PERLHACKTIPS(1perl)

💖 NAME 💖

bestie, you're reading PERLHACKTIPS(1perl) — the perlhacktips page from the Perl Programmers Reference Guide. The actual name line is giving:

perlhacktips - Tips for Perl core C code hacking

✨ DESCRIPTION ✨

okay so 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 perlhack and perlhacktut yet, bestie, you might want to do that first. No cap. 🦋

🔥 COMMON PROBLEMS 🔥

okay so 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. 💀

🫶 Perl environment problems 🫶

• Not compiling with threading — bestie, 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 "How multiple interpreters and concurrency are supported" in perlguts for further discussion about context.

• 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. 🐛
• 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 intrpvar.h (at the very end for binary compatibility).

Introducing read-only (const) globals is okay, as long as you verify with e.g. nm libperl.a|egrep -v ' [TURtr] ' (if your 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 consts:

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

• 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 embed.pl in perlguts.
• 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 embed.pl in 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 embed.pl in perlguts.

🌈 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 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:

• mixed declarations and code
• 64 bit integer types — for consistency with the existing source code, use the typedefs I64 and U64, instead of using long long and unsigned long long directly.
• variadic macros:

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

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

• declarations in for loops:

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

• 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.

• 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 is undefined behaviour accessing beyond 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.

• "//" comments — all compilers we tested support their use. Not all humans we tested support their use. These should be avoided in header files, as there is CPAN code that breaks if it encounters these.
• booleans — you can use bool, true, and false as provided by <stdbool.h> (or natively in C++).

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

• variable length arrays — not supported by 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.

• C99 types in <stdint.h> — use PERL_INT_FAST8_T etc as defined in handy.h.
• C99 format strings in <inttypes.h> — snprintf in the VMS libc only added support for PRIdN etc very recently, meaning that there are live supported installations without this, or formats such as %zu.

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

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

• Probe for it in Configure, set a variable in config.sh, and add fallback logic in the headers for platforms which don't have it.
• Write test code and verify that it works on platforms we need to support, before relying on it unconditionally.

Likely you want to repeat the same plan as we used to get the current C99 feature set. See the message at 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, 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 most C99 (with warnings), but forbid declarations in for loops unless "-std=gnu99" is added. The alternative "-std=c99" might seem better, but using it on some platforms can prevent <unistd.h> declaring some prototypes being declared, which breaks the build. gcc's "-ansi" flag implies "-std=c89" so we can no longer set that, hence the Configure option "-gccansipedantic" now only adds "-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 "-std=gnu99", "-pedantic", and a selection of "-W" flags (see cflags.SH). Files in ext/ dist/ 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 Configure and 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:

• AIX — "-qlanglvl=stdc99"
• HP/UX — "-AC99"
• Solaris — "-xc99"

🏷️ Symbol Names and Namespace Pollution 🏷️

🎀 Choosing legal symbol names 🎀

C saves the symbol names that start with an underscore immediately followed by either a capital letter [A-Z] or another underscore. That's their turf, bestie, not yours. C++ goes even harder: it reserves any symbol with two underscores in a row, and it also reserves, in the global namespace, any symbol starting with an underscore at all — not just the capital-letter ones. And we care about C++ because header files (*.h) have to compile under it, and some people do their entire dev life in a C++ compiler. <span>😭</span>

And the consequences if you ignore this? Probably none. Unless you happen to land on a name the implementation is using, everything just… works. In fact the perl core itself has more than a few examples of using implementation-reserved symbols. (They're being fixed up gradually, slay.) But your code could straight up stop working the moment the implementation decides to use a name you'd already picked — possibly many years after you picked it. <span>💀</span>

So yeah, it's best to:

• Don't start a file-level symbol name with an underscore; (like, don't use: "_FOOBAR")
  • It is totally fine for a symbol inside a function or block to begin with an underscore followed by a lowercase letter, like this:

{
     int _ref;
     ...
}

• Don't use two consecutive underscores anywhere in a symbol name; (like, don't use "FOO__BAR")

POSIX is also out here reserving a whole pile of symbols, no cap. See Section 2.2.2 in <https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html>. And yeah, perl has conflicts with that too. <span>🫠</span>

Perl reserves for its own use any symbol that begins with "Perl", "perl", or "PL_". So every time you drop a macro into a header file that doesn't follow that convention, you're creating the possibility of a namespace clash with an existing XS module — unless you restrict it, like so:

#ifdef PERL_CORE
#  define my_symbol
#endif

There are tons of symbols in header files that aren't of this form and that are still reachable from the XS namespace, intentionally or not — basically anything in config.h, for example. <span>🙈</span>

