PERLHACKTIPS(1) Perl Programmers Reference Guide PERLHACKTIPS(1) #
PERLHACKTIPS(1) Perl Programmers Reference Guide PERLHACKTIPS(1)
NNAAMMEE #
perlhacktips - Tips for Perl core C code hacking
DDEESSCCRRIIPPTTIIOONN #
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.
CCOOMMMMOONN PPRROOBBLLEEMMSS #
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.
PPeerrll eennvviirroonnmmeenntt pprroobblleemmss • 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 _i_n_t_r_p_v_a_r_._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 _e_m_b_e_d_._p_l
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 _e_m_b_e_d_._p_l 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 _e_m_b_e_d_._p_l in
perlguts.
CC9999 #
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
_m_e_m_b_e_r _s_t_r_u_c_t_u_r_e _i_n_i_t_i_a_l_i_s_e_r_s 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 a gcc extension not yet in any published
standard.
• 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"
};
• 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 iiss 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 aannyy 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 _h_a_n_d_y_._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 _C_o_n_f_i_g_u_r_e, set a variable in _c_o_n_f_i_g_._s_h, 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, _C_o_n_f_i_g_u_r_e 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
_m_o_s_t C99 (with warnings), but forbid _d_e_c_l_a_r_a_t_i_o_n_s _i_n _f_o_r _l_o_o_p_s unless
"-std=gnu99" is added. The alternative "-std=c99" mmiigghhtt 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 _e_x_t_/ _d_i_s_t_/ _c_p_a_n_/ etc are compiled with the same
flags as the installed perl would use to compile XS extensions.
Basically, it's safe to assume that _C_o_n_f_i_g_u_r_e and _c_f_l_a_g_s_._S_H 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"
PPoorrttaabbiilliittyy pprroobblleemmss 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 PPTTRR22IIVV(()) macro
that does it right. (Likewise, there are PPTTRR22UUVV(()), PPTTRR22NNVV(()),
IINNTT22PPTTRR(()), and NNUUMM22PPTTRR(()).)
• 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 FFPPTTRR22DDPPTTRR(()) and DDPPTTRR22FFPPTTRR(())
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 nnoott guaranteed to be _e_x_a_c_t_l_y
32 bits, they are _a_t _l_e_a_s_t 32 bits, nor are they guaranteed to be iinntt
or lloonngg. 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 aannyytthhiinngg 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 for usually equivalent to ssiizzeeooff(()) of 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 _u_t_f_8_._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
_r_e_g_e_n_/_u_n_i_c_o_d_e___c_o_n_s_t_a_n_t_s_._p_l, and have Perl create "#define"'s for you,
based on the current platform.
Note that the "is_F_O_O" and "to_F_O_O" macros in _h_a_n_d_y_._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 _u_t_f_8_._h and _u_t_f_e_b_c_d_i_c_._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_/_c_h_a_r_s_e_t___t_o_o_l_s_._p_l 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 BBUURRGGLLEE(()) #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 _c_o_n_f_i_g_._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 pprriinnttff(())'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
PPeerrll__vvaa__ccooppyy(()) 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 #
• Testing for operating systems or versions when should be testing for
features
#ifdef __FOONIX__ /* BAD */
foo = quux();
#endif
Unless you know with 100% certainty that qquuuuxx(()) is only ever
available for the "Foonix" operating system aanndd that is available aanndd
correctly working for aallll past, present, aanndd 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
qquuuuxx(()), 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 qquuuuxx(()) 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
<http://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)))
PPrroobblleemmaattiicc SSyysstteemm IInntteerrffaacceess • 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"_-_f_o_o 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.
• mmaalllloocc(0), rreeaalllloocc(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.)
• ssnnpprriinnttff(()) - the return type is unportable. Use mmyy__ssnnpprriinnttff(())
instead.
SSeeccuurriittyy pprroobblleemmss Last but not least, here are various tips for safer coding. See also perlclib for libc/stdio replacements one should use.
• Do not use ggeettss(())
Or we will publicly ridicule you. Seriously.
• Do not use ttmmppffiillee(())
Use mmkksstteemmpp(()) instead.
• Do not use ssttrrccppyy(()) or ssttrrccaatt(()) or ssttrrnnccppyy(()) or ssttrrnnccaatt(())
Use mmyy__ssttrrllccppyy(()) and mmyy__ssttrrllccaatt(()) instead: they either use the native
implementation, or Perl's own implementation (borrowed from the
public domain implementation of INN).
