PERLHACKTIPS(1) Perl Programmers Reference Guide PERLHACKTIPS(1)
NAME
perlhacktips - Tips for Perl core C code hacking
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 perlhack and perlhacktut yet, you might want to do
that first.
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.
Perl environment problems
• 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 "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 "const"s:
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,
and we use "char" for booleans if "<stdbool.h>" isn't available.
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.
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 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 reserves for its implementation any symbol whose name begins with an
underscore followed immediately by either an uppercase letter "[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 (*.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:
Don't begin a symbol name with an underscore; (e.g., don't use:
"_FOOBAR")
Don't use two consecutive underscores in a symbol name; (e.g., don't use
"FOO__BAR")
POSIX also reserves many symbols. See Section 2.2.2 in
<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 "Perl", "perl", or
"PL_". Any time you introduce a macro into a header file that doesn't
follow that convention, you are creating the possiblity 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 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 "MAX" macro have been problematic, but they were
quickly discovered, and a "#ifdef PERL_CORE" guard added.
So there's no rule imposed about using such symbols, just be aware of
the issues.
Choosing good symbol names
Ideally, a symbol name 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 "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.
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. "last_foo"
could mean either the final "foo" or the previous "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
""av_len"" in perlapi doesn't correspond to what other -len constructs
mean, such as ""sv_len"" in perlapi. Awkward (and controversial)
synonyms were created to use instead that conveyed its true meaning
(""av_top_index"" in perlapi). Eventually, though, someone had the
better idea to create a new name to signify what most people think
"-len" signifies. So ""av_count"" in perlapi was born. And we wish it
had been thought up much earlier.
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
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 "foo" in the macro and the source happens
also to have "foo", the meaning of the macro's "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
"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
"foo" as a parameter, so that "foo" remains freely available to the
caller and the macro interface is explicitly specified.
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
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 "SvPV" Then we could
have
int foo_svpv_ = 0;
This is harder to read than plain "foo", but it is 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 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 "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 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 "STMT_START" .. "STMT_END" construct,
described in perlapi, 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 ""PL_Sv"" in perlintern
and ""PL_na"" in perlapi 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
<https://perlmonks.org/?node_id=11144355>.
Portability problems
The following are common causes of compilation and/or execution
failures, not common 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 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.
• 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().)
• 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.
• Assuming "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, "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.
• 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.
• 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).
• 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. 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.
• 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 "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 config.h):
Uid_t who = ...;
printf("who = %d\n", who); /* BAD */
The problem here is that Uid_t might be not only not "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 is hexadecimal */
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 "Formatted Printing of Size_t and SSize_t" in perlguts for how
to print those.
Also remember that the %p format really does require a void pointer:
U8* p = ...;
printf("p = %p\n", (void*)p);
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 */
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
"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
#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 is 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.
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.
Perl has wrappers for some of these functions. Originally many of
those wrappers returned those volatile pointers. But over time
almost all of them have evolved to return stable copies. To cope
with the remaining ones, do a "savepv" in perlapi to make a copy,
thus avoiding these problems. You will have to free the copy when
you're done to avoid memory leaks. If you don't have control over
when it gets freed, you'll need to make the copy in a mortal scalar,
like so
SvPVX(sv_2mortal(newSVpv(volatile_string, 0)))
Problematic System Interfaces
• 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 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 "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. 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.
• malloc(0), realloc(0), calloc(0, 0) are non-portable. To be
portable allocate at least one byte. (In general you should rarely
need to work at this low level, but instead use the various malloc
wrappers.)
• snprintf() - the return type is unportable. Use my_snprintf()
instead.
Security problems
Last but not least, here are various tips for safer coding. See also
perlclib for libc/stdio replacements one should use.
• Do not use gets()
Or we will publicly ridicule you. Seriously.
• Do not use tmpfile()
Use mkstemp() instead.
• Do not use strcpy() or strcat() or strncpy() or strncat()
Use my_strlcpy() and my_strlcat() instead: they either use the
native implementation, or Perl's own implementation (borrowed from
the public domain implementation of INN).
• Do not use sprintf() or vsprintf()
If you really want just plain byte strings, use my_snprintf() and
my_vsnprintf() instead, which will try to use snprintf() and
vsnprintf() if those safer APIs are available. If you want
something fancier than a plain byte string, use "Perl_form"() or SVs
and Perl_sv_catpvf().