Having to use one of these prefixes lowkey hurts the readability of the code, and it hasn't been an actual issue for non-trivial names. Things like perl defining its own "MAX" macro have been problematic, but people found them fast and slapped a "#ifdef PERL_CORE" guard on. So: there's no rule imposed about using such symbols. Just… be aware of the issues. <span>💅</span>

---

Choosing good symbol names <span>✨</span>

Ideally, a symbol name should correctly and precisely describe its intended purpose. But there's tension there: precise names get long, and long names are awkward to type and read. Metaphors could help (a poetic name, cute!) but metaphors tend to be culturally specific and might not land for someone whose first language isn't English, or who comes from a different cultural background. Also, ngl, the talent for writing poetry seems to be rare among programmers. <span>😭</span>

Some symbol names don't reflect their purpose at all, but they're still fine to use because of long-standing convention. These usually came from Mathematics, where "i" and "j" are frequently used as subscripts and "n" as a population count. Since at least the 1950's, computer programs have used "i", etc. as loop variables. Iconic, honestly.

Our guidance: choose a name that reasonably describes the purpose, and comment its declaration more precisely. That's it, that's the whole vibe.

You for sure shouldn't use misleading or ambiguous names. "last_foo" could mean the final "foo" or the previous "foo", which is confusing for the reader — or even for the writer coming back to the code a few months later after working on something else entirely. Sometimes the programmer has a whole particular line of thought going and it never even occurs to them that ambiguity is present. <span>💀</span>

There are probably still a bunch of off-by-1 bugs floating around because the name ""av_len"" in perlapi doesn't correspond to what other -len constructs mean, like ""sv_len"" in perlapi. Awkward (and controversial) synonyms got created instead to convey its true meaning (""av_top_index"" in perlapi). Eventually though, someone had the better idea: make a new name that signifies what most people think -len signifies. And so ""av_count"" in perlapi was born. We wish it had been thought up much earlier. <span>🎀</span>

Writing safer macros <span>🦋</span>

Macros are used extensively in the Perl core to hide internal details from the caller so the caller doesn't have to worry about them. For example, most lines of code don't need to know whether they're running on a threaded versus unthreaded perl. That detail is automatically, mostly hidden. Very demure. <span>✨</span>

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

That said, there are situations where a function won't do and a macro is required. One example: 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 just be complicating overkill — like when the macro simply creates a mnemonic name for some constant value.

If you do choose to use a non-trivial macro, be aware 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 where it's called. So if you have a token "foo" in the macro and the source happens to also have "foo", the meaning of the macro's "foo" becomes that of the caller's. Sometimes that's exactly the behavior you want, but be aware it tends to get confusing later. It effectively turns "foo" into a reserved word for any code that calls the macro, and this fact is usually not documented nor even considered. It's safer to pass "foo" as a parameter, so "foo" stays freely available to the caller and the macro's interface is explicitly specified. <span>🫶</span>

Worse is when the equivalence between the two "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 "foo" in some way. And it might not be until years later that someone comes along with an instance where "foo" is used. For example, a future caller could do this:

#define foo  bar

Then that declaration of "foo" in the macro suddenly becomes:

int bar

Which could mean something completely different happens than intended. And it's hard to debug — the macro and the call may not even be in the same file, so figuring it out takes digging and gnashing of teeth. <span>😭</span>

So: if a macro does use variables, their names should be such that it is very unlikely they'd 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 "SvPV". Then we could have:

int foo_svpv_ = 0;

This is harder to read than plain "foo", but it's pretty much guaranteed that a caller will never naively use "foo_svpv_" (and run into problems). (The lowercasing makes it clearer that this is a variable, but assumes 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, since those, by convention, signify a private variable name. (See "Choosing legal symbol names" for restrictions on what names you can use.)

This kind of name collision doesn't happen with a macro's formal parameters, so they don't need complicated names. But there are pitfalls when a parameter is an expression or has some Perl magic attached. When you call a function, C evaluates the parameter once and passes the result to the function. But when you call a macro, the parameter is copied as-is by the C preprocessor to each instance inside the macro. Which means: when you evaluate a parameter that has side effects, the function and macro results differ. This is especially fraught when a parameter has overload magic — say it's 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. So 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 they're gradually being converted, usually by swapping them to inline functions instead.

