PERLHACKTIPS(1) Perl Programmers Reference Guide PERLHACKTIPS(1)
NAME
perlhacktips - Tips for Perl core C code hacking
DESCRIPTION
This document will help you learn the best way to go about hacking on the Perl core C code. It covers common
problems, debugging, profiling, and more.
If you haven't read perlhack and perlhacktut yet, you might want to do that first.
COMMON PROBLEMS
Perl source 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
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, 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 data function pointers and data pointers
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 = *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. If a
character doesn't have a trivial input form, you can create a #define for it in both "utfebcdic.h" and
"utf8.h", so that it resolves to different values depending on the character set being used. (There are
three different EBCDIC character sets defined in "utfebcdic.h", so it might be best to insert the #define
three times in that file.)
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.
machines, but as long as the code itself uses the "NATIVE_IS_INVARIANT()" macro appropriately, it works,
even if the comments are wrong.
· 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 is being changed starting in 5.12 so that these
characters will 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).
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
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
#ifdef HAS_QUUX
foo = quux();
#endif
But in any case, try to keep the features and operating systems separate.
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.
· Do not use gets()
Or we will publicly ridicule you. Seriously.
· 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 SVs and Perl_sv_catpvf().
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
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.
· 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
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:
Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1669 if (!sv)
(gdb)
We can now use "Perl_sv_dump" to investigate the SV:
SV = PV(0xa057cc0) at 0xa0675d0
REFCNT = 1
FLAGS = (POK,pPOK)
PV = 0xa06a510 "6XXXX"\0
CUR = 5
LEN = 6
{
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 #
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.
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:
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 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.
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 csh-like shells:
setenv PERL_DESTRUCT_LEVEL 2
For Bourne-type shells:
PERL_DESTRUCT_LEVEL=2
export PERL_DESTRUCT_LEVEL
In Unixy environments you can also use the "env" command:
env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
Purify on Unix
On Unix, Purify creates a new Perl binary. To get the most benefit out of Purify, you should create the perl
to Purify using:
sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
-Uusemymalloc -Dusemultiplicity
where these arguments mean:
· -Accflags=-DPURIFY
Disables Perl's arena memory allocation functions, as well as forcing use of memory allocation functions
derived from the system malloc.
· -Doptimize='-g'
Adds debugging information so that you see the exact source statements where the problem occurs. Without
this flag, all you will see is the source filename of where the error occurred.
· -Uusemymalloc
Disable Perl's malloc so that Purify can more closely monitor allocations and leaks. Using Perl's malloc
will make Purify report most leaks in the "potential" leaks category.
· -Dusemultiplicity
Enabling the multiplicity option allows perl to clean up thoroughly when the interpreter shuts down, which
reduces the number of bogus leak reports from Purify.
Once you've compiled a perl suitable for Purify'ing, then you can just:
make pureperl
which creates a binary named 'pureperl' that has been Purify'ed. This binary is used in place of the standard
'perl' binary when you want to debug Perl memory problems.
As an example, to show any memory leaks produced during the standard Perl testset you would create and run the
Purify'ed perl as:
make pureperl
cd t
../pureperl -I../lib harness
which would run Perl on test.pl and report any memory problems.
Purify outputs messages in "Viewer" windows by default. If you don't have a windowing environment or if you
simply want the Purify output to unobtrusively go to a log file instead of to the interactive window, use
these following options to output to the log file "perl.log":
setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
-log-file=perl.log -append-logfile=yes"
Purify on NT
Purify on Windows NT instruments the Perl binary 'perl.exe' on the fly.
There are several options in the makefile you should change to get the most use out of Purify:
· DEFINES
You should add -DPURIFY to the DEFINES line so the DEFINES line looks something like:
DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
to disable Perl's arena memory allocation functions, as well as to force use of memory allocation
functions derived from the system malloc.
· USE_MULTI = define
Enabling the multiplicity option allows perl to clean up thoroughly when the interpreter shuts down, which
reduces the number of bogus leak reports from Purify.
· #PERL_MALLOC = define
Disable Perl's malloc so that Purify can more closely monitor allocations and leaks. Using Perl's malloc
will make Purify report most leaks in the "potential" leaks category.
· CFG = Debug
Adds debugging information so that you see the exact source statements where the problem occurs. Without
this flag, all you will see is the source filename of where the error occurred.
As an example, to show any memory leaks produced during the standard Perl testset you would create and run
Purify as:
cd win32
make
cd ../t
purify ../perl -I../lib harness
which would instrument Perl in memory, run Perl on test.pl, then finally report any memory problems.
valgrind
The excellent valgrind tool can be used to find out both memory leaks and illegal 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.
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.
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, it uses statistical time-sampling.
You can build a profiled version of perl called "perl.gprof" by invoking the make target "perl.gprof" (What
is required is that Perl must be compiled using the "-pg" flag, you may need to re-Configure). Running the
profiled version of Perl will create an output file called gmon.out is created which contains the profiling
data collected during the execution.
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.
quick hint:
Running the profiled version of Perl will cause profile output to be generated. For each source file an
accompanying ".da" 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.
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 3.0 see
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
and its section titled "8. gcov: a Test Coverage Program"
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
quick hint:
$ sh Configure -des -Dusedevel -Doptimize='-g' \
-Accflags='-fprofile-arcs -ftest-coverage' \
-Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
$ rm -f regexec.c.gcov regexec.gcda
$ ./perl.gcov
$ gcov regexec.c
$ view regexec.c.gcov
MISCELLANEOUS TRICKS
PERL_DESTRUCT_LEVEL
If you want to run any of the tests yourself manually using e.g. valgrind, or the pureperl or perl.third
executables, 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 "third-degreed" Perl:
env PERL_DESTRUCT_LEVEL=2 ./perl.third -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.
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", both memory and SV allocations go through logging functions, which is handy
for breakpoint setting.
Unless "-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.
Poison
· 1
Only sets read-only on all slabs of ops at "CHECK" time, hence ops allocated later via "require" or "eval"
will be re-write
· 2
Turns an entire slab of ops read-write if the refcount of any op in the slab needs to be decreased.
· 3
Turns an entire slab of ops read-write if any op from the slab is freed.
It's not possible to turn the slabs to read-only after an action requiring read-write access, as either can
happen during op tree building time, so there may still be legitimate write access.
However, as an 80% solution it is still effective, as currently it catches a write access during the
generation of Config.pm, which means that we can't yet build perl with this enabled.
The .i Targets
You can expand the macros in a foo.c file by saying
make foo.i
which will expand the macros using cpp. Don't be scared by the results.
AUTHOR
This document was originally written by Nathan Torkington, and is maintained by the perl5-porters mailing
list.
perl v5.16.3 2013-03-04 PERLHACKTIPS(1)
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