Note that glibc printf(), sprintf(), etc. are buggy before glibc
version 2.17. They won't allow a "%.s" format with a precision to
create a string that isn't valid UTF-8 if the current underlying
locale of the program is UTF-8. What happens is that the %s and its
operand are simply skipped without any notice.
<https://sourceware.org/bugzilla/show_bug.cgi?id=6530>.
• Do not use atoi()
Use grok_atoUV() instead. atoi() has ill-defined behavior on
overflows, and cannot be used for incremental parsing. It is also
affected by locale, which is bad.
• Do not use strtol() or strtoul()
Use grok_atoUV() instead. strtol() or strtoul() (or their
IV/UV-friendly macro disguises, Strtol() and Strtoul(), or Atol()
and Atoul() are affected by locale, which is bad.
DEBUGGING
You can compile a special debugging version of Perl, which allows you to
use the "-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.
Poking at Perl
To really poke around with Perl, you'll probably want to build Perl for
debugging, like this:
./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). It will also turn on the "DEBUGGING"
compilation symbol which enables all the internal debugging code in
Perl. There are a whole bunch of things you can debug with this: perlrun
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'
Using a source-level debugger
If the debugging output of "-D" doesn't help you, it's time to step
through perl's execution with a source-level debugger.
• 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.
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)
"help" will get you into the documentation, but here are the most useful
commands:
• run [args]
Run the program with the given arguments.
• break function_name
• break source.c:xxx
Tells the debugger that we'll want to pause execution when we reach
either the named function (but see "Internal Functions" in perlguts!)
or the given line in the named source file.
• step
Steps through the program a line at a time.
• next
Steps through the program a line at a time, without descending into
functions.
• continue
Run until the next breakpoint.
• finish
Run until the end of the current function, then stop again.
• 'enter'
Just pressing Enter will do the most recent operation again - it's a
blessing when stepping through miles of source code.
• 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;
} *
• print
Execute the given C code and print its results. WARNING: Perl makes
heavy use of macros, and gdb does not necessarily support macros (see
later "gdb macro support"). You'll have to substitute them yourself,
or to invoke cpp on the source code files (see "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)
You may find it helpful to have a "macro dictionary", which you can
produce by saying "cpp -dM perl.c | sort". Even then, cpp won't
recursively apply those macros for you.
gdb macro support
Recent versions of 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 gcc version 3.1, this means
configuring with "-Doptimize=-g3". Other compilers might use a
different switch (if they support debugging macros at all).
Dumping Perl Data Structures
One way to get around this macro hell is to use the dumping functions in
dump.c; these work a little like an internal 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 "$x = $y + $z" we used before, but give
it a bit of context: "$y = "6XXXX"; $z = 2.3;". Where's a good place to
stop and poke around?
What about "pp_add", the function we examined earlier to implement the
"+" operator:
(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
Notice we use "Perl_pp_add" and not "pp_add" - see "Internal Functions"
in perlguts. With the breakpoint in place, we can run our program:
(gdb) run -e '$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 "dPOPTOPnnrl_ul"
arranges for two "NV"s to be placed into "left" and "right" - 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 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. 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 "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 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
"PL_op", and we can dump it with "Perl_op_dump". This'll give us
similar output to 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 knew to look for "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 "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 $i == 50;
}
Using gdb to look at what the parser/lexer are doing
If you want to see what perl is doing when parsing/lexing your code, you
can use "BEGIN {}":
print "Before\n";
BEGIN { study; }
print "After\n";
And in gdb:
(gdb) break Perl_pp_study
If you want to see what the parser/lexer is doing inside of "if" blocks
and the like you need to be a little trickier:
if ($x && $y && do { BEGIN { study } 1 } && $z) { ... }
SOURCE CODE STATIC ANALYSIS
Various tools exist for analysing C source code statically, as opposed
to 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.
lint
The good old C code quality inspector, "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 "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.
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 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 "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. 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 (<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
Though 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), 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.
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 -Aldflags=-fsanitize=address \
-Alddlflags=-shared\ -fsanitize=address \
-fsanitize-blacklist=`pwd`/asan_ignore
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
Compile perl and extensions sources with AddressSanitizer.
• -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).
• -fsanitize-blacklist=`pwd`/asan_ignore
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.)
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/>.
PROFILING
Depending on your platform there are various ways of profiling Perl.
There are two commonly used techniques of profiling executables:
statistical time-sampling and 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 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 instrumenting the
code by adding 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.