Above we said "first make a copy". In a macro, that's easier said than done, because macros are normally expressions, and declarations aren't allowed in expressions. But the "STMT_START" .. "STMT_END" construct, described in perlapi, lets you have declarations in most contexts, as long as you don't need a return value. If you do need a value returned, you can shape the interface so a pointer is passed to the construct, which then stores its result there. (Or you can use GCC brace groups. But those 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 ""PL_Sv"" in perlintern and ""PL_na"" in perlapi to use (with a slight performance penalty) for some such common cases. But beware: a call chain involving multiple macros using them will zap the other's use. These have been very difficult to debug. <span>💀</span>

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

Portability problems <span>🌈</span>

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. <span>🙈</span>

Also study 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.

• <mark>Casting pointers to integers or casting integers to pointers</mark>

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

or

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

okay bestie so both of those? so bad, so broken, so unportable 😭 the vibe is giving "I learned C in 1987 and refuse to evolve." Use the PTR2IV() macro that does it right, periodt. (And yes, there's a whole girl squad: PTR2UV(), PTR2NV(), INT2PTR(), and NUM2PTR() 💅)

• 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. Still, you should use the FPTR2DPTR() and DPTR2FPTR() macros, not vibes. Sometimes you can also play games with unions.
• Assuming sizeof(int) == sizeof(long) — bestie 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, long long is not a portable way to specify 64 bits, and long long is not even guaranteed to be any wider than long.) 😳

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

• 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. Rude? Yes. Correct? Also yes.

• Lvalue casts

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

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

• Assume anything about structs (especially the ones you don't control, like the ones coming from the system headers)
  • That a certain field exists in a struct
  • That no other fields exist besides the ones you know of
  • That a field is of certain signedness, sizeof, or type
  • That the fields are in a certain order
    • While C guarantees the ordering specified in the struct definition, between different platforms the definitions might differ
  • That the sizeof(struct) or the alignments are the same everywhere
    • There might be padding bytes between the fields to align the fields - the bytes can be anything
    • Structs are required to be aligned to the maximum alignment required by the fields - which for native types is usually equivalent to sizeof(the_field).

tl;dr: do not trust structs you didn't birth yourself. 🫶

• Assuming the character set is ASCIIish

Perl can compile and run under EBCDIC platforms. See 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 'A', but not 0x41. You can safely say '\n', but not "\012". However, you can use macros defined in utf8.h to specify any code point portably. 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 LATIN1_TO_NATIVE are from 0x00 through 0xFF. If your input isn't guaranteed to be in that range, use UNICODE_TO_NATIVE instead. NATIVE_TO_LATIN1 and 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 regen/unicode_constants.pl, and have Perl create #define's for you, based on the current platform. ✨

Note that the isFOO and toFOO macros in 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 utf8.h and 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 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 "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, hibit. The situation differs and is not so simple on EBCDIC machines, but as long as the code itself uses the NATIVE_IS_INVARIANT() macro appropriately, it works, even if the comments are wrong. 🦋

As noted in "TESTING" in perlhack, when writing test scripts, the file 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 "\0", "\r", and "\N{VT}" (also specifiable as "\cK", "\x0B", "\N{U+0B}", or "\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. 🧚

• 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). ✨

• 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, bestie. 😭 For example in the above you need two separate BURGLE() #defines, one for each #ifdef branch.

• 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 -Wendif-labels warns about the bad variant (by default on starting from Perl 5.9.4).

• 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).

• 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.

• 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. (Not AIX doing the most, that's just AIX.)

• Using printf formats for non-basic C types

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

While this might by accident work on some platform (where IV happens to be an "int"), in general it cannot. IV might be something larger. And it gets even worse with the more specific types (the ones Perl's configuration step defines in config.h):

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

The drama here is that Uid_t might not only be not-"int"-wide, it might also be unsigned — in which case big uids print as negative values. 💀

There is no simple solution to this, because printf() only has so much brainpower, but for many types the right format is right there with either an 'f' or '_f' suffix, like:

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);

For how to print Size_t and SSize_t, go peek at "Formatted Printing of Size_t and SSize_t" in perlguts. 💖

Also bestie, remember that the %p format really does require a void pointer:

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

And the gcc option -Wformat scans for such problems. ✨

• 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.
• Using gcc statement expressions

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

Okay it's a nice extension, lowkey, but 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 "static inline" functions, so use them instead. Declare functions as "PERL_STATIC_INLINE" to transparently fall back to emulation where needed. 🦋

• Binding together several statements in a macro — use the macros "STMT_START" and "STMT_END".