• Do not use sspprriinnttff(()) or vvsspprriinnttff(())
If you really want just plain byte strings, use mmyy__ssnnpprriinnttff(()) and
mmyy__vvssnnpprriinnttff(()) instead, which will try to use ssnnpprriinnttff(()) and
vvssnnpprriinnttff(()) 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 aattooii(())
Use ggrrookk__aattooUUVV(()) instead. aattooii(()) 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 ssttrrttooll(()) or ssttrrttoouull(())
Use ggrrookk__aattooUUVV(()) instead. ssttrrttooll(()) or ssttrrttoouull(()) (or their
IV/UV-friendly macro disguises, SSttrrttooll(()) and SSttrrttoouull(()), or AAttooll(()) and
AAttoouull(()) are affected by locale, which is bad.
DDEEBBUUGGGGIINNGG #
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.
PPookkiinngg aatt PPeerrll 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 '$a + 1'
....
(-e:1) gvsv(main::a)
=> 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'
UUssiinngg aa ssoouurrccee--lleevveell ddeebbuuggggeerr 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. WWAARRNNIINNGG: Perl makes
heavy use of macros, and _g_d_b 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, _c_p_p won't
recursively apply those macros for you.
ggddbb mmaaccrroo ssuuppppoorrtt Recent versions of _g_d_b 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 _g_c_c version 3.1, this means configuring with “-Doptimize=-g3”. Other compilers might use a different switch (if they support debugging macros at all).
DDuummppiinngg PPeerrll DDaattaa SSttrruuccttuurreess One way to get around this macro hell is to use the dumping functions in _d_u_m_p_._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 “$a = $b + $c” we used before, but give it a bit of context: “$b = “6XXXX”; $c = 2.3;”. Where’s a good place to stop and poke around?
What about "pp_add", the function we examined earlier to implement the
"+" operator:
(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
Notice we use "Perl_pp_add" and not "pp_add" - see "Internal Functions"
in perlguts. With the breakpoint in place, we can run our program:
(gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
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 $b, 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 #
UUssiinngg ggddbb ttoo llooookk aatt ssppeecciiffiicc ppaarrttss ooff aa pprrooggrraamm 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 $c (1..100) {
study if $c == 50;
}
UUssiinngg ggddbb ttoo llooookk aatt wwhhaatt tthhee ppaarrsseerr//lleexxeerr aarree ddooiinngg If you want to see what perl is doing when parsing/lexing your code, you can use “BEGIN {}”:
print "Before\n";
BEGIN { study; }
print "After\n";
And in gdb:
(gdb) break Perl_pp_study
If you want to see what the parser/lexer is doing inside of "if" blocks
and the like you need to be a little trickier:
if ($a && $b && do { BEGIN { study } 1 } && $c) { ... }
SSOOUURRCCEE CCOODDEE SSTTAATTIICC AANNAALLYYSSIISS #
Various tools exist for analysing C source code ssttaattiiccaallllyy, as opposed to
ddyynnaammiiccaallllyy, 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.
lliinntt 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).
CCoovveerriittyy Coverity (http://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>
HHPP--UUXX ccaaddvviissee ((CCooddee AAddvviissoorr)) 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”.
ccppdd ((ccuutt--aanndd--ppaassttee ddeetteeccttoorr)) 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://pmd.github.io/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 ...
ggcccc wwaarrnniinnggss 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.
WWaarrnniinnggss ooff ootthheerr CC ccoommppiilleerrss Other C compilers (yes, there aarree 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.
MMEEMMOORRYY DDEEBBUUGGGGEERRSS #
NNOOTTEE 11: 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 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. The said test must be doing
something that is quite unfriendly for memory debuggers. If you don't
feel like waiting, that you can simply kill away the perl process.
Roughly valgrind slows down execution by factor 10, AddressSanitizer by
factor 2.
NNOOTTEE 22: 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 ...
NNOOTTEE 33: 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.
NNOOTTEE 44: DynaLoader will not clean up after itself completely unless Perl
is built with the Configure option "-Accflags=-DDL_UNLOAD_ALL_AT_EXIT".
vvaallggrriinndd 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 _t_e_s_t_f_i_l_e_._v_a_l_g_r_i_n_d 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_/_p_e_r_l_._s_u_p_p.
To get valgrind and for more information see
http://valgrind.org/
AAddddrreessssSSaanniittiizzeerr 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>.
PPRROOFFIILLIINNGG #
Depending on your platform there are various ways of profiling Perl.
