably
leaking SVs that are still reachable and will be properly cleaned up
during destruction of the interpreter.  In such cases, using the C<-Dm>
switch can point you to the source of the leak.  If the executable was
built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV
allocations in addition to memory allocations.  Each SV allocation has
a distinct serial number that will be written on creation and
destruction of the SV.  So if you're executing the leaking code in a
loop, you need to look for SVs that are created, but never destroyed
between each cycle.  If such an SV is found, set a conditional
breakpoint within C<new_SV()> and make it break only when
C<PL_sv_serial> is equal to the serial number of the leaking SV.  Then
you will catch the interpreter in exactly the state where the leaking
SV is allocated, which is sufficient in many cases to find the source
of the leak.

As C<-Dm> is using the PerlIO layer for output, it will by itself
allocate quite a bunch of SVs, which are hidden to avoid recursion. You
can bypass the PerlIO layer if you use the SV logging provided by
C<-DPERL_MEM_LOG> instead.

=for apidoc_section $debugging
=for apidoc Amnh||PL_sv_serial

=head2 Leaked SV spotting: sv_mark_arenas() and sv_sweep_arenas()

These functions exist only on C<DEBUGGING> builds. The first marks all
live SVs which can be found in the SV arenas with the C<SVf_BREAK> flag.
The second lists any such SVs which don't have the flag set, and resets
the flag on the rest. They are intended to identify SVs which are being
created, but not freed, between two points in code. They can be used
either by temporarily adding calls to them in the relevant places in the
code, or by calling them directly from a debugger.

For example, suppose the following code was found to be leaking:

    while (1) { eval '\(1..3)' }

A F<gdb> session on a threaded perl might look something like this:

    $ gdb ./perl
    (gdb) break Perl_pp_entereval
    (gdb) run -e'while (1) { eval q{\(1..3)} }'
    ...
    Breakpoint 1, Perl_pp_entereval ....
    (gdb) call Perl_sv_mark_arenas(my_perl)
    (gdb) continue
    ...
    Breakpoint 1, Perl_pp_entereval ....`
    (gdb) call Perl_sv_sweep_arenas(my_perl)
    Unmarked SV: 0xaf23a8: AV()
    Unmarked SV: 0xaf2408: IV(1)
    Unmarked SV: 0xaf2468: IV(2)
    Unmarked SV: 0xaf24c8: IV(3)
    Unmarked SV: 0xace6c8: PV("AV()"\0)
    Unmarked SV: 0xace848: PV("IV(1)"\0)
    (gdb) 

Here, at the start of the first call to pp_entereval(), all existing SVs
are marked. Then at the start of the second call, we list all the SVs
which have been since been created but not yet freed. It is quickly clear
that an array and its three elements are likely not being freed, perhaps
as a result of a bug during constant folding. The final two SVs are just
temporaries created during the debugging output and can be ignored.

This trick relies on the C<SVf_BREAK> flag not otherwise being used. This
flag is typically used only during global destruction, but also sometimes
for a mark and sweep operation when looking for common elements on the two
sides of a list assignment. The presence of the flag can also alter the
behaviour of some specific actions in the core, such as choosing whether to
copy or to COW a string SV. So turning it on can occasionally alter the
behaviour of code slightly.

=head2 PERL_MEM_LOG

If compiled with C<-DPERL_MEM_LOG> (C<-Accflags=-DPERL_MEM_LOG>), both
memory and SV allocations go through logging functions, which is handy
for breakpoint setting.

Unless C<-DPERL_MEM_LOG_NOIMPL> (C<-Accflags=-DPERL_MEM_LOG_NOIMPL>) is
also compiled, the logging functions read $ENV{PERL_MEM_LOG} to
determine whether to log the event, and if so how:

    $ENV{PERL_MEM_LOG} =~ /m/           Log all memory ops
    $ENV{PERL_MEM_LOG} =~ /s/           Log all SV ops
    $ENV{PERL_MEM_LOG} =~ /c/           Additionally log C backtrace for
                                        new_SV events
    $ENV{PERL_MEM_LOG} =~ /t/           include timestamp in Log
    $ENV{PERL_MEM_LOG} =~ /^(\d+)/      write to FD given (default is 2)

Memory logging is somewhat similar to C<-Dm> but is independent of
C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(), and
Safefree() are logged with the caller's source code file and line
number (and C function name, if supported by the C compiler).  In
contrast, C<-Dm> is directly at the point of C<malloc()>.  SV logging
is similar.

Since the logging doesn't use PerlIO, all SV allocations are logged and
no extra SV allocations are introduced by enabling the logging.  If
compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for each SV
allocation is also logged.

The C<c> option uses the C<Perl_c_backtrace> facility, and therefore
additionally requires the Configure C<-Dusecbacktrace> compile flag in
order to access it.