STMT_START {
   ...
} STMT_END

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

• Testing for operating systems or versions when you should be testing for features — the classic oopsie:

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

Unless you know with 100% certainty that quux() is only ever available for the "Foonix" operating system and that it's available and correctly working for all past, present, 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. periodt.

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

• 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. 😭

"Dealing with embedded perls and threads" in perlclib contains a list of problematic functions with good advice as to how to cope with them.

🌈 Problematic System Interfaces 🌈

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

Remember that Perl strings are NOT the same as C strings: they may contain "NUL" characters, whereas a C string is terminated by the first "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 that's the reason so 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 "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 "NUL"s in your tests. Now it's fairly rare in most real world cases to get "NUL"s, so your code may seem to work, until one day a "NUL" comes along. 🙈

Here's an example, bestie. It used to be a common paradigm, for decades, in the perl core to use "strchr("list", c)" to see if the character "c" is any of the ones given in "list", a double-quote-enclosed string of the set of characters that we are seeing if "c" is one of. As long as "c" isn't a "NUL", it works. But when "c" is a "NUL", "strchr" returns a pointer to the terminating "NUL" in "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 "mem"-foo C library functions instead. In this case "memchr" can be used to see if "c" is in "list" and works even if "c" is "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 "String Handling" in perlapi. 💅

---

🐛 DEBUGGING 🐛

Most code won't work as intended the first time it is run, and so needs to be debugged and revised. Scarier is code that seems to work but has hidden flaws that a hacker could use to perform malicious actions. 😱 There are ways to detect both types.

🔍 Poking at Perl 🔍

We will start with the first kind. You can compile a special debugging version of Perl, which enables many built-in checks that things are going as expected. When one fails, the interpreter abruptly stops with a message as to what went wrong. Many people always use this option during code development. It also allows you to enable tracing of one or more specific areas of the execution, and to use a debugger much more conveniently.

To create this kind of Perl, you will need to:

./Configure -d -DDEBUGGING
make

"-DDEBUGGING" turns on the C compiler's "-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). See "Using a source-level debugger" for more on this.

"-DDEBUGGING" also causes each compiled file to have the symbol "DEBUGGING" #defined. This enables all the internal debugging code in Perl, including the ability to trace a whole bunch of things. perlrun lists them all, and the best way to find out about them is to play about with them. The most useful options for a general overview of what is going on 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)

All the options are listed in "-Dletters" in perlrun. Note that "-DU" is reserved for your (temporary) use; you can add your own debugging statements controlled just by that switch. 👑

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'

More details can be found in "Tracing the interpreter execution" in perlguts.

🕵️ Catching hidden flaws 🕵️

Various tools exist to find hidden flaws in your code, but these typically cause an unacceptable slowdown, and so tend not to get used until a problem is found, which may be after a hacker has caused damage. 😬 See "MEMORY DEBUGGERS" for more on these.

But perl does have a compilation option that catches many instances of the important flaw of using uninitialized values, and slows things down by much less than these tools — by around 5%. To enable it, add this option to your Configure call

./Configure ... -Accflags=-DPERL_POISON
make

This causes perl to set any created or freed memory to specific bit patterns that should be illegal as pointers (and floating point numbers), and also hopefully surprising enough as integers, so that any code attempting to use the data without forethought will break sooner rather than later. 💥

You may wish to always cause this to happen for particular memory allocations. The following macros (which have similar arguments to Zero()) accomplish this:

PoisonWith(dst, n, t, b)    scribble memory with byte b
PoisonNew(dst, n, t)        equal to PoisonWith(dst, n, t, 0xAB)
PoisonFree(dst, n, t)       equal to PoisonWith(dst, n, t, 0xEF)
Poison(dst, n, t)           equal to PoisonFree(dst, n, t)

🧚 Using a source-level debugger 🧚

If the debugging output of "-D" doesn't help you, it's time to dive in with a source-level debugger, either to see the stack trace of a core dump (very useful in a bug report), or to try to figure out what went wrong before the core dump by stepping through perl's execution.

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

Like, to actually fire up the debugger bestie, you type:

gdb ./perl

Or if you've got a core dump lying around:

gdb ./perl core

Do it inside your Perl source tree so the debugger can actually read the source code (it's nosy like that 💅). You should see the copyright message, and then this little prompt shows up:

(gdb)

Typing help drops you into the docs, but here are the commands you'll actually be obsessed with:

• run [args] — runs the program with whatever arguments you hand it.
• break function_name
• break source.c:xxx — tells the debugger to pause execution when it reaches either that named function (but peek at "Internal Functions" in perlguts first, iykyk!) or that given line in that named source file.
• step — walks through the program one line at a time.
• next — same energy, one line at a time, except it does not descend into functions.
• continue — runs until it hits the next breakpoint.
• finish — runs until the end of the current function, then stops again.
• 'enter' — literally just smashing Enter repeats the most recent operation, and it's a whole blessing when you're stepping through miles and miles of source code 😭
• ptype — prints out the C definition of whatever argument you give it.