There are two commonly used techniques of profiling executables:
_s_t_a_t_i_s_t_i_c_a_l _t_i_m_e_-_s_a_m_p_l_i_n_g and _b_a_s_i_c_-_b_l_o_c_k _c_o_u_n_t_i_n_g.
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 _b_a_s_i_c _b_l_o_c_k_s. Basic
blocks are sections of code that are entered only in the beginning and
exited only at the end. For example, a conditional jump starts a basic
block. Basic block profiling usually works by _i_n_s_t_r_u_m_e_n_t_i_n_g the code by
adding _e_n_t_e_r _b_a_s_i_c _b_l_o_c_k _#_n_n_n_n 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.
GGpprrooff PPrrooffiilliinngg _g_p_r_o_f is a profiling tool available in many Unix platforms which uses _s_t_a_t_i_s_t_i_c_a_l _t_i_m_e_-_s_a_m_p_l_i_n_g. You can build a profiled version of _p_e_r_l by compiling using gcc with the flag “-pg”. Either edit _c_o_n_f_i_g_._s_h or re-run _C_o_n_f_i_g_u_r_e. Running the profiled version of Perl will create an output file called _g_m_o_n_._o_u_t 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 _g_p_r_o_f tool can then display the collected data in various ways.
Usually _g_p_r_o_f 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 _g_m_o_n_._s_u_m 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 _g_p_r_o_f.
GGCCCC ggccoovv PPrrooffiilliinngg _b_a_s_i_c _b_l_o_c_k _p_r_o_f_i_l_i_n_g is officially available in gcc 3.0 and later. You can build a profiled version of _p_e_r_l by compiling using gcc with the flags “-fprofile-arcs -ftest-coverage”. Either edit _c_o_n_f_i_g_._s_h or re-run _C_o_n_f_i_g_u_r_e.
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 _._g_c_d_a file will be
created.
To display the results you use the _g_c_o_v utility (which should be
installed if you have gcc 3.0 or newer installed). _g_c_o_v is run on source
code files, like this
gcov sv.c
which will cause _s_v_._c_._g_c_o_v to be created. The _._g_c_o_v files contain the
source code annotated with relative frequencies of execution indicated by
"#" markers. If you want to generate _._g_c_o_v 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 _g_c_o_v 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 _g_c_o_v and basic block profiling with gcc, see the latest GNU CC manual.
As of gcc 4.8, this is at
<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>
ccaallllggrriinndd pprrooffiilliinngg 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 _c_a_l_l_g_r_i_n_d_._o_u_t_._P_I_D, 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
_c_a_l_l_g_r_i_n_d___a_n_n_o_t_a_t_e. In it's 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.
MMIISSCCEELLLLAANNEEOOUUSS TTRRIICCKKSS #
PPEERRLL__DDEESSTTRRUUCCTT__LLEEVVEELL #
If you want to run any of the tests yourself manually using e.g.
valgrind, please note that by default perl ddooeess nnoott explicitly cleanup
all the memory it has allocated (such as global memory arenas) but
instead lets the eexxiitt(()) 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 also this environment variable for
its own purposes and extended its semantics. Refer to the mod_perl
documentation 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 _s_c_a_l_a_r_s _l_e_a_k_e_d, 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 increases
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.
PPEERRLL__MMEEMM__LLOOGG #
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} =~ /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 NNeewwxx(()), RReenneeww(()), and
SSaaffeeffrreeee(()) 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.
DDDDDD oovveerr ggddbb 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.
CC bbaacckkttrraaccee 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 mmuusstt 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 oonnllyy if you have the
Developer Tools installed. (BFD is nnoott 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 ffrroomm wwiitthhiinn 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
PPooiissoonn If you see in a debugger a memory area mysteriously full of 0xABABABAB or 0xEFEFEFEF, you may be seeing the effect of the PPooiissoonn(()) macros, see perlclib.
RReeaadd--oonnllyy ooppttrreeeess 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.
WWhheenn iiss aa bbooooll nnoott aa bbooooll?? 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)” _e_x_p_r> should now always work.
There are no plans to remove any of "TRUE", "FALSE", nor "cBOOL".
FFiinnddiinngg uunnssaaffee ttrruunnccaattiioonnss 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.
TThhee ..ii TTaarrggeettss You can expand the macros in a _f_o_o_._c file by saying
make foo.i
which will expand the macros using cpp. Don't be scared by the results.
AAUUTTHHOORR #
This document was originally written by Nathan Torkington, and is
maintained by the perl5-porters mailing list.
perl v5.36.3 2023-02-15 PERLHACKTIPS(1)