=head2 DDD over gdb

Those debugging perl with the DDD frontend over gdb may find the
following useful:

You can extend the data conversion shortcuts menu, so for example you
can display an SV's IV value with one click, without doing any typing.
To do that simply edit ~/.ddd/init file and add after:

  ! Display shortcuts.
  Ddd*gdbDisplayShortcuts: \
  /t ()   // Convert to Bin\n\
  /d ()   // Convert to Dec\n\
  /x ()   // Convert to Hex\n\
  /o ()   // Convert to Oct(\n\

the following two lines:

  ((XPV*) (())->sv_any )->xpv_pv  // 2pvx\n\
  ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx

so now you can do ivx and pvx lookups or you can plug there the sv_peek
"conversion":

  Perl_sv_peek(my_perl, (SV*)()) // sv_peek

(The my_perl is for threaded builds.)  Just remember that every line,
but the last one, should end with \n\

Alternatively edit the init file interactively via: 3rd mouse button ->
New Display -> Edit Menu

Note: you can define up to 20 conversion shortcuts in the gdb section.

=head2 C backtrace

On some platforms Perl supports retrieving the C level backtrace
(similar to what symbolic debuggers like gdb do).

The backtrace returns the stack trace of the C call frames, with the
symbol names (function names), the object names (like "perl"), and if
it can, also the source code locations (file:line).

The supported platforms are Linux, and OS X (some *BSD might work at
least partly, but they have not yet been tested).

This feature hasn't been tested with multiple threads, but it will only
show the backtrace of the thread doing the backtracing.

The feature needs to be enabled with C<Configure -Dusecbacktrace>.

The C<-Dusecbacktrace> also enables keeping the debug information when
compiling/linking (often: C<-g>).  Many compilers/linkers do support
having both optimization and keeping the debug information.  The debug
information is needed for the symbol names and the source locations.

Static functions might not be visible for the backtrace.

Source code locations, even if available, can often be missing or
misleading if the compiler has e.g. inlined code.  Optimizer can make
matching the source code and the object code quite challenging.

=over 4

=item Linux

You B<must> have the BFD (-lbfd) library installed, otherwise C<perl>
will fail to link.  The BFD is usually distributed as part of the GNU
binutils.

Summary: C<Configure ... -Dusecbacktrace> and you need C<-lbfd>.

=item OS X

The source code locations are supported B<only> if you have the
Developer Tools installed.  (BFD is B<not> needed.)

Summary: C<Configure ... -Dusecbacktrace> and installing the Developer
Tools would be good.

=back

Optionally, for trying out the feature, you may want to enable
automatic dumping of the backtrace just before a warning or croak (die)
message is emitted, by adding C<-Accflags=-DUSE_C_BACKTRACE_ON_ERROR>
for Configure.

Unless the above additional feature is enabled, nothing about the
backtrace functionality is visible, except for the Perl/XS level.

Furthermore, even if you have enabled this feature to be compiled, you
need to enable it in runtime with an environment variable:
C<PERL_C_BACKTRACE_ON_ERROR=10>.  It must be an integer higher than
zero, telling the desired frame count.

Retrieving the backtrace from Perl level (using for example an XS
extension) would be much less exciting than one would hope: normally
you would see C<runops>, C<entersub>, and not much else.  This API is
intended to be called B<from within> the Perl implementation, not from
Perl level execution.

The C API for the backtrace is as follows:

=over 4

=item get_c_backtrace

=item free_c_backtrace

=item get_c_backtrace_dump

=item dump_c_backtrace

=back

=head2 Poison

If you see in a debugger a memory area mysteriously full of 0xABABABAB
or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
L<perlclib>.

=head2 Read-only optrees

Under ithreads the optree is read only.  If you want to enforce this,
to check for write accesses from buggy code, compile with
C<-Accflags=-DPERL_DEBUG_READONLY_OPS> to enable code that allocates op
memory via C<mmap>, and sets it read-only when it is attached to a
subroutine. Any write access to an op results in a C<SIGBUS> and abort.

This code is intended for development only, and may not be portable
even to all Unix variants.  Also, it is an 80% solution, in that it
isn't able to make all ops read only.  Specifically it does not apply
to op slabs belonging to C<BEGIN> blocks.

However, as an 80% solution it is still effective, as it has caught
bugs in the past.

=head2 When is a bool not a bool?

There wasn't necessarily a standard C<bool> type on compilers prior to
C99, and so some workarounds were created.  The C<TRUE> and C<FALSE>
macros are still available as alternatives for C<true> and C<false>.
And the C<cBOOL> macro was created to correctly cast to a true/false
value in all circumstances, but should no longer be necessary.  Using
S<C<(bool)> I<expr>> should now always work.

There are no plans to remove any of C<TRUE>, C<FALSE>, nor C<cBOOL>.

=head2 Finding unsafe truncations

You may wish to run C<Configure> with something like

    -Accflags='-Wconversion -Wno-sign-conversion -Wno-shorten-64-to-32'

or your compiler's equivalent to make it easier to spot any unsafe
truncations that show up.

=head2 The .i Targets

You can expand the macros in a F<foo.c> file by saying

    make foo.i

which will expand the macros using cpp.  Don't be scared by the
results.

=head1 AUTHOR

This document was originally written by Nathan Torkington, and is
maintained by the perl5-porters mailing list.