(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;
} *

• print — executes the C code you give it and prints the results. WARNING: Perl leans heavily on macros, and gdb doesn't necessarily support macros (see the later "gdb macro support" bit). You'll have to substitute them yourself, or invoke cpp on the source code files (see "The .i Targets"). So for instance, you can not say

print SvPV_nolen(sv)

but you do have to say

print Perl_sv_2pv_nolen(sv)

Ngl, having a "macro dictionary" is genuinely helpful, and you can make one by saying cpp -dM perl.c | sort. Even then though, cpp won't recursively apply those macros for you. Sad but true 🫶

👑 gdb macro support 👑

Recent versions of gdb have pretty good macro support, but to actually use it you'll need to compile perl with the macro definitions included in the debugging information. Using gcc version 3.1, that means configuring with -Doptimize=-g3. Other compilers might use a different switch (if they even support debugging macros at all, no cap).

🌸 Dumping Perl Data Structures 🌸

One way to dodge this whole macro hellscape is to use the dumping functions sitting in dump.c. They work a little like an internal Devel::Peek, except they also cover OPs and other structures you simply can't get at from Perl. Let's take an example ✨. We're using that same $x = $y + $z from before, but giving it some context: $y = "6XXXX"; $z = 2.3;. So where's a good spot to stop and poke around at?

What about pp_add — the function we dug into earlier that implements the + operator:

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

Notice we use Perl_pp_add and not pp_add — see "Internal Functions" in perlguts. With that breakpoint in place, we can run our program:

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

Lots of junk goes flying past while gdb reads in all 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 dPOPTOPnnrl_ul arranges for two NVs to be placed into left and right — so let's slightly expand it:

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

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

Since we don't have an NV for $y, we'll have to use sv_2nv to convert it. Step again and boom, we find ourselves right there:

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

Now we can use 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 already know we're getting 6 out of 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 PL_op, and we can dump it with Perl_op_dump. This gives us output similar to the CPAN module B::Debug.

(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 #

🦋 Using gdb to look at specific parts of a program 🦋

With the example above, you already knew to look for Perl_pp_add, but what if there were multiple calls to it scattered all over the place? Or what if you had no clue which op you were even hunting for? 🙈

One move is to inject a rare call somewhere near what you're looking for. For example, you could add study before your method:

study;

And in gdb do:

(gdb) break Perl_pp_study

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

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

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

If you want to see what perl is doing while it's parsing/lexing your code, you can use 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 are doing inside of if blocks and the like, you have to get a little trickier:

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

✨ SOURCE CODE STATIC ANALYSIS ✨

okay so bestie, static analysis is when you analyse the C source code without actually running it 🧐 — it's the opposite of dynamic analysis, and the whole point is that you can sniff out resource leaks, undefined behaviour, type mismatches, portability problems, code paths that would cause illegal memory accesses, and other similar chaos, just by parsing the C code and staring at the resulting graph to see what it says about the execution and data flows. and honestly? that's literally exactly how C compilers know to give you warnings about dubious code. no cap 💅

🌸 lint 🌸

the good old C code quality inspector, lint, is available on several platforms — BUT bestie, be aware that there are several different implementations of it by different vendors, which means the flags are not identical across different platforms 🙈 keep that in mind.

there's a lint target in Makefile, but you may have to diddle with the flags (see above).

🌸 Coverity 🌸

Coverity (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. cute of them honestly 🎀

there is Coverity setup for the perl5 project: https://scan.coverity.com/projects/perl5

🌸 HP-UX cadvise (Code Advisor) 🌸

HP has a C/C++ static analyzer product for HP-UX called 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, iykyk 🔍)

the use of the cadvise_cc recipe with Configure ... -Dcc=./cadvise_cc (see cadvise "User Guide") is recommended; as is the use of +wall.

🌸 cpd (cut-and-paste detector) 🌸

the cpd tool detects cut-and-paste coding. and like, if one instance of the cut-and-pasted code changes, all the other spots should probably be changed too, right? so therefore such code should probably be turned into a subroutine or a macro. periodt ✨

cpd (https://docs.pmd-code.org/latest/pmd_userdocs_cpd.html) is part of the pmd project (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 ...