Gprof Profiling
gprof is a profiling tool available in many Unix platforms which uses
statistical time-sampling. You can build a profiled version of perl by
compiling using gcc with the flag "-pg". Either edit config.sh or re-
run Configure. Running the profiled version of Perl will create an
output file called 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 "-shared" to the <-Alddlflags> line until RT
#118199 is resolved)
The gprof tool can then display the collected data in various ways.
Usually gprof understands the following options:
• -a
Suppress statically defined functions from the profile.
• -b
Suppress the verbose descriptions in the profile.
• -e routine
Exclude the given routine and its descendants from the profile.
• -f routine
Display only the given routine and its descendants in the profile.
• -s
Generate a summary file called gmon.sum which then may be given to
subsequent gprof runs to accumulate data over several runs.
• -z
Display routines that have zero usage.
For more detailed explanation of the available commands and output
formats, see 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 using gcc with 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 will cause profile output to be
generated. For each source file an accompanying .gcda file will be
created.
To display the results you use the gcov utility (which should be
installed if you have gcc 3.0 or newer installed). gcov is run on
source code files, like this
gcov sv.c
which will cause sv.c.gcov to be created. The .gcov files contain the
source code annotated with relative frequencies of execution indicated
by "#" markers. If you want to generate .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 gcov include "-b" which will summarise the basic
block, branch, and function call coverage, and "-c" which instead of
relative frequencies will use the actual counts. For more information
on the use of gcov and basic block profiling with gcc, see the latest
GNU CC manual. As of gcc 4.8, this is at
<https://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>.
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 ("-g"), you can view the
annotated source and click around, much like Devel::NYTProf's HTML
output.
For basic usage:
valgrind --tool=callgrind ./perl ...
By default it will write 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 prefer to 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
Percentage of counts (of primary sort event) we are interested in.
The default is 99%, 100% might show things that seem to be missing.
• --auto
Annotate 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 perform sample based profiling, it
requires no special preparation of the executables beyond debugging
symbols.
This produces sampling data which can be processed with "gprof".
There is limited documentation <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 will
need to build perl with "CFG=Debug" or "CFG=DebugSymbols".
The Visual Studio profiler is a sampling profiler.
See the visual studio documentation
<https://github.com/MicrosoftDocs/visualstudio-
docs/blob/main/docs/profiling/beginners-guide-to-performance-
profiling.md> to get started.
MISCELLANEOUS TRICKS
PERL_DESTRUCT_LEVEL
If you want to run any of the tests yourself manually using e.g.
valgrind, please note that by default perl does not explicitly clean up
all the memory it has allocated (such as global memory arenas) but
instead lets the 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 "PERL_DESTRUCT_LEVEL" to a non-zero value. The 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 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 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 such 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 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 new_SV() and
make it break only when "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 "-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 marks all
live SVs which can be found in the SV arenas with the "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 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 "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.
PERL_MEM_LOG
If compiled with "-DPERL_MEM_LOG" ("-Accflags=-DPERL_MEM_LOG"), both
memory and SV allocations go through logging functions, which is handy
for breakpoint setting.
Unless "-DPERL_MEM_LOG_NOIMPL" ("-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 "-Dm" but is independent of
"-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,
"-Dm" is directly at the point of 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 "-DDEBUG_LEAKING_SCALARS", the serial number for each SV
allocation is also logged.
The "c" option uses the "Perl_c_backtrace" facility, and therefore
additionally requires the Configure "-Dusecbacktrace" compile flag in
order to access it.
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.
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 "Configure -Dusecbacktrace".
The "-Dusecbacktrace" also enables keeping the debug information when
compiling/linking (often: "-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.
Linux
You must have the BFD (-lbfd) library installed, otherwise "perl"
will fail to link. The 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, 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 "-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:
"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 "runops", "entersub", and not much else. This API is intended
to be called from within the Perl implementation, not from Perl level
execution.
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 see in a debugger a memory area mysteriously full of 0xABABABAB
or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
perlclib.
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
"-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.
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 "BEGIN" blocks.
However, as an 80% solution it is still effective, as it has caught bugs
in the past.
When is a bool not a bool?
There wasn't necessarily a standard "bool" type on compilers prior to
C99, and so some workarounds were created. 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 should no longer be necessary. Using
"(bool)" expr> should now always work.
There are no plans to remove any of "TRUE", "FALSE", nor "cBOOL".
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.
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.
AUTHOR
This document was originally written by Nathan Torkington, and is
maintained by the perl5-porters mailing list.
perl v5.40.1 2026-08-30 PERLHACKTIPS(1)
Generated by dwww version 1.16 on Fri Oct 2 04:10:48 CEST 2026.