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 plays by ANSI C89 rules: no C99 (or C++) extensions. In
some cases we have to take pre-ANSI requirements into consideration.
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 (or a dVAR) 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"};
There is a way to completely hide any modifiable globals (they are
all moved to heap), the compilation setting
"-DPERL_GLOBAL_STRUCT_PRIVATE". It is not normally used, but can
be used for testing, read more about it in "Background and
PERL_IMPLICIT_CONTEXT" in perlguts.
* 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.
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.
If using gcc, you can add the "-std=c89" option which will hopefully
catch most of these unportabilities. (However it might also catch
incompatibilities in your system's header files.)
Use the Configure "-Dgccansipedantic" flag to enable the gcc "-ansi
-pedantic" flags which enforce stricter ANSI rules.
If using the "gcc -Wall" note that not all the possible warnings (like
"-Wunitialized") are given unless you also compile with "-O".
Note that if using gcc, starting from Perl 5.9.5 the Perl core source
code files (the ones at the top level of the source code distribution,
but not e.g. the extensions under ext/) are automatically compiled with
as many as possible of the "-std=c89", "-ansi", "-pedantic", and a
selection of "-W" flags (see cflags.SH).
Also study perlport carefully to avoid any bad assumptions about the
operating system, filesystems, character set, and so forth.
You may once in a while try a "make microperl" to see whether we can
still compile Perl with just the bare minimum of interfaces. (See
README.micro.)
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 really explicitly need 64-bit variables, use
I64 and U64, but only if guarded by HAS_QUAD.
* 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 for usually equivalent to sizeof() 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 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).
* Using //-comments
// This function bamfoodles the zorklator. /* BAD */
That is C99 or C++. Perl is C89. Using the //-comments is
silently allowed by many C compilers but cranking up the ANSI C89
strictness (which we like to do) causes the compilation to fail.
* Mixing declarations and code
void zorklator()
{
int n = 3;
set_zorkmids(n); /* BAD */
int q = 4;
That is C99 or C++. Some C compilers allow that, but you
shouldn't.
The gcc option "-Wdeclaration-after-statements" scans for such
problems (by default on starting from Perl 5.9.4).
* Introducing variables inside for()
for(int i = ...; ...; ...) { /* BAD */
That is C99 or C++. While it would indeed be awfully nice to have
that also in C89, to limit the scope of the loop variable, alas, we
cannot.
* 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);
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 using variadic macros
gcc has had them for a while with its own syntax, and C99 brought
them with a standardized syntax. Don't use the former, and use the
latter only if the HAS_C99_VARIADIC_MACROS is defined.
* 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. The Perl code does
admittedly use them if available to gain some extra speed
(essentially as a funky form of inlining), but you shouldn't.
* 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 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
<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 light-weight wrappers for some of these
functions, and which don't make copies of the static memory. A
good example is the interface to the environment variables that are
in effect for the program. Perl has "PerlEnv_getenv" to get values
from the environment. But the return is a pointer to static memory
in the C library. If you are using the value to immediately test
for something, that's fine, but if you save the value and expect it
to be unchanged by later processing, you would be wrong, but
perhaps you wouldn't know it because different C library
implementations behave differently, and the one on the platform
you're testing on might work for your situation. But on some
platforms, a subsequent call to "PerlEnv_getenv" or related
function WILL overwrite the memory that your first call points to.
This has led to some hard-to-debug problems. 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:
if ((s = PerlEnv_getenv("foo") == NULL) {
... /* handle NULL case */
}
else {
s = SvPVX(sv_2mortal(newSVpv(s, 0)));
}
The above example works only if "s" is "NUL"-terminated; otherwise
you have to pass its length to "newSVpv".
Problematic System Interfaces
* 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 -D optimize=-g
make
"-g" is a flag to the C compiler 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).
Configure 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
t Trace execution
o Method and overloading resolution
c String/numeric conversions
Some of the functionality of the debugging code can be achieved 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 "$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
309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
(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 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 $c (1..100) {
study if $c == 50;
}
Using gdb to look at what the parser/lexer are doing
If you want to see what perl is doing when parsing/lexing your code,
you can use "BEGIN {}":
print "Before\n";
BEGIN { study; }
print "After\n";
And in gdb:
(gdb) break Perl_pp_study
If you want to see what the parser/lexer is doing inside of "if" blocks
and the like you need to be a little trickier:
if ($a && $b && do { BEGIN { study } 1 } && $c) { ... }
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, splint
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 variant called "splint" (Secure Programming Lint)
available from http://www.splint.org/ that should compile on any Unix-
like platform.
There are "lint" and <splint> targets in Makefile, but you may have to
diddle with the flags (see above).
Coverity
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>
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 (http://pmd.sourceforge.net/cpd.html) is part of the pmd project
(http://pmd.sourceforge.net/). 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.
The "-ansi" (and its sidekick, "-pedantic") would be nice to be on
always, but unfortunately they are not safe on all platforms, they can
for example cause fatal conflicts with the system headers (Solaris
being a prime example). If Configure "-Dgccansipedantic" is used, the
"cflags" frontend selects "-ansi -pedantic" for the platforms where
they are known to be safe.
Starting from Perl 5.9.4 the following extra flags are added:
* "-Wendif-labels"
* "-Wextra"
* "-Wdeclaration-after-statement"
The following flags would be nice to have but they would first need
their own Augean stablemaster:
* "-Wpointer-arith"
* "-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 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.
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
http://valgrind.org/
AddressSanitizer
AddressSanitizer is a clang and gcc extension, included in clang since
v3.1 and gcc since v4.8. It checks illegal heap pointers, global
pointers, stack pointers and use after free errors, and is fast enough
that you can easily compile your debugging or optimized perl with it.
It does not check memory leaks though. AddressSanitizer is available
for Linux, Mac OS X and soon on Windows.
To build perl with AddressSanitizer, your Configure invocation should
look like:
sh Configure -des -Dcc=clang \
-Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \
-Alddlflags=-shared\ -faddress-sanitizer
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=-faddress-sanitizer
Compile perl and extensions sources with AddressSanitizer.
* -Aldflags=-faddress-sanitizer
Link the perl executable with AddressSanitizer.
* -Alddlflags=-shared\ -faddress-sanitizer
Link dynamic extensions with AddressSanitizer. You must manually
specify "-shared" because using "-Alddlflags=-shared" will prevent
Configure from setting a default value for "lddlflags", which
usually contains "-shared" (at least on Linux).
See also
<http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>.
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
<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>
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 cleanup
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 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 scalars leaked, you can
recompile with "-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.
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} =~ /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.
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?
On pre-C99 compilers, "bool" is defined as equivalent to "char".
Consequently assignment of any larger type to a "bool" is unsafe and
may be truncated. The "cBOOL" macro exists to cast it correctly.
On those platforms and compilers where "bool" really is a boolean (C++,
C99), it is easy to forget the cast. You can force "bool" to be a
"char" by compiling with "-Accflags=-DPERL_BOOL_AS_CHAR". You may also
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.
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