🌸 gcc warnings 🌸

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

the -Wall is by default on.

it would be nice for -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 -Dgccansipedantic is used, the cflags frontend selects -pedantic for the platforms where it is known to be safe.

the following extra flags are added:

• -Wendif-labels
• -Wextra
• -Wc++-compat
• -Wwrite-strings
• -Werror=pointer-arith
• -Werror=vla

the following flags would be nice to have but they would first need their own Augean stablemaster 💀

• -Wshadow
• -Wstrict-prototypes

the -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.

🌸 Warnings of other C compilers 🌸

other C compilers (yes, there 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 -Xa mode on (though implicitly), or the DEC (these days, HP...) has its -std1 mode on.

---

🦋 MEMORY DEBUGGERS 🦋

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 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.

(also see "Catching hidden flaws".)

NOTE 2: to minimize the number of memory leak false alarms (see "PERL_DESTRUCT_LEVEL" for more information), you have to set the environment variable PERL_DESTRUCT_LEVEL to 2. for example, like this:

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

NOTE 3: there are known memory leaks when there are compile-time errors within eval or require; seeing 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.

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

🌸 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 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 t/perl.supp.

to get valgrind and for more information see https://valgrind.org/

🌸 AddressSanitizer 🌸

AddressSanitizer ("ASan") consists of a compiler instrumentation module and a run-time 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 ASAN_OPTIONS=detect_leaks=1.

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

sh Configure -des -Dcc=clang \
 -Accflags="-fsanitize=address -fsanitize-ignorelist=$PWD/asan_ignore" \
 -Aldflags="-fsanitize=address" -Alddlflags="-shared -fsanitize=address"

where these arguments mean:

• -Dcc=clang — this should be replaced by the full path to your clang executable if it is not in your path.
• -Accflags="-fsanitize=address -fsanitize-ignorelist=$PWD/asan_ignore" — compile perl and extensions sources with AddressSanitizer. AddressSanitizer will ignore functions listed in the asan_ignore file. (this file should contain a short explanation of why each of the functions is listed.)
• -Aldflags="-fsanitize=address" — link the perl executable with AddressSanitizer.
• -Alddlflags="-shared -fsanitize=address" — link dynamic extensions with AddressSanitizer. you must manually specify -shared because using -Alddlflags=-shared will prevent Configure from setting a default value for lddlflags, which usually contains -shared (at least on Linux).

see also https://github.com/google/sanitizers/wiki/AddressSanitizer

🌸 Dr Memory 🌸

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

it supports heap checking like memcheck from valgrind. there are also other tools included.

see https://drmemory.org/

💅 Gprof Profiling

gprof is a profiling tool that's available on a bunch of Unix platforms, and it uses statistical time-sampling. To build a profiled version of perl you compile with gcc using the flag -pg. Either edit config.sh or re-run Configure. Then running that profiled Perl creates an output file called gmon.out, which holds all the profiling data collected while it ran. 🫶

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 -shared to the -Alddlflags line until RT #118199 is resolved)

Then gprof can show you all that collected data in various ways. Usually gprof understands these options:

• -a — suppresses the statically defined functions from the profile. bye bestie 💀
• -b — suppresses the verbose descriptions in the profile.
• -e routine — excludes the given routine and its descendants from the profile.
• -f routine — displays only the given routine and its descendants in the profile. main character only. 👑
• -s — generates a summary file called gmon.sum, which you can then hand to subsequent gprof runs to accumulate data across several runs.
• -z — displays routines that have zero usage.

For a more detailed explanation of the available commands and output formats, go check your own local documentation of gprof. ✨

🌈 GCC gcov Profiling

basic block profiling is officially available in gcc 3.0 and later. You can build a profiled version of perl by compiling with gcc using the flags -fprofile-arcs -ftest-coverage. Either edit config.sh or re-run 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 -shared to the -Alddlflags line until RT #118199 is resolved)

Running the profiled version of Perl makes profile output get generated. For each source file, an accompanying .gcda file gets created. 🍓

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

gcov sv.c

which creates sv.c.gcov. The .gcov files contain the source code annotated with relative frequencies of execution, marked up with "#" markers. And if you want .gcov files for all the 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 gcov include -b, which summarises the basic block, branch, and function call coverage, and -c, which uses the actual counts instead of relative frequencies. For more info on using gcov and basic block profiling with gcc, see the latest GNU CC manual. As of gcc 4.8, that lives at https://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro. 📖

🦋 callgrind profiling

callgrind is a valgrind tool for profiling source code. Paired up with kcachegrind (a Qt based UI), it gives you the whole overview of where your code is eating up time, plus the ability to examine callers, call trees, and more. One of its perks? You can use it on perl and XS modules that were not compiled with debugging symbols. slay. 💅

If perl is compiled with debugging symbols (-g), you can view the annotated source and click around, a lot like Devel::NYTProf's HTML output. iconic.

For basic usage:

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

By default it writes output to callgrind.out.PID, but you can change that with --callgrind-out-file=...

To view the data, do:

kcachegrind callgrind.out.PID

If you'd rather view the data in a terminal, you can use callgrind_annotate. In its basic form:

callgrind_annotate callgrind.out.PID | less

Some useful options are:

• --threshold — the percentage of counts (of the primary sort event) we're interested in. The default is 99%, and 100% might show things that seem to be missing.
• --auto — annotates all source files containing functions that helped reach the event count threshold.

💜 "profiler" profiling (Cygwin)

Cygwin allows for "gprof" profiling and "gcov" coverage testing, but this only profiles the main executable. 🙈

You can use the "profiler" tool to do sample based profiling, and it needs no special preparation of the executables beyond debugging symbols.

This produces sampling data which can then be processed with "gprof".

There's limited documentation at https://www.cygwin.com/cygwin-ug-net/profiler.html on the Cygwin web site.

⭐ 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'll need to build perl with CFG=Debug or CFG=DebugSymbols.

The Visual Studio profiler is a sampling profiler.

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

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🎀 MISCELLANEOUS TRICKS 🎀

🌸 PERL_DESTRUCT_LEVEL

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

There is a way to tell perl to do a complete cleanup though: set the environment variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper does set this to 2, and that's exactly 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 the mod_perl documentation at 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 N scalars leaked, you can recompile with -DDEBUG_LEAKING_SCALARS (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 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 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 -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 those cases, using the -Dm switch can point you to the source of the leak. If the executable was built with -DDEBUG_LEAKING_SCALARS, -Dm will output SV allocations in addition to memory allocations. Each SV allocation has a distinct serial number that gets 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 you find such an SV, set a conditional breakpoint inside new_SV() and make it break only when PL_sv_serial equals the serial number of the leaking SV. Then you'll catch the interpreter in exactly the state where the leaking SV is allocated, which is enough in many cases to find the source of the leak. 😭

Since -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 -DPERL_MEM_LOG instead.

🧚 Leaked SV spotting: sv_mark_arenas() and sv_sweep_arenas()

These functions exist only on DEBUGGING builds. The first one marks all live SVs which can be found in the SV arenas with the SVf_BREAK flag. The second one lists any such SVs which don't have the flag set, and resets the flag on the rest. They're intended to identify SVs which are being created, but not freed, between two points in code. You can use them 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 gdb session on a threaded perl might look something like this:

okay bestie, picking right back up mid-gdb-session, no context lost, we're literally in the middle of the vibe 💅

$ 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)

so here's the tea ☕ at the start of that first call to pp_entereval(), literally all the SVs that exist get marked. then at the start of the second call, we dump every SV that's been created since then but not freed yet. and it's instantly obvious that an array and its three elements are probably not getting freed — like, maybe a bug during constant folding situation. the final two SVs are just temporaries made while we were generating the debugging output, so ignore them, they're innocent 🫶

heads up though — this whole trick relies on the SVf_BREAK flag not otherwise being used. it's normally only used during global destruction, but sometimes also for a mark and sweep operation when hunting for common elements on the two sides of a list assignment. and the flag being set can also alter the behaviour of some specific actions in the core — like choosing whether to copy or to COW a string SV. so turning it on can occasionally change how your code behaves, slightly. lowkey risky, highkey useful 💀

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🧠 PERL_MEM_LOG 🧠

if you compile with -DPERL_MEM_LOG (that's -Accflags=-DPERL_MEM_LOG), then both memory and SV allocations get routed through logging functions — super handy for setting breakpoints, no cap.

unless -DPERL_MEM_LOG_NOIMPL (aka -Accflags=-DPERL_MEM_LOG_NOIMPL) is also compiled in, those logging functions read $ENV{PERL_MEM_LOG} to figure out 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 kind of like -Dm, but it doesn't care about -DDEBUGGING and it sits at a higher level — every single use of Newx(), Renew(), and Safefree() gets logged along with the caller's source code file and line number (plus the C function name, if your C compiler supports that). meanwhile -Dm is down at the raw point of malloc(). SV logging is similar, same energy.

and because the logging doesn't use PerlIO, all SV allocations get logged and no extra SV allocations sneak in just because you turned logging on. 🎀 and if you compiled with -DDEBUG_LEAKING_SCALARS, the serial number for each SV allocation gets logged too.

the "c" option uses the Perl_c_backtrace facility, which means it additionally needs the Configure -Dusecbacktrace compile flag in order to access it.

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🐞 DDD over gdb 🐞

if you're debugging perl with the DDD frontend sitting on top of gdb, you might find this cute:

you can extend the data conversion shortcuts menu, so for example you can display an SV's IV value with one click, zero typing. to do it, just edit the ~/.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 the sv_peek "conversion" in there instead:

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\ — iykyk.

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. that's the limit, bestie, no more. 🫶

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🌈 C backtrace 🌈

on some platforms, perl can retrieve the C level backtrace (basically the same kind of thing symbolic debuggers like gdb do for you).

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 — so like, don't @ us).

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

the feature needs to be enabled with Configure -Dusecbacktrace.

-Dusecbacktrace also makes sure debug information gets kept when compiling/linking (often -g). lots of compilers/linkers support having both optimization and debug info at the same time. and you need that debug info for the symbol names and the source locations.

static functions might not be visible to the backtrace, unfortunately. 🙈

and source code locations, even when they're available, can often be missing or misleading if the compiler has, like, inlined code. optimizer can make matching the source code to the object code quite the challenge.

🐧 Linux 🐧

you must have the BFD (-lbfd) library installed, otherwise perl will fail to link. BFD is usually distributed as part of the GNU binutils.

summary: Configure ... -Dusecbacktrace and you need -lbfd.

🍎 OS X 🍎

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

summary: Configure ... -Dusecbacktrace and installing the Developer Tools would be good.

---

optionally, if you just wanna try the feature out, you might want to enable automatic dumping of the backtrace right before a warning or croak (die) message gets emitted, by adding -Accflags=-DUSE_C_BACKTRACE_ON_ERROR for Configure.

unless that extra feature above is enabled, literally nothing about the backtrace functionality is visible, except for the Perl/XS level.

and even if you did enable this feature to be compiled, you still have to enable it at runtime with an environment variable: PERL_C_BACKTRACE_ON_ERROR=10. it has to be an integer higher than zero, telling it the frame count you want.

retrieving the backtrace from Perl level (say, using an XS extension) would be much less exciting than you'd hope: normally you'd just see runops, entersub, and not much else. 😭 this API is intended to be called from within the Perl implementation, not from Perl level execution. it's an inside-job situation.

the C API for the backtrace is as follows:

• get_c_backtrace
• free_c_backtrace
• get_c_backtrace_dump
• dump_c_backtrace

---

☠️ Poison ☠️

if you spot a memory area in a debugger that's mysteriously full of 0xABABABAB or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros — go read perlclib. 💀

---

🔒 Read-only optrees 🔒

under ithreads the optree is read only. if you want to enforce that, to check for write accesses coming from buggy code, compile with -Accflags=-DPERL_DEBUG_READONLY_OPS to enable code that allocates op memory via mmap, and sets it read-only when it is attached to a subroutine. any write access to an op results in a SIGBUS and abort. 💥 dramatic but effective.

this code is intended for development only, and may not be portable even to all Unix variants. also it's an 80% solution — it isn't able to make all ops read only. specifically, it does not apply to op slabs belonging to BEGIN blocks.

however, as an 80% solution it's still very much effective — it has caught bugs in the past. 👑

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🤔 When is a bool not a bool? 🤔

there wasn't necessarily a standard "bool" type on compilers before C99, so some workarounds got invented. the "TRUE" and "FALSE" macros are still available as alternatives for true and false. and the "cBOOL" macro was created to correctly cast to a true/false value in all circumstances — but it should no longer be necessary. using (bool) expr should now always work.

there are no plans to remove any of "TRUE", "FALSE", nor "cBOOL". rest easy, they're staying. 🫶

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🔍 Finding unsafe truncations 🔍

you may wish to run 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. little warning-magnet for your bug hunt. 🔥

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🎯 The .i Targets 🎯

you can expand the macros in a foo.c file by saying

make foo.i

which will expand the macros using cpp. don't be scared by the results, bestie, it's just a lot of text. 🙈

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✍️ AUTHOR ✍️

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

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and that's a wrap on the whole page, lovelies 💖 this was perl v5.44.0, dated 2026-10-17 — PERLHACKTIPS(1perl), slay, go hack on perl, mwah 💅✨